## Abstracts and Presentations

### Invited talks

Marek Abramowicz: Innermost part of accretion disks around black holes

Marek Abramowicz (Göteborg University, Physics Department)

Innermost part of accretion disks around black holes

Authors: M. Abramowicz

Abstract: Einstein's general relativity makes fundamental predictions about the latest, strong-field, stages of accretion, when matter plunges from the accretion disk into the black hole. They include \begin{itemize} \item location of the plunge-in region, \item efficiency of the black hole accretion, \item nature of torques in the plunge-in region, \item extracting energy from the black hole. \end{itemize} The 1977 Blandford \& Znajek prediction that energy may be extracted from a rotating black hole by an electromagnetic version of the Penrose process was convincingly proven in recent super-computer simulations of the "arrested" accretion of magnetized matter by Tchekhovskoy, McKinney and Narayan${}^*$. Another version of the Penrose process, involving energetic particle collisions near the horizon, has been discussed by Piran and collaborators already in the 1970. These classic works (and their recent follow-up, also by Piran and collaborators) proved that this process can neither extract a significant amount of energy from the black hole nor accelerate particles to large energies. ${}^*$ Ramesh Narayan's lecture describes these simulations.

Presentation:

Back to list of abstracts
Lars Andersson: Cosmological models and stability

Cosmological models and stability

Abstract: In this talk I will discuss some mathematical results on inhomogeneous cosmological models, focusing on late time behavior and the issues of nonlinear stability versus instability.

Presentation:

Back to list of abstracts
Abhay Ashtekar: Loop quantum gravity and the very early universe

Abhay Ashtekar (Institute for Gravitation & the Cosmos, Penn State)

Loop quantum gravity and the very early universe

Authors: Abhay Ashtekar

Abstract: Since the standard cosmological perturbation theory is based on QFT on curved space-times, it is not applicable in the Planck era. Using techniques from loop quantum gravity, the theory is extended to overcome this limitation. The new framework sharpens conceptual issues by distinguishing between true and apparent trans-Planckian difficulties and shows that the true difficulties can be generically overcome in the standard inflationary scenario, with interesting lessons for both theory and observations. The talk will be based largely on some recently completed joint work with Ivan Agullo and William Nelson.

Presentation:

Back to list of abstracts
Leor Barack: Gravitational self-force: orbital mechanics beyond the geodesic approximation

Leor Barack (University of Southampton)

Gravitational self-force: orbital mechanics beyond the geodesic approximation

Authors: Leor Barack

Abstract: The fundamentally simple question of motion in a gravitationally-bound two-body system is a longstanding open problem of General Relativity. When the mass ratio is large the problem lends itself to a perturbative treatment, whereby corrections to the geodesic motion of the smaller object (due to radiation reaction, internal structure, etc.) are accounted for order by order in a small-mass-ratio expansion, using the language of an effective gravitational self-force''. The prospect for observing gravitational waves from compact objects inspiralling into massive black holes in the foreseeable future has in the past 15 years motivated a program to obtain a rigorous formulation of the self-force and compute it for astrophysically interesting systems. I will begin this talk by reviewing the general theory of the gravitational self-force in curved spacetime, and proceed to describe how this theory is being implemented today in actual calculations of the self force for inspiral orbits. I will discuss recent calculations of some conservative post-geodesic effects of the self-force (including the finite-mass correction to the precession rate of the periastron), and highlight the way in which these calculations allow us to make a fruitful contact with post-Newtonian theory and with Numerical Relativity, and also inform the development of a universal Effective One Body model of the two-body dynamics.

Presentation:

Back to list of abstracts
Julian Barbour: Prague and the conception of general relativity: Kepler, Mach and Einstein

Julian Barbour (Department of Physics, University of Oxford)

Prague and the conception of general relativity: Kepler, Mach and Einstein

Authors: J. Barbour

Abstract: In the first part of my talk, I shall argue that Kepler's discovery of his first two laws of planetary motion in Prague in the years 1600 to 1605 can be seen as the first success of Mach's principle. Kepler's intuition was quite unlike that of his predecessors and Newton and strikingly similar to Mach's. In view of the considerable confusion surrounding Mach's principle, I shall try to identify his key ideas, which included a relational treatment of not only position but also time. I will also discuss a penetrating analysis by Poincare of a predictive defect that is revealed when one expresses Newtonian dynamics in relative quantities. On this basis one can formulate a precise criterion that a Machian theory of motion should satisfy. I shall then consider why Einstein did not make any serious attempt to implement a theory of relativity directly along the lines suggested by Mach's and Poincare's ideas and instead followed a brilliantly successful indirect strategy. Finally, I shall consider whether purely historical accidents - such as the creation of general relativity before quantum mechanics - could be closing our minds to new conceptions of time and motion.

Presentation:

Back to list of abstracts
Jiří Bičák: Einstein's Days and Works in Prague: Relativity Then and Now

Jiří Bičák (Charles University in Prague, Institute of Theoretical Physics)

Einstein's Days and Works in Prague:\\ Relativity Then and Now

Authors: J. Bi\v{c}\'{a}k

Abstract: It was during his stay in Prague when Einstein started in earnest to develop his ideas about general relativity. I will evoke those days in 1911 and 1912, discuss Einstein's papers on gravitation from that period, emphasize which new concepts and ideas he introduced. I also want to indicate how the main themes that preoccupied him then, the principle of equivalence, bending of light, gravitational redshift and frame dragging effects are alive in contemporary relativity.

Presentation:

Back to list of abstracts
Donato Bini: Observers, observables and measurements in general relativity

Donato Bini (Istituto per le Applicazioni del Calcolo "M. Picone," CNR, Rome)

Observers, observables and measurements in general relativity

Authors: D. Bini

Abstract: The definition of special observers and frames, geometrically or physically motivated, is discussed. Applications to black hole spacetimes and other astrophysically relevant contexts are also presented.

Presentation:

Back to list of abstracts
Chris Clarkson: Dark Energy and Inhomogeneity

Chris Clarkson (University of Cape Town)

Dark Energy and Inhomogeneity

Authors: Chris Clarkson

Abstract: Most aspects of structure formation in the late universe are treated using Newtonian gravity, either using perturbation theory or N-body simulations. General relativistic corrections to this picture are generally assumed to be 'small'. Surprisingly, not a lot is known about them however, which has led to some speculation that dark energy may in fact be relativistic aspects of structure formation in disguise. While this idea seems implausible to many, it does appear that relativistic corrections to the standard model come in at the percent level or larger. This will be important for precision cosmology and our interpretation of dark energy.

Presentation:

Back to list of abstracts
Thibault Damour: The Two-Body Problem in General Relativity

Thibault Damour (Institut des Hautes Etudes Scientifiques)

The Two-Body Problem in General Relativity

Authors: Thibault Damour

Abstract: The two-body problem has a long history in General Relativity. It has recently acquired a renewed practical importance in view of the development of interferometric detectors of gravitational waves. Indeed, a network of ground-based interferometric gravitational wave detectors (LIGO/VIRGO...) is currently being upgraded, and should, in a few years, reach a sensitivity enabling them to detect the gravitational waves emitted by coalescing compact binaries: i.e. binary systems made of black holes and/or neutron stars. This prospect has motivated renewed theoretical studies of the motion and radiation of relativistic two-body systems. I will review the recent analytical studies of (comparable-mass) two-body systems, and their comparison to numerical relativity results. Particular attention will be given to the recently developed "effective one body" approach to the motion and radiation of binary systems.

Presentation:

Back to list of abstracts
John Friedman: Stability of relativistic stars

John Friedman (University of Wisconsin-Milwaukee)

Stability of relativistic stars

Authors: John L. Friedman

Abstract: The stable relativistic stars form a two-dimensional family parametrized by mass and spin. Radial instabilities to collapse and to explosive expansion set upper and lower limits on their mass; and an instability driven by gravitational waves may limit on their spin. Gravitational waves from unstable nonaxisymmetric modes of nascent neutron stars, old stars spun up by accretion, and the hypermassive remnants of binary mergers, are all candidates for gravitational wave detectors, but major uncertainties in the microphysics persist.

Presentation:

Back to list of abstracts
Helmut Friedrich: The large scale Einstein evolution problem

Helmut Friedrich (Max-Planck-Institut fuer Gravitationsphysik)

The large scale Einstein evolution problem

Authors: H. Friedrich

Abstract: Asymptotic considerations do not only play an important role in the interpretation of gravitational fields but also in deriving large scale existence results. With this in mind we revisit certain existence and non-linear stability results for solutions to Einstein's field equations which have been obtained under various assumptions on the matter fields and the sign of the cosmological constant and discuss various questions concerning the specific asymptotic behaviour of the fields, which has been derived or imposed in the different cases.

Presentation:

Back to list of abstracts
Valeri Frolov: Black holes, hidden symmetry and complete integrability

Valeri Frolov (University of Alberta, Edmonton, Canada)

Black holes, hidden symmetry and complete integrability

Authors: V. Frolov

Abstract: In physics and mathematics the symmetry allows one to simplify a problem, and often to make it solvable. According to the Noether theorem, symmetries are responsible for conservation laws. Besides evident (explicit) spacetime symmetries, responsible, for example, for the conservation of energy, momentum, and angular momentum of a system, there also exist what is called hidden symmetries, which are connected with higher order in momentum integrals of motion. A remarkable fact is that stationary black holes with spherical topology of the horizon in four and higher dimensions always possess a set (tower') of explicit and hidden symmetries which make the equations of motion of particles and light in their spacetime completely integrable. The talk gives a general review of the recently obtained results.

Presentation:

Back to list of abstracts
Gary Gibbons: Links between general relativity and other parts of physics

Gary Gibbons (University of Cambridge)

Links between general relativity and other parts of physics

Authors: G. Gibbons

Abstract: Now that General Relativity has become such a central part of modern physics, its geometrical formalism being taught as part of almost all undergraduate physics courses, it is natural to ask: how can its basic concepts and techniques be used to illuminate areas of physics which have no connection with gravity. Another way of asking this question is: are the analogues situations to those occurring in General Relativity. The search for such analogues is of course an old one, but recently, because of advances in technology, these questions have become more topical. In this talk I will illustrate this theme by examples drawn from optics, acoustics, liquid crystals, graphene and the currently popular topic of cloaking.

Presentation:

Back to list of abstracts
Gabriela Gonzalez: Gravitational Wave Astronomy with LIGO and Virgo detectors

Gabriela Gonzalez (Louisiana State University)

Gravitational Wave Astronomy with LIGO and Virgo detectors

Authors: Gabriela Gonzalez, for the LIGO Scientific Collaboration and the Virgo Collaboration.

Abstract: I will present the latest results from the searches for gravitational waves in LIGO and Virgo data taken in the last several years, including the results from a blind injection experiment in 2010 on the ability to make a discovery, and on the efforts to search for electromagnetic and high-energy counterparts of gravitational wave triggers. I will also present the status of the Advanced LIGO and Virgo detectors, and review the bright prospects for gravitational wave astronomy with the international network.

Presentation:

Back to list of abstracts
Gerhard Heinzel: Low-frequency gravitational wave detectors in space

Gerhard Heinzel (Albert-Einstein-Institut Hannover)

Low-frequency gravitational wave detectors in space

Authors: G. Heinzel

Abstract: The low-frequency part of the gravitational wave spectrum, from 100 micro-Hertz up to 1 Hz, contains the most spectacular sources of gravitational waves. Really high precision measure-ments are possible here, making this frequency range very interesting for both Astronomy and Fundamental Physics. To open this window for observations, we need an observatory in space!

Presentation:

Back to list of abstracts
Michael Kramer: Einstein's gravity as seen by a cosmic lighthouse keeper

Einstein's gravity as seen by a cosmic lighthouse keeper

Authors: M. Kramer

Abstract: We can only speculate but presumably Albert Einstein would be delighted to see the experiments possible today that are made to test his theory of gravity. Among the most precise ones are tests with binary pulsars, which provide in particular information about the strong-field regime. From observations of pulsars we can test a large variety of relativistic effects or concepts deeply embedded in the framework of theories of gravity, including the existence of gravitational waves or the validity of the strong equivalence principle. This talk summarizes the latest experiments and tests provided by the cosmic lighthouses that we call pulsars. The corresponding observations provide the best limits on the validity of general relativity and alternative theories of gravity.

Presentation:

Back to list of abstracts
Jerzy Lewandowski: Loop Quantum Gravity - where are we?

Jerzy Lewandowski (Uniwersytet Warszawski)

Loop Quantum Gravity - where are we?

Authors: Jerzy Lewandowski

Abstract: For several models of gravity coupled to other fields, the algorithm of the canonical quantization has been completed and performed to an end. It gave rise to well defined, exact quantum theories. The Dirac observables are provided by the relational and the deparametrization frameworks. The quantum states, Hilbert spaces and concrete quantum operators are furnished by the canonical Loop Quantum Gravity framework. The models are not confirmed experimentally and admit ambiguities, but they are there, available for further study and applications.

Presentation:

Back to list of abstracts
Marc Mars: Stability of marginally outer trapped surfaces and geometric inequalities

Marc Mars (University of Salamanca)

Stability of marginally outer trapped surfaces and geometric inequalities

Authors: M. Mars

Abstract: Marginally outer trapped surfaces (MOTS) admit a notion of stability that in many respects generalizes a similar notion for minimal hypersurfaces. Stable MOTS play an interesting role in a number of geometric inequalities involving physical parameters such as area, mass, charge or, in the axially symmetric case, angular momentum. Some of those inequalities are global in nature while others are local, with interesting relationships between them. In this talk I will introduce the notion of stable MOTS and describe some of the geometric inequalities conjectured for them.

Presentation:

Back to list of abstracts
Ramesh Narayan: Energy Extraction from Spinning Black Holes: Relativistic Jets

Ramesh Narayan (Harvard University - Department of Astronomy)

Energy Extraction from Spinning Black Holes: Relativistic Jets

Authors: R. Narayan

Abstract: A deep idea in black hole physics is that it is possible to extract energy from a spinning black hole. It has for long been an article of faith among astrophysicists that black hole spin power is somehow responsible, perhaps via magnetic fields, for the relativistic jets seen in accreting black holes. Two recent advances have strengthened the case for this thesis. First, spin parameters of a number of accreting stellar-mass black holes have been measured. It is found that relativistic jets from more rapidly spinning black holes have substantially larger radio power than those from slowly spinning black holes. This observational evidence strongly suggests a causal relationship between black hole spin and jets. Second, numerical magnetohydrodynamic simulations of accreting black holes show that relativistic jets appear spontaneously in such systems. For a while it was unclear whether the power source for the jets is the accretion disk or the spin of the black hole. Recent work has produced unambiguous evidence that much of the energy flows out of the black hole and into the jet via magnetic fields.

Presentation:

Back to list of abstracts
Gernot Neugebauer: Stationary two-black-hole configurations: A non-existence proof for disconnected horizons

Gernot Neugebauer (Univesitaet Jena, Theoretisch-Physikalisches Institut)

Stationary two-black-hole configurations: A non-existence proof for disconnected horizons

Authors: Gernot Neugebauer

Abstract: We resume former discussions of the question, whether the spin-spin repulsion and the gravitational attraction of two aligned black holes can balance each other. Based on the solution of a boundary problem for disconnected (Killing) horizons and the resulting violation of characteristic black hole properties, we present a non-existence proof for the equilibrium configuration in question. From a mathematical point of view, this result is a further example for the efficiency of the inverse (´scattering´) method in non-linear theories.

Presentation:

Back to list of abstracts
Hermann Nicolai: Quantum Gravity: the view from particle physics

Hermann Nicolai (Max Planck Institute for Gravitational Physics, Golm)

Quantum Gravity: the view from particle physics

Authors: H. Nicolai

Abstract: In this talk I will review some facts and lessons that particle physics can offer to help in the search for a fully consistent theory of quantum gravity, including a brief discussion of recent LHC data from this perspective.

Presentation:

Back to list of abstracts
Harvey Reall: Higher dimensional black holes

Harvey Reall (Cambridge University)

Higher dimensional black holes

Authors: H. Reall

Abstract: I shall review what is known about black holes in higher dimensions. I shall discuss the known explicit solutions, results concerning the classification of stationary black holes, and instabilities of rotating black holes in higher dimensions.

Presentation:

Back to list of abstracts
Luciano Rezzolla: Using numerical relativity to explore fundamental physics and astrophysics

Luciano Rezzolla (Albert Einstein Institute)

Using numerical relativity to explore fundamental physics and astrophysics

Authors: L. Rezzolla

Abstract: Recent years have seen a major progress in numerical relativity and the solution of the simplest and yet among the most challenging problems in classical general relativity: that of the evolution of two objects interacting only gravitationally. I will review the results obtained so far when modelling binaries of black holes or of neutron stars and also discuss the impact these studies have in detection of gravitational-waves, in astrophysics, and in our understanding of general relativity.

Presentation:

Back to list of abstracts
Andrzej Rostworowski: Instability of anti de-Sitter spacetime

Andrzej Rostworowski (Jagiellonian University)

Instability of anti de-Sitter spacetime

Authors: P. Bizoń, A. Rostworowski

Abstract: In a joint work with Piotr Bizoń, we study the nonlinear evolution of a weakly perturbed anti-de Sitter (AdS) space by solving numerically the spherically symmetric Einstein-massless-scalar field equations with a negative cosmological constant. Our results suggest that AdS spacetime is unstable against a black hole formation under arbitrarily small generic perturbations. We conjecture that this instability is triggered by a resonant mode mixing which gives rise to diffusion of energy from low to high frequencies. A partial summary of our results can be found in Phys.Rev.Lett 107 031102 [arXiv:1104.3702].

Presentation:

Back to list of abstracts
Misao Sasaki: Inflation and birth of cosmological perturbations

Misao Sasaki (Yukawa Institute for Theoretical Physics, Kyoto University)

Inflation and birth of cosmological perturbations

Authors: Misao Sasaki

Abstract: The idea that there was a period of accelerated expansion in the very early universe, inflation, has been extremely successful in explaining the large scale structure of the present universe. In this talk, I review recent developments in the theory of inflation and its predictions, particularly those on cosmological perturbations.

Presentation:

Back to list of abstracts
Gerhard Schäfer: Hamiltonian formalism of spinning black holes in general relativity

Gerhard Schäfer (Friedrich-Schiller-Universtät Jena)

Hamiltonian formalism of spinning black holes in general relativity

Authors: G. Schäfer

Abstract: The mathematical treatment of the motion and spin precession of selfgravitating spinning compact objects in general relativity is manageable to the order linear in spin with the aid of the Tulczyjew stress-energy tensor in pole-dipole approximation, also applying regularization techniques. In my talk a Hamilton canonical treatment of gravitationally interacting spinning black holes will be presented using a tetrad-generalization of the Arnowitt-Deser-Misner canonical formulation of general relativity. The formalism is valid through linear order in the single spins. For binary systems, higher-order post-Newtonian Hamiltonians will be given in explicit analytic form. Higher order in spin generalizations will be discussed too.

Presentation:

Back to list of abstracts
Peter Schneider: Gravitational light bending: a powerful astrophysical tool

Peter Schneider (Argelander-Institut f. Astronomie, Univ. Bonn)

Gravitational light bending: a powerful astrophysical tool

Authors: P. Schneider

Abstract: Originally being the first crucial test of General Relativity, light deflection in a gravitational field has become one of the prime tool for astrophysics and cosmology, with applications ranging from the detection of extra-Solar planets, the study of the (dark) matter distribution in galaxies and galaxy clusters, the use as natural telescopes which come for free, the biasing properties of galaxies, to constraining the equation-of-state of dark energy and testing the law of gravity on cosmic scales. In this talk, a selection of recent research highlight will be presented, and prospects for future developments will be given.

Presentation:

Back to list of abstracts
Bernard Schutz: Gravity talks: observing the Universe with gravitational waves

Bernard Schutz (Albert Einstein Institute)

Gravity talks: observing the Universe with gravitational waves

Authors: B. Schutz

Abstract: Einstein's work on gravity was inspired by the need to limit the speed of transmission of gravitational influences to the speed of light, for consistency with special relativity. This leads inevitably to gravitational waves. But the weakness of the waves' interaction with matter has made them hard to detect, so that the first detections are expected in the next few years. By the end of this decade there might be a network of 5 powerful detectors around the world, and a space-based detector might be under construction. When gravity begins to talk back to us, to tell us about strong fields directly through the waves they produce, astronomy will gain a completely new way of gathering information. We will test gravity theory in strong fields, verify directly the existing of horizons, finally learn how often black holes are produced when stars die, study the structure of neutron stars, directly probe the black-hole engines of gamma-ray bursts, and independently measure the rate of expansion of the universe.

Presentation:

Back to list of abstracts
Alexei Starobinsky: f(R) gravity - the most straightforward generalization of the Einstein gravity

Alexei Starobinsky (Landau Institute for Theoretical Physics RAS, Moscow)

f(R) gravity - the most straightforward generalization of the Einstein gravity

Authors: A. A. Starobinsky

Abstract: f(R) gravity where R is the Ricci scalar represents the simplest non-perturbative generally covariant generalization of the Einstein gravity where it is possible to avoid the appearance of new ghost and tachyon degrees of freedom. Thus, this theory can be considered at the same level of generality as general relativity, not in some perturbative regime only. It represents a particular case of scalar-tensor gravity in the limit of the zero Brans-Dicke parameter, but with a non-zero scalar field potential. Its most interesting applications in cosmology are related to the possibility to use it for description of both types of dark energy which have appeared during the Universe evolution: primordial dark energy driving inflation in the early Universe and present dark energy which has much smaller effective energy density. In the case of inflation, the simplest $(R+R^2)$ model proposed already in 1980 is internally consistent, has a graceful exit to the radiation-dominated FRW stage via the period of reheating in which all matter in the Universe arises as a result of gravitational particle creation, and remains in agreement with the most recent observational data. Moreover, this form of f(R) may be justified by a number of microscopic models. In particular, it describes the gravitational sector of the Higgs inflation. It is possible to construct models describing the present dark energy in f(R) gravity which satisfy all present observational tests. However, these models require a much more complicated form of f(R) and a very low energy scale, so there is no microscopic justification of them at present. More critical is that these models generically cannot reproduce the correct evolution of the Universe in the past due to formation of additional weak singularities and other problems. Thus, to construct complete cosmological models of present dark energy not destroying all previous achievements of the early Universe cosmology including the recombination, the correct BBN and inflation of any kind, one has to change the behaviour of f(R) at large positive R and to extend f(R) to the region of negative R. I describe correct ways to do it. Combined description of primordial and present dark energy using one f(R) function is possible, too, but it leads to completely different reheating after inflation during which strongly non-linear oscillations of R occur.

Presentation:

Back to list of abstracts
Daniel Sudarsky: The quantum gravity interface and the origin of the seeds of cosmic structure during inflation

Daniel Sudarsky (Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México)

The quantum gravity interface and the origin of the seeds of cosmic structure during inflation

Authors: Daniel Sudarsky

Abstract: The observations of the first traces of cosmic structure in the Cosmic Microwave Background are in excellent agreement with the predictions of Inflation. However, as we shall see, that account is not fully satisfactory, as it does not address the transition from an homogeneous and isotropic early stage to a later one lacking those symmetries. We will argue that New Physics is needed to account for such transition and that Quantum Gravity might be the place from where this new physics emerges. Moreover, we will show that the observations can be used to constrain the various phenomenological proposals made in this regard.

Presentation:

Back to list of abstracts
Robert Wald: Stability of Black Holes and Black Branes

Robert Wald (University of Chicago)

Stability of Black Holes and Black Branes

Authors:

Abstract: I describe recent work with Stefan Hollands (arXiv:1201.0463) that establishes a close relationship between dynamical stability and thermodynamic stability for black holes and black branes in classical general relativity in spacetime dimensions $D \geq 4$. We show that for axisymmetric perturbations of an arbitrary stationary, axisymmetric black hole, dynamical stability is equivalent to the positivity of canonical energy of perturbations that have vanishing linearized ADM mass and angular momentum at infinity. We further show that positivity of canonical energy is equivalent to thermodynamic stability. A thermodynamically unstable black hole may be dynamically stable (as is the case for a Schwarzschild black hole) if the only perturbations with negative canonical energy have nonvanishing linearized mass and/or angular momentum. However, we show that all black branes associated with thermodynamically unstable black holes must be dynamically unstable, as conjectured by Gubser and Mitra. We also prove that positivity of canonical energy for perturbations with vanishing linearized mass and angular momentum is equivalent to the satisfaction of a "local Penrose inequality," thus showing that satisfaction of this local Penrose inequality is necessary and sufficient for dynamical stability. Although we explicitly consider vacuum general relativity, most of our results are derived using general Lagrangian and Hamiltonian methods and therefore can be generalized to allow for the presence of matter fields and/or to the case of an arbitrary diffeomorphism covariant gravitational action.

Presentation:

Back to list of abstracts
Clifford Will: Testing general relativity: Centenary highlights and future prospects

Clifford Will (Washington University, St. Louis; & University of Florida)

Testing general relativity: Centenary highlights and future prospects

Authors: Clifford M. Will

Abstract: During the latter part of the 20th century, general relativity was well-tested in the weak-field slow-motion regime of the solar system. In binary pulsar systems, some tests of strong-field aspects of the theory were carried out. We will give a brief overview of these achievements. As we look to the post-centenary future, testing GR in the strong-field, highly dynamical regime will be an important theme in experimental relativity. We will describe a number of possible tests that could be carried out, including tests using astrophysical phenomena around black holes, tests using gravitational waves, and tests of black hole no-hair theorems using high-precision observations of stars orbiting our galactic center black hole.

Presentation:

Back to list of abstracts

### Contributed talks

Andres Anabalon: Exact Black Holes and Universality in the Backreaction of non-linear Sigma Models with a potential in (A)dS4

Exact Black Holes and Universality in the Backreaction of non-linear Sigma Models with a potential in (A)dS4

Authors: A. Anabalon.

Abstract: The aim of the talk is to describe the construction of accelerated, stationary and axisymmetric exact solutions of the Einstein theory with self interacting scalar fields in (A)dS4. To warm up, the backreaction of the (non)-minimally coupled scalar field is solved, the scalar field equations are integrated and all the potentials compatible with the metric ansatz and Einstein gravity are found. With these results at hand the non-linear sigma model is tackled. The scalar field Lagrangian is generic; neither the coupling to the curvature, neither the metric in the scalar manifold nor the potential, are fixed ab initio. The unique assumption in the analysis is the metric ansatz: it has the form of the most general Petrov type D vacuum solution of general relativity; it is a a cohomogeneity two Weyl rescaling of the Carter metric and therefore it has the typical Plebanski-Demianski form with two arbitrary functions of one variable and one arbitrary functions of two variables. It is shown, by an straightforward manipulation of the field equations, that the metric is completely integrable without necessity of specifiying anything in the scalar Lagrangian. This results in that the backreaction of the scalar fields, within this class of metrics, is universal. The metric functions generically show an explicit dependence on a dynamical exponent that allows to smoothly connect this new family of solutions with the actual Plebanski-Demianski spacetime. The remaining field equations imply that the scalar fields follow geodesics in the scalar manifold with an affine parameter given by a non-linear function of the spacetime coordinates and define the on-shell form of the potential plus a functional equation that it has to satisfy. Finally, a general family of (A)dS4 static hairy black holes is explicitly constructed and its properties are outlined.

Presentation:

Back to list of abstracts
Hakan Andreasson: Black hole formation from a complete regular past for collisionless matter

Hakan Andreasson (Dept. of Mathematics, University of Gothenburg)

Black hole formation from a complete regular past for collisionless matter

Authors: H. Andreasson

Abstract: Initial data for the spherically symmetric Einstein-Vlasov system is constructed whose past evolution is regular and whose future contains a black hole. This is the first example of initial data with these properties for the Einstein-matter system with a "realistic" matter model. One consequence of the result is that there exists a class of initial data for which the ratio of the Hawking mass and the area radius is arbitrarily small everywhere, such that a black hole forms in the evolution. Another consequence is that there exist black hole initial data such that the solutions exist for all Schwarzschild time.

Presentation:

Back to list of abstracts
Thomas Bäckdahl: How to measure deviation from the Kerr initial data -- recent progress

Thomas Bäckdahl (Max-Planck-Institut für Gravitationsphysik, Albert-Einstein-Institut)

How to measure deviation from the Kerr initial data -- recent progress

Authors: T. Backdahl and J. A. Valiente Kroon

Abstract: In this talk I will present recent progress concerning a construction of a geometric invariant for initial data sets for the Einstein vacuum field equations. This geometric invariant vanishes if and only if the initial data set corresponds to data for the Kerr spacetime, and thus, it characterizes this type of data. This construction is based on Killing spinors and have now been carried out for compact domains and domains reaching the asymptotically flat ends. We investigate properties of this invariant.

Presentation:

Back to list of abstracts
Vladimír Balek: From 'nothing' to inflation and back again

Vladimír Balek (Department of Theoretical Physics, Comenius University, Bratislava)

From 'nothing' to inflation and back again

Authors: V. Balek

Abstract: Solutions of Wheeler-DeWitt equation in a minisuperspace with massive scalar field are constructed, following step by step the procedure developed for the description of a particle escaping from a two-dimensional potential well by Banks, Bender and Wu. For an inflationary universe driven by an unstable and metastable false vacuum, the solution describing tunneling of a universe from 'nothing' and the no-boundary solution are obtained, respectively. New features of the solutions coming from the indefinite metric in the kinetic term are pointed out and possible implications of the solutions for the theory of vacuum decay via Hawking-Moss and Coleman-de Lucia instantons are discussed.

Presentation:

Back to list of abstracts
Enrico Barausse: The complete non-spinning effective-one-body metric at linear order in the mass ratio

Enrico Barausse (University of Guelph)

The complete non-spinning effective-one-body metric at linear order in the mass ratio

Authors: Enrico Barausse, Alessandra Buonanno, Alexandre Le Tiec

Abstract: Using the results of related work, in which the binding energy of a circular-orbit non-spinning compact binary system is computed at leading-order beyond the test-particle approximation, the exact expression of the effective-one-body (EOB) metric component $g^{eff}_{tt}$ is obtained through first order in the mass ratio. Combining these results with the recent gravitational self-force calculation of the periastron advance for circular orbits in the Schwarzschild geometry, the EOB metric component $g^{eff}_{rr}$ is also determined at linear order in the mass ratio. These results assume that the mapping between the real and effective Hamiltonians at the second and third post-Newtonian (PN) orders holds at all PN orders. Our findings also confirm the advantage of resumming the PN dynamics around the test-particle limit if the goal is to obtain a flexible model that can smoothly connect the test-mass and equal-mass limits.

Presentation:

Back to list of abstracts
Yuri Bonder: Quantum Gravity Phenomenology without Lorentz Invariance Violations

Yuri Bonder (Instituto de Ciencias Nucleares, UNAM)

Quantum Gravity Phenomenology without Lorentz Invariance Violations

Authors: Yuri Bonder and Daniel Sudarsky

Abstract: In the last years the phenomenology of quantum gravity has been dominated by the search of violations of Lorentz symmetry. However, there are very serious arguments that led us to assume that Lorentz invariance is a real symmetry in Nature. This motivated us to construct a phenomenological model describing how a Lorentz invariant discrete structure of spacetime could become manifest. The proposal is fully observer covariant, it involves non-trivial couplings of curvature to matter fields and leads to a well defined phenomenology. In fact, an experiment specially designed to test the model has been performed by the Eöt-Wash group allowing to put bounds on some of the model's free parameters.

Presentation:

Back to list of abstracts
Marco Cariglia: Dirac equation in curved spacetime and hidden symmetries

Marco Cariglia (Universidade Federal de Ouro Preto)

Dirac equation in curved spacetime and hidden symmetries

Authors: M. Cariglia, P. Krtous, D. Kubiznak

Abstract: I will discuss the importance of hidden symmetries in the study of the Dirac equation curved spacetime. Conformal Killing-Yano special tensors are associated to symmetries of the Dirac equation, and in some notable cases like the higher dimensional Kerr-NUT-(A)dS black holes lead to full separation of variables. I will discuss in this case how the symmetries operators can be simultaneously diagonalised. Conformal Killing-Yano tensors and their related symmetry operators admit a generalisation in the case of metrics with fluxes that are of relevance for supergravity theories. Finally, if time permits I will mention the Eisenhart lift of a spacetime and its relation to the Dirac equation.

Presentation:

Back to list of abstracts
Saulo Carneiro: A cosmological concordance model with particle creation

Saulo Carneiro (Federal University of Bahia)

A cosmological concordance model with particle creation

Authors: J. S. Alcaniz, H. A. Borges, S. Carneiro, J. C. Fabris, C. Pigozzo and W. Zimdahl

Abstract: We show that creation of dark-matter particles at a constant rate implies the existence of a vacuum term that decays linearly with the Hubble rate. We discuss the cosmological model that arises in this context and test it against observations of the first acoustic peak in the cosmic microwave background (CMB) anisotropy spectrum, the Hubble diagram for supernovas of type Ia (SNIa), the distance scale of baryonic acoustic oscillations (BAO) and the distribution of large scale structures. We show that a good concordance is obtained, albeit with a higher value of the present matter abundance than in the standard model.

Presentation:

Back to list of abstracts
Carla Cederbaum: The Geometry of Static Spacetimes: Geometrostatics

Carla Cederbaum (Duke University)

The Geometry of Static Spacetimes: Geometrostatics

Authors: Carla Cederbaum

Abstract: Geometrostatics is an important subdomain of Einstein's General Relativity. It describes the mathematical and physical properties of static isolated relativistic systems such as stars, galaxies or black holes. For example, geometrostatic systems have a well-defined ADM-mass (Chrusciel, Bartnik) and (if this is nonzero) also a center of mass (Huisken-Yau, Metzger) induced by a CMC-foliation at infinity. We will present localized surface integral formulas for these physical properties in general geometrostatic systems. Together with an asymptotic analysis, these can be used to prove that ADM-mass and center of mass 'converge' to the Newtonian mass and center of mass in the Newtonian limit $c\to\infty$ (using Ehler's frame theory). We will discuss geometric similarities of geometrostatic and classical static Newtonian systems along the way.

Presentation:

Back to list of abstracts
Cecilia Chirenti: Quasinormal modes from a naked singularity

Quasinormal modes from a naked singularity

Authors: C. Chirenti, A. Saa and J. Skakala

Abstract: What should be the quasinormal modes associated with a spacetime that contains a naked singularity instead of a black hole? In the present work we address this problem by studying the scattering of scalar waves on a curved background described by a Reissner-Nordstr\"om spacetime with $q > m$. We show that there is a qualitative difference between cases with $1 < q^2/m^2 < 9/8$ and cases with $q^2/m^2 > 9/8$. We discuss the necessary conditions for the well-posedness of the problem, and present results for the low $\ell$ and large $\ell$ limits.

Presentation:

Back to list of abstracts
Alejandro Corichi: Effective Dynamics of Anisotropic Cosmologies in Loop Quantum Cosmology

Alejandro Corichi (Center for Mathematical Sciences, UNAM)

Effective Dynamics of Anisotropic Cosmologies in Loop Quantum Cosmology

Authors: A. Corichi, A. Karami, E. Montoya

Abstract: We present results of numerical evolutions of effective equations for anisotropic cosmologies with spatial curvature in loop quantum cosmology. We address the issue of singularity resolution for different types of initial conditions and study the behavior of geometrical scalar quantities.

Presentation:

Back to list of abstracts
Mariateresa Crosta: Tracing a relativistic Milky Way within the RAMOD measurement protocol

Mariateresa Crosta (L' Osservatorio Astrofisico di Torino, Istituto Nazionale di Astrofisica)

Tracing a relativistic Milky Way within the RAMOD measurement protocol

Authors: M. Crosta

Abstract: Advancement in astronomical observations and technical instrumentation implies taking into account the general relativistic effects due the dynamical gravitational fields encountered by the light while propagating from the star to the observer. Therefore, data exploitation for Gaia-like space astrometric mission (ESA, launch 2013) requires a fully relativistic interpretation of the inverse ray-tracing problem, namely the development of a highly accurate astrometric models, named RAMOD, in accordance with the geometrical environment affecting light propagation itself and the precepts of the theory of measurement. This could open a new rendition of the stellar distances and proper motions, or even an alternative detection perspective of many subtle relativistic effects suffered by light while it is propagating and subsequently reordered in the physical measurements.

Presentation:

Back to list of abstracts
Naresh Dadhich: Universal Features of the Lovelock Gravity

Naresh Dadhich (Jamia Millia Islamia and Inter-University Center for Astronomy and Astrophysics)

Universal Features of the Lovelock Gravity

Abstract: We will explore the universal properties of the Lovelock gravity. It turns out that for the static pure Lovelock black holes, the thermodynamical parameters always bear the same relation to the horizon radius indicating the thermodynamical universality. Further the pure Lovelock vacuum in the odd critical $d=2n+1$ dimension is defined by the vanishing of the higher order Riemann analogue tensor. However the spacetime is not Riemann flat and it is characterized by the solid angle deficit which gives rise to the Einstein stresses conforming to that of a global monopole asymptotically. By adding $\Lambda$, we obtain the BTZ black hole in $d=2n+1$ dimension.

Presentation:

Back to list of abstracts
Sergio Dain: Geometric inequalities for black holes

Sergio Dain (Universidad Nacional de Córdoba)

Geometric inequalities for black holes

Authors: Sergio Dain

Abstract: A geometric inequality in General Relativity relates quantities that have both a physical interpretation and a geometrical definition. It is well known that the parameters that characterize the Kerr-Newman black hole satisfy several important geometric inequalities. Remarkably enough, some of these inequalities also hold for dynamical black holes. This kind of inequalities, which are valid in the dynamical and strong field regime, play an important role in the characterization of the gravitational collapse. They are closed related with the cosmic censorship conjecture. In this talk I will review recent results in this subject.

Presentation:

Back to list of abstracts
Gustavo Dotti: Gravitational instabilities and cosmic censorship

Gustavo Dotti (FaMAF, Universidad Nacional de Córdoba)

Gravitational instabilities and cosmic censorship

Authors: G.Dotti, R.J.Gleiser, I.F.Ranea-Sandoval, J.Pullin, H.Vucetich

Abstract: I will review the results that we have obtained in the last few years on linear perturbations in the Kerr-Newman family. It is proved that the naked singularities arising in the super-extreme (charge or angular momentum larger than mass) cases are unstable, and that the stationary regions beyond the inner horizon of black holes are also unstable. These results have implications on cosmic censorship in both its weak and strong forms, since the inner horizon is also a Cauchy horizon for an appropriate data surface on the black hole exterior. The dynamics of gravitational perturbations in these non globally hyperbolic spacetimes can be uniquely defined in terms of data on a partial Cauchy surface thanks to the fact that there is a single choice of boundary conditions at the singularity that makes the linear scheme self consistent. References: Class.Quant.Grav in press; ibid v. 27 (2010) 187005; ibid v. 26 (2009) 215002; ibid v.25 (2008) 2450012; ibid v. 26 (2006) 5063; Phys.Lett. B644 (2007) 289

Presentation:

Back to list of abstracts
Maite Dupuis: Loop Quantum gravity in terms of spinors and harmonic oscillators

Maite Dupuis (Institute for Theoretical Physics III, University of Erlangen-Nürnberg)

Loop Quantum gravity in terms of spinors and harmonic oscillators

Authors: Maïté Dupuis

Abstract: Loop Quantum Gravity is an attempt to quantize general relativity. Its kinematical aspects are well understood and yield a description of space in terms of quanta. Spinorial tools provide a really nice geometrical picture of the classical phase space of Loop Gravity. Moving to the quantum level, spinors are simply quantized as harmonic oscillators. They are then the building blocks to define coherent states for Loop Quantum Gravity and to build spinfoam models which is a regularized path integral for general relativity. I will recall the main results of the spinorial formalism in the context of Loop Quantum Gravity and Spinfoam models and explain how it can be generalized to introduce a cosmological constant into the game.

Presentation:

Back to list of abstracts
Gyula Fodor: Scalar fields on anti-de Sitter background

Gyula Fodor (Wigner Research Centre for Physics, Budapest)

Scalar fields on anti-de Sitter background

Authors: G. Fodor, P. Forgács and P. Grandclément

Abstract: Because of the implication of anti-de Sitter spacetime instability, there has been much interest recently in scalar fields coupled to gravity when there is a negative cosmological constant. It is an interesting question how different the scalar field evolution is when the background is a fixed AdS metric. On the other hand, it is known that self-interacting massive real scalar fields on flat Minkowski background can form long living oscillating localized objects, called oscillons. In the flat background case these objects radiate energy extremely slowly, in a rate which is exponentially suppressed in terms of the central amplitude. As a result their oscillation frequency slowly increases. On AdS background the situation is different, because then there are localized exactly time-periodic solutions for massive or massless linear Klein-Gordon fields. In my talk I plan to discuss the influence of the AdS background on the structure and stability of oscillons.

Presentation:

Back to list of abstracts
Janusz Garecki: Canonical superenergy tensors: a reappraisal

Janusz Garecki (Institute of Mathematics, University of Szczecin)

Canonical superenergy tensors: a reappraisal

Authors: Janusz Garecki

Abstract: In the framework of general relativity (GR) the gravitational field has no energy-momentum tensor. But it is very easy to attach to this field a superenergy tensor''. In the Lecture we present an universal and constructive definition of such a tensor. This definition uses locally Minkowskian structure of the spacetime in GR and canonical energy-momentum complex for matter and gravity in this theory. The obtained canonical superenergy tensor for gravity is very closely related to Appel's energy of acceleration'' in classical mechanics. Applied to matter tensor our procedure leads to superenergy tensor for matter. We have used in past the superenergy tensors, gravitation and matter, to analysis of the majority solutions to the Einstein equations which are interesting in astrophysics and cosmology. The obtained results were interesting (they were published). By slightly changing our constructive definition of the superenrgy tensors one can obtain the averaged relative energy-momentum tensors. These tensors have proper dimensions and they differ from the superenergy tensors only by a dimensional factor which needs fixing. We have given a proposal how to establish this factor.

Presentation:

Back to list of abstracts
Steffen Gielen: Spontaneous breaking of Lorentz symmetry for canonical gravity

Steffen Gielen (Perimeter Institute)

Spontaneous breaking of Lorentz symmetry for canonical gravity

Authors: S. Gielen, D. K. Wise

Abstract: In Hamiltonian formulations of general relativity, in particular Ashtekar variables which serve as the classical starting point for loop quantum gravity, Lorentz covariance is a subtle issue which has been the focus of some debate, while at the same time being crucial with regard to possible experimental tests. After reviewing the sources of difficulty, we present a Lorentz covariant formulation in which we generalise the notion of a foliation of spacetime usually used in the Hamiltonian formalism to a field of ”local observers” which specify a time direction only locally. This field spontaneously breaks the local SO(3,1) symmetry down to a subgroup SO(3), in a way similar to systems in condensed matter and particle physics. The formalism is analogous to that in MacDowell-Mansouri gravity, where SO(4,1) is spontaneously broken to SO(3,1). We show that the apparent breaking of SO(3,1) to SO(3) is not in conflict with Lorentz covariance. We close by outlining other possible applications of the formalism of local observer, especially with regard to phenomenology of quantum gravity.

Presentation:

Back to list of abstracts
Florian Girelli: Geometric operators in loop quantum gravity with a cosmological constant

Florian Girelli (University of Waterloo)

Geometric operators in loop quantum gravity with a cosmological constant

Authors: Florian Girelli

Abstract: Loop quantum gravity is a candidate to describe the quantum gravity regime with zero cosmological constant. One of its key results is that geometric operators, such as area, angle, volume, are quantized. Not much is known when the cosmological constant is not zero. It is usually believed that to introduce this parameter in the game, we need to use quantum groups. However due to the complicated algebraic structure inherent to quantum groups not much is known in this case. Apart from the area operator, the geometric operators are not yet defined. I will discuss how the use of tensor operators can circumvent the difficulties and allow to construct a natural set of observables. In particular, I will construct the natural geometric observables such as angle or volume and discuss some of their properties.

Presentation:

Back to list of abstracts
Domenico Giulini: Another perspective on Einstein's "Prague" field equation of 1912

Domenico Giulini (Leibniz University Hannover and ZARM Bremen)

Another perspective on Einstein's "Prague" field equation of 1912

Authors: Domenico Giulini

Abstract: During his time in Prague, Einstein searched for modifications of Newtonian gravitational field equation for static fields and arrived at a non-linear equation for a scalar field that, as it turns out retrospectively, shares some qualitative features with General Relativity (GR). In my talk I will show how to arrive at this equation form a procedure that merely requires a self consistent implementation of the gravitational field's self-energy in Newtonian gravity. This procedure is the analog of the derivation of GR starting from a Poincare invariant zero-mass spin-2 theory in Minkowski space (known as the "flat" approach to GR) and the requirement of consistent self coupling.

Presentation:

Back to list of abstracts
Andrzej Gorlich: A transfer matrix model of volume fluctuations in 4D Causal Dynamical Triangulations

Andrzej Gorlich (Niels Bohr Institute, Copenhagen)

A transfer matrix model of volume fluctuations in 4D Causal Dynamical Triangulations

Authors: J. Ambjorn, A. Gorlich, J. Jurkiewicz, J. Gizbert-Studnicki

Abstract: Causal Dynamical Triangulation (CDT) is a background independent approach to quantum gravity. We introduce a phenomenological transfer matrix model, where at each time step we use a reduced set of quantum states characterized solely by the discretized spatial volume. Using computer simulations we determine the transfer matrix elements in this representation and extract the effective action for the scale factor in the de Sitter phase of the 4D Causal Dynamical Triangulations. In this framework no degrees of freedom are frozen, however, the obtained effective action agrees with the 'minisuperspace' model. We show that the observed probability distribution of spatial volume in the 'stalk' region is dominated by the quantum state with the largest eigenvalue and that the structure of the covariance matrix can be fully explained in the language of the transfer matrix.

Presentation:

Back to list of abstracts
Samuel Gralla: Second-Order Gravitational Self-Force

Samuel Gralla (University of Maryland)

Second-Order Gravitational Self-Force

Authors: Samuel E. Gralla

Abstract: The prospect of gravitational-wave astronomy has renewed interest in the old problem of the motion of a particle taking into account the effects of its self-field. In the context of a small mass described by the theory of general relativity, roughly fifteen years of work has established---and in some cases solved---an equation of motion valid to first order in the size/mass of the body (i.e., taking into account the leading order effects of the self-field). However, for the gravitational-wave application, second-order effects may in fact be required, i.e., it may be necessary to include self-field effects corresponding to non-linear terms in the Einstein equation. I report on recent work, building on previous work with R. Wald at first order, that rigorously derives an equation of motion (in the form of a very complicated prescription!) valid through second-order in the size/mass of a small body. Key elements of the approach at second order are the use of "effective source" techniques to handle the non-linearity and the identification of a class of gauges where a notion of center of mass position may be sensibly defined.

Presentation:

Back to list of abstracts
Norman Gürlebeck: Source integrals for Geroch's Multipole Moments

Norman Gürlebeck (ZARM, University Bremen)

Source integrals for Geroch's Multipole Moments

Authors: N. Gürlebeck

Abstract: Geroch's multipole moments of a stationary spacetime are defined by the asymptotic behavior of its metric near spatial infinity. They can be used to describe the spacetime sufficiently far away from the source. However, their relation to the source itself is only clarified for the leading multipole moments -- the mass and, in case of axially symmetry, the angular momentum -- via the Komar integrals. We describe here an algorithm to derive such source integrals for stationary and axially symmetric spacetimes also for higher multipole moments and give the first few examples explicitly. In the special case of staticity, we give all multipole moments in such a local form.

Presentation:

Back to list of abstracts
Eva Hackmann: Geodesic equations and algebro-geometric methods

Eva Hackmann (ZARM, University of Bremen)

Geodesic equations and algebro-geometric methods

Authors: V. Enolski, E. Hackmann, V. Kagramanova, J. Kunz, and C. L\"ammerzahl

Abstract: For an investigation of the physical properties of gravitational fields the observation of massive test particles and light is very useful. The characteristic features of a given space-time may be decoded by studying the complete set of all possible geodesic motions. Such a thorough analysis can be accomplished most effectively by using analytical methods to solve the geodesic equation. In this contribution, we will present the use of elliptic functions and their generalizations for solving the geodesic equation in a wide range of well known space-times, which are part of the general Pleba\'nski-Demia\'nski family of solutions. In addition, we address the definition and calculation of observable effects like the perihelion shift and outline further applications of the presented methods.

Presentation:

Back to list of abstracts
Andrew Hamilton: Generalized thermodynamics inside black holes

Generalized thermodynamics inside black holes

Authors: Andrew J. S. Hamilton

Abstract: There is persistent and endemic confusion between the true (or future) horizon that an infaller passes when they fall into a black hole, and the illusory (or past) horizon, which is the exponentially redshifted image of the object that collapsed to a black hole long ago. I will use a general relativistically accurate interactive Black Hole Flight Simulator to illustrate the distinction between the true and illusory horizons. I will argue that (it is obvious that): (a) Hawking radiation arises from the illusory horizon, for both inside and outside observers; (b) the entropy of the black hole is a quarter of the area of the illusory horizon, for both inside and outside observers; (c) the illusory horizon is intrintrinsically nonlocal, and is at the root of the nonlocality (information) paradox; (d) when an infaller reaches the singularity, their states merge with the illusory horizon.

Presentation:

Back to list of abstracts
Phillip Helbig: Is there a flatness problem in classical cosmology?

Phillip Helbig ()

Is there a flatness problem in classical cosmology?

Authors: P. Helbig

Abstract: I briefly review the flatness problem within the context of classical cosmology and examine some of the debate in the literature with regard to its definition and even the question whether it exists. I then present some new calculations for cosmological models which will collapse in the future; together with previous work by others for models which will expand forever, this allows one to examine the flatness problem quantitatively for all cosmological models. This leads to the conclusion that the flatness problem does not exist, not only for the cosmological models corresponding to the currently popular values of $\lambda_0$ and $\Omega_0$ but indeed for all Friedmann-Lema\^itre models.

Presentation:

Back to list of abstracts
Franz Hinterleitner: Quantization of plane gravitational waves

Franz Hinterleitner (Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Brno )

Quantization of plane gravitational waves

Authors: F. Hinterleitner, S. Major

Abstract: A long-standing problem in Loop Qauntum Gravity (LQG) is the semiclassical limit and the question of Lorentz invariance violation due to the "granularity" of quantum space-time. In full 3+1 LQG there are strong indications for such violations, but no definitve answer to this issue has been given so far. Unidirectional plane gravitational waves are 1+1 dimensional fully general-relativistic systems, which are convenient for an investigation of possible dispersion of gravitational radiation, quantum fluctuations of flat space, and the speed of light in a quantum space-time environmant. In a recent paper a classical canonical approach to plane waves was found, where the reduction from arbitrarily forth- and back running waves to unidirectional ones is formulated in terms of first-class constraints. This means that this step of symmetry reduction can be carried out after quantization. The presently ongoing work deals with the formulation of the corresponding quantum constraint operators and the construction of solutions.

Presentation:

Back to list of abstracts
Lee Hodgkinson: Static, stationary and inertial Unruh-DeWitt detectors on the BTZ black hole

Lee Hodgkinson (University of Nottingham)

Static, stationary and inertial Unruh-DeWitt detectors on the BTZ black hole

Authors: L. Hodgkinson and J. Louko

Abstract: We examine an Unruh-DeWitt particle detector coupled to a scalar field in three-dimensional curved spacetime. We first obtain a regulator-free expression for the transition probability in an arbitrary Hadamard state, working within first-order perturbation theory and assuming smooth switching, and we show that both the transition probability and the instantaneous transition rate remain well-defined and finite in the sharp switching limit. We then analyse the detector for a massless conformally coupled field in the Hartle-Hawking vacua on the Ba\~nados-Teitelboim-Zanelli black hole, under both transparent and reflective boundary conditions. A~selection of stationary and freely-falling detector trajectories are examined, including the co-rotating trajectories, for which the response is shown to be thermal. Analytic results in a number of asymptotic regimes, including those of large and small mass, are complemented by numerical results in the interpolating regimes. The boundary condition at infinity is seen to have a significant effect on the detector.

Presentation:

Back to list of abstracts
Jiří Horák: Corotation instability in black-hole accretion disks

Jiří Horák (Astronomical Institute, Czech Academy of Sciences )

Corotation instability in black-hole accretion disks

Authors: J. Horak and D. Lai

Abstract: Basic theory of accretion disk oscillations will be reviewed. We will concentrate on the mechanism of the corotation instability. We will present new results for p-modes trapped close to the inner edge of the disk and briefly discuss conditions under which these modes become unstable.

Presentation:

Back to list of abstracts
Balasubramanian Iyer: The 2.5PN linear momentum flux and associated recoil from inspiralling compact binaries in quasi-circular orbits: Nonspinning case

Balasubramanian Iyer (Raman Research Institute, Bangalore)

The 2.5PN linear momentum flux and associated recoil from inspiralling compact binaries in quasi-circular orbits: Nonspinning case

Authors: Chandra Kant Mishra, K. G. Arun, Bala Iyer

Abstract: Anisotropic emission of gravitational-waves (GWs) from inspiralling compact binaries leads to the loss of linear momentum and gravitational recoil of the system. The loss rate of linear momentum for a non-spinning binary system of black holes in quasi-circular orbit is obtained at the 2.5 post-Newtonian (PN) order and used to provide an analytical expression for the 2.5PN accurate recoil velocity of the binary in the inspiral phase. The maximum recoil velocity of the binary system at the innermost stable circular orbit (ISCO)) estimated by the 2.5PN formula is of the order of 4 km/s which is smaller than the 2PN estimate of 22 km/s. This indicates the importance of higher order post-Newtonian (PN) corrections. Going beyond inspiral, we provide an estimate of the more important contribution to the recoil velocity from the plunge phase. The maximum recoil velocity at the end of the plunge, involving contributions both from inspiral and plunge phase, for a binary with symmetric mass ratio $\nu=0.2$ is of the order of 182 km/s.

Presentation:

Back to list of abstracts
Michael Jasiulek: Novel geometric methods for quasi-local mass and spin via isometric embeddings and curvature invariants

Michael Jasiulek (Albert-Einstein-Institute Potsdam)

Novel geometric methods for quasi-local mass and spin via isometric embeddings and curvature invariants

Authors: M. Jasiulek and M. Korzynski

Abstract: Quasi-local mass and angular momentum of bounded regions in numerical binary black hole (BBH) simulations provide necessary information to assemble complete waveforms of BBH inspirals, since the system parameters of the post-Newtonian part and the fully relativistic part of the waveform have to agree. Meaningful definitions of quasi-local mass and spin are typically based on non-linear elliptic conditions on the geometry and location of closed 2-surfaces in an ambient spatial 3-slice, in order to fix certain gauge freedoms. To access quasi-local mass of arbitrary bounded regions in numerical relativistic simulation we introduce a novel geometric method to find the isometric embedding of a 2-surface in Euclidean three space through linearised embedding flow which is necessary to determine the Liu-Yau or Brown-York masses. To access quasi-local mass and angular momentum of axial black hole horizons we introduce a method to read off angular momentum and higher multipole moments through invariant curvature averages. The method does not require a solution of the Killing equation and yields well-defined generalised axial multipole moments for perturbed axial 2-metrics by averaging the contained axial information.

Presentation:

Back to list of abstracts
Igor Khavkine: Time delay observable in classical and quantum geometries

Igor Khavkine (Institute for Theoretical Physics, Utrecht)

Time delay observable in classical and quantum geometries

Authors: I. Khavkine

Abstract: A class of diffeomorphism invariant, physical observables, so-called astrometric observables, is introduced. A particularly simple example, the time delay, which expresses the difference between two initially synchronized proper time clocks in relative inertial motion, is analyzed in detail. It is found to satisfy some interesting inequalities related to the causal structure of classical Lorentzian spacetimes. Thus it can serve as a probe of causal structure and in particular of violations of causality. A quantum model of this observable as well as the calculation of its variance due to vacuum fluctuations in quantum linearized gravity are sketched. The question of whether the causal inequalities are still satisfied by quantized gravity, which is pertinent to the nature of causality in quantum gravity, is raised, but it is shown that perturbative calculations cannot provide a definite answer. Some potential applications of astrometric observables in quantum gravity are discussed. [arXiv:1111.7127]

Presentation:

Back to list of abstracts
Deborah Konkowski: Quantum singularities in static and conformally static space-times

Deborah Konkowski (Department of Mathematics, U.S. Naval Academy, Annapolis)

Quantum singularities in static and conformally static space-times

Authors: D. A. Konkowski and T. M. Helliwell

Abstract: The definition of quantum singularity is extended from static space-times to conformally static space-times. After the usual definitions of classical and quantum singularities are reviewed, examples of quantum singularities in static space-times are given. These include asymptotically power-law space-times, space-times with diverging higher-order differential invariants, and a space-time with a 2-sphere singularity. The theory behind quantum singularities in conformally static space-times is followed by several examples: a Friedmann-Robertson-Walker space-time with cosmic string, Roberts space-time, Fornav space-time, and the HMN metric. Future areas of research are discussed.

Presentation:

Back to list of abstracts
Ondřej Kopáček: Regular and Chaotic Motion in General Relativity: The Case of a Massive Magnetic Dipole

Ondřej Kopáček (Astronomical Institute, Czech Academy of Sciences)

Regular and Chaotic Motion in General Relativity: The Case of a Massive Magnetic Dipole

Authors: O. Kopáček, V. Karas, Y. Kojima, J. Kovář, P. Slaný, and Z. Stuchlík

Abstract: Circular motion of particles, dust grains and fluids in the vicinity of compact objects has been investigated as a model for accretion of gaseous and dusty environment. Here we further discuss, within the framework of general relativity, figures of equilibrium of matter under the influence of combined gravitational and large-scale magnetic fields, assuming that the accreted material acquires a small (but non-vanishing) electric charge due to interplay of plasma processes and photoionization. In particular, we employ exact solution describing the massive magnetic dipole and we identify the regions of stability. We also investigate situations when the motion exhibits the onset of chaos. In order to characterize the measure of chaoticness we employ techniques of Poincare surfaces of section and of recurrence plots.

Presentation:

Back to list of abstracts
Tim Koslowski: Shape Dynamics

Tim Koslowski (Perimeter Institute for Theoretical Physics)

Shape Dynamics

Authors: T. A. Koslowski

Abstract: Shape Dynamics is a reformulation of General Relativity where refoliation invariance is traded for local spatial conformal invariance. This is an example of the concept of equivalence of Hamiltonian gauge theories, where two theories are called equivalent if and only if each can be gauged such that in these two particular gauges the two theories admit identical initial value problems and identical equations of motion. The BRST formulation of this duality suggests a novel definition of gravity theories, in particular in the effective field theory framework.

Presentation:

Back to list of abstracts
Manuel Krämer: Can effects of quantum gravity be observed in the cosmic microwave background?

Manuel Krämer (Institute for Theoretical Physics, University of Cologne)

Can effects of quantum gravity be observed in the cosmic microwave background?

Authors: C. Kiefer and M. Krämer

Abstract: We investigate the question whether small quantum-gravitational effects can be observed in the anisotropy spectrum of the cosmic microwave background radiation. An observation of such an effect is needed in order to discriminate between different approaches to quantum gravity. Using canonical quantum gravity with the Wheeler-DeWitt equation, we find a suppression of power at large scales. Current observations only lead to an upper bound on the energy scale of inflation, but the framework is general enough to study other situations in which such effects might indeed be seen. Reference: C. Kiefer and M. Krämer, Quantum Gravitational Contributions to the Cosmic Microwave Background Anisotropy Spectrum, Phys. Rev. Lett. 108, 021301 (2012).

Presentation:

Back to list of abstracts
David Kubiznak: On integrability of spinning particle motion in higher-dimensional rotating black hole spacetimes

David Kubiznak (Perimeter Institute for Theoretical Physics)

On integrability of spinning particle motion in higher-dimensional rotating black hole spacetimes

Authors: M. Cariglia, D. Kubiznak

Abstract: In this talk, I shall review several various approaches for describing a spinning particle in curved rotating black hole background and discuss their integrability properties'. In particular, I will concentrate on a semiclassical theory, where the spin degrees of freedom are described by a vector of Grassmann variables. I will show that for rotating black hole spacetimes in any dimension n there exist n bosonic functionally independent integrals of spinning particle motion, corresponding to explicit and hidden symmetries generated from the principal Killing-Yano tensor. Moreover, in 4, 5, 6, and 7 dimensions such integrals are in involution, making the bosonic part of the motion integrable. This is conjectured to be valid in any dimension. The presented construction generalizes the result of Page et. al. [hep-th/0611083] on complete integrability of geodesic motion in these spacetimes.

Presentation:

Back to list of abstracts
Julien Larena: A cosmological lattice model

Julien Larena (Department of Mathematics, Rhodes University)

A cosmological lattice model

Authors: Julien Larena

Abstract: We study a cosmological model consisting of an infinite number of masses M placed on a cubic lattice of size L. The purpose of the study is to obtain a toy-model for a Universe made of localized masses, in order to test the usual fluid approximation of cosmology, as well as the Lindquist-Wheeler tesselated approximation that has recently been employed to explore the possibility to replace Dark Energy by an effect of inhomogeneities. We find a solution that is exact at order M/L, thus representing adequately a lattice of galactic-size objects separated by inter-galactic distances. The kinematics of the solution matches exactly the one of the corresponding Friedmann-Lemaitre-Robertson-Walker (FLRW) model with a dust matter component having the equivalent energy density. This supports the fluid approximation. Nevertheless, differences arise in the propagation of light and the values of observables, between the lattice model and the kinematically equivalent smoothed one. We comment on these effects and their potential observability, and we compare our results to the ones obtained in Wheeler-Lindquist models and in the FLRW context when using the Dyer-Roeder equation to take into account the clumping of matter.

Presentation:

Back to list of abstracts
Alexandre Le Tiec: The first law of binary black hole mechanics

Alexandre Le Tiec (University of Maryland, College Park)

The first law of binary black hole mechanics

Authors: A. Le Tiec, L. Blanchet, and B. F. Whiting

Abstract: First laws of black hole mechanics, or thermodynamics, come in a variety of different forms. We establish a first law of mechanics for binary systems of point masses moving along circular orbits. This relation is derived from first principles in General Relativity, and is explicitly shown to hold up to very high orders in the post-Newtonian approximation. Analogies are drawn with the single and binary black hole cases, revealing intriguing formal relations between point masses and black holes. Several applications to gravitational-wave source modeling are discussed, such as the computation of the binding energy E and total angular momentum J of the binary system, at leading order beyond the test-particle approximation. The resulting expression for the coordinate invariant relation E(J) is shown to agree remarkably well with the exact results from recent numerical simulations of comparable-mass non-spinning black hole binaries.

Presentation:

Back to list of abstracts
Georgios Loukes-Gerakopoulos: Non-linear effects in non-Kerr spacetimes

Georgios Loukes-Gerakopoulos (Theoretisch-Physikalisches Institut, Friedrich-Schiller-Universität, Jena)

Non-linear effects in non-Kerr spacetimes

Authors: G. Lukes-Gerakopoulos

Abstract: The absence of a Carter-like constant in stationary axisymmetric perturbations of black hole spacetimes allows the appearance of chaos. Although, chaos in frequency analysis corresponds to noise, certain non-linear effects of these non-integrable systems can be observed. I will present some of these effects and their corresponding imprints on the frequency spectra.

Presentation:

Back to list of abstracts
Christian Luebbe: The conformal Einstein field equations for trace-free perfect fluids

Christian Luebbe (University of Leicester)

The conformal Einstein field equations for trace-free perfect fluids

Authors: C. Luebbe, J.A. Valiente-Kroon

Abstract: This talk will discuss the conformal formulation of the Einstein field equations introduced by Helmut Friedrich in the 80s and used to derive existence, uniqueness and stability results for the spacetimes containing vacuum or Einstein-Maxwell-Yang-Mills. These results were recently extended by the authors to radiation fluids (trace-free perfect fluids) to analyse the stability of FLRW spacetimes. I shall give a brief overview over these latest results.

Presentation:

Back to list of abstracts
Luca Lusanna: Canonical gravity, non-inertial frames, relativistic metrology and dark matter

Luca Lusanna (Istituto Nazionale di Fisica Nucleare)

Canonical gravity, non-inertial frames, relativistic metrology and dark matter

Authors: Luca Lusanna

Abstract: Clock synchronization leads to the definition of instantaneous 3-spaces (to be used as Cauchy surfaces) in non-inertial frames, the only ones allowed by the equivalence principle. ADM canonical tetrad gravity in asymptotically Minkowskian space-times can be described in this framework. This allows to find the York canonical basis in which the inertial (gauge) and tidal (physical) degrees of freedom of the gravitational field can be identified. A Post-Minkowskian linearization with respect to the asymptotic Minkowski metric (asymptotic background) allows to solve the Dirac constraints in non-harmonic 3-orthogonal gauges and to find non-harmonic TT gravitational waves. The inertial gauge variable York time (the trace of the extrinsic curvature of the 3-space) describes the general relativistic freedom in clock synchronization. After a digression on the gauge problem in general relativity and its connection with relativistic metrology, it is shown that dark matter, whose experimental signatures are the rotation curves and the mass of galaxies, may be described (at least partially) as an inertial relativistic effect (absent in Newton gravity) connected with the York time, namely with the non-Euclidean nature of 3-spaces as 3-sub-manifolds of space-time.

Presentation:

Back to list of abstracts
Donald Lynden-Bell: Strong Gravomagnetism and Gravitational Solenoids

Donald Lynden-Bell (University of Cambridge)

Strong Gravomagnetism and Gravitational Solenoids

Authors: D Lynden-Bell

Abstract: In electromagnetism a current along a wire tightly wound on a torus makes a solenoid whose magnetic field is confined within the torus. In Einstein's gravity we give a corresponding solution in which a current of matter moves up on the inside of a toroidal shell and down on the outside, rolling around the torus by the short way. The metric is static outside the torus but stationary inside with the gravomagnetic field confined inside the torus, running around it by the long way. This exact solution of Einstein's equations is found by fitting Bonnor's solution for the metric of a light beam, which gives the required toroidal gravomagnetic field inside the torus, to the general Weyl static external metric in toroidal coordinates, which we develop. We deduce the matter tensor on the torus and find when it obeys the energy conditions. We also give the equipotential shells that generate the simple Bach–Weyl metric externally and find which shells obey the energy conditions.

Presentation:

Back to list of abstracts
Maxim Lyutikov: Hair of astrophysical black holes

Maxim Lyutikov (Purdue University)

Hair of astrophysical black holes

Authors: Maxim Lyutikov

Abstract: The no hair'' theorem is not applicable if the black hole is surrounded by highly conducting plasma. Astrophysical black holes formed from the collapse of a rotating magnetized neutron stars, which can self-produce particles via vacuum breakdown and form a highly conducting plasma magnetosphere, can keep the effectively frozen-in'' magnetic field lines both during and after the collapse. This introduces a topological constraint which prohibits the open magnetic field lines from sliding off the newly-formed event horizon. During collapse of a neutron star into the black hole, the latter conserves the number of magnetic flux tubes open to infinity. As a result, an isolated black hole can spin down electromagnetically.

Presentation:

Back to list of abstracts
Hideki Maeda: Exact dynamical AdS black holes and wormholes with a Klein-Gordon field

Hideki Maeda (Centro de Estudios Cientificos)

Exact dynamical AdS black holes and wormholes with a Klein-Gordon field

Authors: Hideki Maeda

Abstract: We present a set of exact solutions in the Einstein-Klein-Gordon system with a cosmological constant in arbitrary dimensions. The spacetime has spherical, plane, or hyperbolic symmetry and a class of solutions represents an asymptotically locally AdS dynamical black hole or wormhole. In four and higher dimensions, the quasi-local mass blows up at the AdS infinity, suggesting that the slow fall-off is realized. In three dimensions, the scalar field becomes trivial and the solution reduces to the BTZ black hole.

Presentation:

Back to list of abstracts
Seth Major: On the Observability of Granularity of Spatial Geometry

Seth Major (Hamilton College)

On the Observability of Granularity of Spatial Geometry

Authors: S. A. Major and J. Zappala

Abstract: If quantum geometry is an accurate model of microscopic spatial geometry then two related questions arise, one observational and one theoretical: How and at what scale is the discreteness manifest? How is the general relativistic limit achieved? These questions will be discussed in the context of studies of a single atom of geometry. It will be shown that the effective scale of the discreteness could be much larger than the Planck scale. Before this scale can be predicted, the relations between discrete geometry, coherent states, and the semiclassical limit need to be clarified. Work towards this goal, using coherent states in spin foams, the spin geometry theorem of Penrose and Moussouris, and twisted geometries will be described.

Presentation:

Back to list of abstracts
Daniele Malafarina: Equilibrium configurations from gravitational collapse

Daniele Malafarina (Tata Institute of Fundamental Research)

Equilibrium configurations from gravitational collapse

Authors: Daniele Malafarina

Abstract: We study how equilibrium configurations can be obtained as the result of gravitational collapse from regular initial conditions within the general theory of relativity. Assuming that the collapsing cloud is composed by a perfect fluid we show that the equilibrium geometries generated by this procedure form a subset of static interior solutions to the Einstein equations. We further show that these static configuration can be either regular or develop a naked singularity at the center, where the presence of a naked singularity is given a precise physical interpretation. We then study the properties of stable circular orbits around and inside such equilibrium configurations and show that in the case where a naked singularity is present there are key observational differences with respect to the properties of a Schwarzschild black hole with the same mass. We conclude that if similar objects can form in the universe they could be observationally distinguished from a black hole of the same mass.

Presentation:

Back to list of abstracts
Giovanni Marozzi: Backreaction effects on the luminosity-redshift relation in inhomogeneous cosmology

Giovanni Marozzi (College de France, Paris)

Backreaction effects on the luminosity-redshift relation in inhomogeneous cosmology

Authors: G. Marozzi

Abstract: I will show a general gauge invariant formalism for defining cosmological averages that are relevant for observations based on light-like signals. Such averages involve either null hypersurfaces corresponding to a family of past light-cones or compact surfaces given by their intersection with timelike hypersurfaces. Afterwards, using such formalism, together with adapted "geodesic light-cone" coordinates, I will show as backreaction effect emerges in the evaluation of the luminosity distance-redshift relation in an inhomogeneous Universe. To conclude, considering a realistic stochastic spectrum of inhomogeneities of primordial (inflationary) origin, I will show the magnitude and behaviour of such backreaction effects. Talk based on the following papers: M. Gasperini, G. Marozzi, F. Nugier and G. Veneziano, JCAP 1107, 008 (2011), arXiv:1104.1167 [astro-ph.CO]; I. Ben-Dayan, M. Gasperini, G. Marozzi, F. Nugier and G. Veneziano, arXiv:1202.1247 [astro-ph.CO]; I. Ben-Dayan, M. Gasperini, G. Marozzi, F. Nugier and G. Veneziano, in preparation.

Presentation:

Back to list of abstracts
Jakub Mielczarek: Signature change in loop quantum cosmology

Jakub Mielczarek (Jagiellonian University; National Centre for Nuclear Research)

Signature change in loop quantum cosmology

Authors: Jakub Mielczarek

Abstract: The Wick rotation is commonly considered only as an useful computational trick. However, as it was suggested by Hartle and Hawking already in early eighties, Wick rotation may gain physical meaning at the Planck epoch. While such possibility is conceptually interesting, leading to no-boundary proposal, mechanism behind the signature change remains mysterious. In this talk we show that the signature change anticipated by Hartle and Hawking may occur in result of the loop quantum gravity effects. Theory of cosmological perturbations with the effects of quantum holonomies is constructed. It is shown that such theory can be uniquely formulated in the anomaly-free manner. The algebra of quantum constraints turns out to be modified such that the signature is changing from Lorentzian in low curvature regime to Euclidean in high curvature regime. Implications of this phenomenon on propagation of cosmological perturbations are discussed. Possible relations with other approaches to quantum gravity are also outlined.

Presentation:

Back to list of abstracts
James Nester: A reference for the covariant Hamiltonian boundary term

James Nester (Department of Physics, National Central University)

A reference for the covariant Hamiltonian boundary term

Authors: James M. Nester

Abstract: The Hamiltonian for dynamic geometry generates the evolution of a spatial region along a vector field. It includes a boundary term which determines both the value of the Hamiltonian and the boundary conditions. The value gives the quasi-local quantities: energy-momentum, angular-momentum/center-of-mass. The boundary term depends not only on the dynamical variables but also on their reference values, the latter determine the ground state (having vanishing quasi-local quantities). For our preferred boundary term for Einstein's GR we propose 4D isometric matching and extremizing the energy to determine the reference metric and connection values.

Presentation:

Back to list of abstracts
Herbert Pfister: Gravitomagnetism: From Einstein's 1912 paper to the satellites LAGEOS and Gravity Probe B

Herbert Pfister (Insitut für Theoretische Physik, Universität Tübingen)

Gravitomagnetism: From Einstein's 1912 paper to the satellites LAGEOS and Gravity Probe B

Authors: Herbert Pfister

Abstract: In a short 1912 paper Einstein asked whether there exists a gravitational action analogous to electrodynamic induction. He introduced the model of an infinitely thin spherical mass shell, and derived that a test mass at its center is dragged along if the mass shell is linearly accelerated. Later Einstein and Besso (in the Entwurf theory) and Lense and Thirring (in GR) derived a rotational dragging of test masses inside a mass shell and outside a rotating full body (e.g. the earth), but only in the weak field and small rotation limit. Brill, Cohen (1966) and Pfister, Braun (1985) generalized this to strong fields and to higher order rotation, in this way confirming Machian ideas in GR, and proposing a "quasiglobal principle of equivalence". Recently this new "gravitomagnetic force" was experimentally confirmed by the satellites LAGEOS and Gravity Probe B.

Presentation:

Back to list of abstracts
Nelson Pinto-Neto: The quantum-to-classical transition of primordial cosmological perturbations

Nelson Pinto-Neto (Centro Brasileiro de Pesquisas Físicas)

The quantum-to-classical transition of primordial cosmological perturbations

Authors: N. Pinto-Neto, G. Santos, and W. Struyve

Abstract: There is a widespread belief that the classical small inhomogeneities which gave rise to all structures in the Universe through gravitational instability originated from primordial quantum cosmological fluctuations. However, this transition from quantum to classical fluctuations is plagued with important conceptual issues, most of them related to the application of standard quantum theory to the Universe as a whole. In this paper, we show how these issues can easily be overcome in the framework of the de Broglie-Bohm quantum theory. This theory is an alternative to standard quantum theory that provides an objective description of physical reality, where rather ambiguous notions of measurement or observer play no fundamental role, and which can hence be applied to the Universe as a whole. In addition, it allows for a simple and unambiguous characterization of the classical limit.

Presentation:

Back to list of abstracts
Jiří Podolský: Geodesic deviation in Kundt spacetimes of any dimension

Jiří Podolský (Charles University in Prague, Institute of Theoretical Physics)

Geodesic deviation in Kundt spacetimes of any dimension

Authors: R. \v{S}varc and J. Podolsk\'y

Abstract: Using the invariant form of equation of geodesic deviation, that describes relative motion of free test particles, we investigate a completely general family of $D$-dimensional Kundt spacetimes. We demonstrate that local influence of the gravitational field can be naturally decomposed into Newton-type tidal effects typical for type~II spacetimes, longitudinal deformations mainly present in spacetimes of algebraic type~III, and type~N purely transverse effects corresponding to gravitational waves with $D(D-3)/2$ independent polarization states. We also explicitly study the most important examples, namely exact pp-waves, gyratons, and VSI spacetimes. This analysis helps us to clarify the geometrical and physical interpretation of the Kundt class of nonexpanding, nontwisting, and shearfree geometries.

Presentation:

Back to list of abstracts
Alena Pravdová: On the Goldberg-Sachs theorem in five dimensions

Alena Pravdová (Institute of Mathematics, Czech Academy of Sciences, Prague)

On the Goldberg-Sachs theorem in five dimensions

Authors: M. Ortaggio, V. Pravda, A. Pravdová, H. S. Reall

Abstract: We discuss generalization of the Godlberg-Sachs theorem to higher dimensions. After working out some general constraints that hold in arbitrary dimensions, we determine necessary algebraic conditions on the optical matrix of a multiple WAND in a five-dimensional Einstein spacetime. We prove that there are three canonical classes of the optical matrices. We provide explicit examples of spacetimes corresponding to each form discussed.

Presentation:

Back to list of abstracts
Dimitrios Psaltis: Testing the no-hair theorem with astrophysical black holes

Dimitrios Psaltis (University of Arizona)

Testing the no-hair theorem with astrophysical black holes

Authors: Dimitrios Psaltis

Abstract: The Kerr spacetime of spinning black holes is one of the most intriguing predictions of Einstein's theory of general relativity. The special role this spacetime plays in the theory of gravity is encapsulated in the no-hair theorem, which states that the Kerr metric is the only realistic black-hole solution of the vacuum field equations. Recent and anticipated advances in the observations of black holes throughout the electromagnetic spectrum have secured our understanding of their basic properties while opening up new opportunities for devising tests of the Kerr metric. In this talk, I will show how imaging and dynamical observations of accreting black-holes with current and future instruments will lead to the first direct test of the no-hair theorem with an astrophysical object. I will also discuss the current state of the Event Horizon Telescope, which will obtain, in the near future, the first horizon-scale image of the black hole in the center of the Milky Way.

Presentation:

Back to list of abstracts
Jorge Pullin: A local Hamiltonian for spherically symmetric gravity coupled to a scalar field

Jorge Pullin (Louisiana State University)

A local Hamiltonian for spherically symmetric gravity coupled to a scalar field

Authors: Nestor Alvarez, Rodolfo Gambini, Jorge Pullin

Abstract: We present a gauge fixing for gravity coupled to a scalar field in spherical symmetry that leads to a true Hamiltonian that is the integral over space of a local density. We discuss its potential use to study black hole evaporation.

Presentation:

Back to list of abstracts
István Rácz: Superradiance or total reflection?

István Rácz (Wigner RCP, Budapest)

Authors: A. László and I. Rácz

Abstract: The evolution of a massless scalar field on Kerr background is considered. In particular, the time evolution of initial data specifications with compact support in the distant region is investigated. The inward sent wave packet is tuned to maximize the effect of superradiance. Evidences are shown indicating that instead of the occurrence of energy extraction from black hole the inward sent radiation fail to reach the ergoregion rather it suffers total reflection.

Presentation:

Back to list of abstracts
Oliver Rinne: Evolution of the Einstein equations to future null infinity

Oliver Rinne (Albert Einstein Institute)

Evolution of the Einstein equations to future null infinity

Authors: O. Rinne and V. Moncrief

Abstract: Given that gravitational radiation is only defined unambiguously at future null infinity $\cal J^+$, it is very desirable to include $\cal J^+$ in numerical evolutions of the Einstein equations. We choose to work directly with the Einstein equations (in an ADM-like reduction with elliptic gauge conditions) expressed in terms of a conformal metric. The resulting equations develop apparently singular terms at $\cal J^+$ that can nevertheless be evaluated in a regular way through an enforcement of the constraint equations. Stable numerical evolutions of a perturbed Schwarzschild black hole in axisymmetry have been obtained. We also show how matter can be included in our formalism and present numerical Einstein-matter evolutions in spherical symmetry.

Presentation:

Back to list of abstracts
Thomas Roman: Probability Distributions of Quantum Stress Tensors in Two and Four Dimensions

Thomas Roman (Central Connecticut State University)

Probability Distributions of Quantum Stress Tensors in Two and Four Dimensions

Authors: Chris Fewster, Larry Ford, and Tom Roman

Abstract: This talk discusses recent work with Chris Fewster and Larry Ford on probability distributions for smeared quantum fields in the vacuum in two and four-dimensional Minkowski spacetime. These distributions have the feature that there is a lower bound at a finite negative value, but no upper bound. The lower bound of the distribution gives the optimal quantum inequality bound, thus illustrating a deep connection between these probability distributions and quantum inequalities. However, arbitrarily large positive energy density fluctuations are possible. In two dimensions, the unique exact analytic form for the distribution has been found for the stress tensor of a massless scalar field in the vacuum state. In four dimensions, we are not able to give closed form expressions for the probability distribution, but rather use calculations of a finite number of moments to estimate the lower bounds, the asymptotic forms for large positive argument, and possible fits to the intermediate region. All of our four-dimensional results are subject to the caveat that these distributions are not uniquely determined by the moments. We apply the asymptotic form of the electromagnetic energy density distribution to estimate the nucleation rates of black holes and of Boltzmann brains.

Presentation:

Back to list of abstracts

Marcelo Salgado (Institute for Nuclear Sciences, UNAM)

f(R) cosmology revisited

Authors: M. Salgado, L. Jaime, L. Patiño

Abstract: I shall present various results concerning the analysis of several f(R) models in a FRW spacetime. The analysis is based on an approach where the f(R) theory is not mapped to a Scalar-Tensor Theory in order to avoid the use of potentials that may be ill defined (e.g multivalued). Thus, the Ricci scalar itself is one of the fundamental variables instead of the scalar f'(R). The system of equations is then solved numerically as an initial value problem constrained by the modified-gravity Hamiltonian. This in turn is used to monitor the accuracy of the numerical integration at every time step. Among the results I shall present are the behaviour of the matter-dominated and accelerated eras, the age of the Universe and the confrontation with SNIa data. I intend to discuss the differences and similarities of our findings relative to previous results.

Presentation:

Back to list of abstracts
Sudipta Sarkar: Entropy increase during physical processes for black holes in Lanczos-Lovelock gravity

Sudipta Sarkar (Institute of Mathematical Sciences, Chennai)

Entropy increase during physical processes for black holes in Lanczos-Lovelock gravity

Authors: Sanved Kolekar, T. Padmanabhan, Sudipta Sarkar

Abstract: We study quasi-stationary physical process for black holes within the context of Lanczos-Lovelock gravity. We show that the Wald entropy of stationary black holes in Lanczos-Lovelock gravity monotonically increases for quasi-stationary physical processes in which the horizon is perturbed by the accretion of positive energy matter and the black hole ultimately settles down to a stationary state. This result reinforces the physical interpretation of Wald entropy for Lanczos-Lovelock models and takes a step towards proving the analogue of the black hole area increase-theorem in a wider class of gravitational theories.

Presentation:

Back to list of abstracts
Christoph Schmid: Exact dragging of inertial axes by cosmic energy currents on the past light-cone

Christoph Schmid (ETH Zurich)

Exact dragging of inertial axes by cosmic energy currents on the past light-cone

Authors: C. Schmid

Abstract: We prove exact rotational dragging of spin axes of gyroscopes (= inertial axes) by cosmic energy-currents on the past light-cone of a gyroscope for linear perturbations with arbitrary energy-flows of arbitrary matter. Our proof demonstrates that the principle postulated by Mach holds for linear cosmological perturbations. Gyroscopes are "in the absolute grip of the distant universe" with the weight function peaked for matter (galaxies etc) at redshift $z = 1$. For sub-Hubble distances between gyroscope and energy-currents, the mechanism for dragging of the gyroscope's spin axis is the same as in Ampere's law of magnetism except for a sign change. For distances larger than the Hubble radius, $z \approx 1$, and back to the big bang, there is an exponential suppression of the influence of energy currents on our gyroscopes. For arbitrary fields of energy currents, we show that the precession of the spin axis of a gyroscope can be caused only by the vector-spherical-harmonic components of the energy currents in the toroidal vorticity sector and with angular momentum $\ell = 1$ relative to the gyroscope. For every infinitesimal distance interval, this harmonic component is equal to the gravito-magnetic moment and also equal to half of the kinetic angular momentum of the arbitrary energy-current distribution. The corresponding Einstein equation is the Ricci $R^{v \phi} = C T^{v \phi}$ equation, where $v$ is the retarded time, which is constant on the past light-cone of the gyroscope. This Einstein equation is the angular momentum constraint on the past light-cone. A crucial element is our proof that the intrinsic geometry of any light-cone cannot be perturbed by toroidal vorticity currents. We choose coordinates on the past light-cone of the gyroscope such that the metric coefficients $g_{\mu \nu}$ are unperturbed.

Presentation:

Back to list of abstracts
Frederic Schuller: Geometrodynamics beyond Einstein

Frederic Schuller (Albert Einstein Institute, Max Planck Institute for Gravitational Physics, Golm)

Geometrodynamics beyond Einstein

Authors: F. P. Schuller

Abstract: Which alternative geometries on a smooth manifold can serve as a spacetime structure? And what are their gravitational dynamics? In this talk I will show that these questions surprisingly have rather comprehensive answers, if one employs an intriguing interplay of real algebraic geometry, convex analysis and the theory of hyperbolic polynomials.

Presentation:

Back to list of abstracts
José M Senovilla: On the stability operator for MOTS and the core' of Black Holes

José M Senovilla (Universidad del País Vasco, Bilbao)

On the stability operator for MOTS and the core' of Black Holes

Authors: José M M Senovilla

Abstract: I will consider small deformations of marginally (outer) trapped surfaces (MOTS) by using the stability operator introduced by Andersson-Mars-Simon. In the case of spherical symmetry, one can use these deformations on any marginally trapped round sphere placed at the spherically symmetric marginally trapped tube (MTT) -defined by $r=2m$- to prove several interesting results as well as the following surprising and fundamental theorem: "In spherically symmetric spacetimes, there are closed trapped surfaces (topological spheres) penetrating both sides of the spherical (non-null) MTT with arbitrarily small portions inside the region $r<2m$". Then, the concept of core' of a black hole is introduced: it is the minimal region that one should remove from the spacetime in order to get rid of all possible closed trapped surfaces. In spherical symmetry, and using the previous theorem, one can prove that the spherical MTT is the boundary of a core. By using a novel formula for the principal eigenvalue of the stability operator, I will argue that similar results may hold in general black hole spacetimes.

Presentation:

Back to list of abstracts
Leszek Sokolowski: The twin paradox in static spacetimes and Jacobi fields

Leszek Sokolowski (Astronomical Observatory, Jagiellonian University, Krakow)

The twin paradox in static spacetimes and Jacobi fields

Authors: L. M. Sokolowski

Abstract: The twin paradox in special relativity has a clear geometrical meaning, yet in most textbooks its resolution is given only in the simplest case and provides no deeper understanding of the effect. Vaguely speaking, in flat spacetime the twin moving at a higher nonuniform velocity is the younger one. In curved spacetimes a multitude of possibilities occur and the simplest case in Schwarzschild spacetime, a non-geodesic twin at (absolute) rest and a geodesic twin moving on a circular orbit, is in some sense misleading. In general one can only determine which worldline connecting the separation and the reunion point makes the twin following it the oldest one. This is the timelike geodesic without points conjugate to the initial (separation) point on the segment ending at the reunion point. The conjugate points exist if any Jacobi vector field (any solution of the geodesic deviation equation) vanishes at a point provided that the field vanishes at the initial point. We therefore investigate Jacobi fields on timelike geodesics in a number of static space- times. For physical reasons we study only simplest geodesic curves: circular orbits (if they exist) and radial trajectories (flights upwards and downwards) and calculate their lengths. For comparison we compute the length of a non-geodesic worldline of a static observer between the separation and the reunion. In all the spacetimes under consideration the radial geodesics are the longest ones and except de Sitter space they contain conjugate points outside the relevant segment; there may be two points or an infinite sequence of conjugate points. All static spherically symmetric spacetimes have the same properties concerning circular geodesics (if they exist): there exist two infinite sequences of conjugate points. We also compare the radial time- like geodesics in Robertson-Walker spacetime with these in static metrics and find qualitative similarities. The Jacobi fields may be effectively studied only in spacetimes with a high symmetry since the existence of integrals of motion generated by Killing vector fields is crucial for solving the geodesic deviation equation.

Presentation:

Back to list of abstracts
Roberto Sussman: The averaging problem in Szekeres dust models

Roberto Sussman (Instituto de Ciencias Nucleares, UNAM)

The averaging problem in Szekeres dust models

Authors: R. A. Sussman

Abstract: We consider a general formalism of scalar averaging for the study of the dynamics of Szekeres dust models. Although these models do not admit (in general) Killing vectors, their averaged scalars behave as spherically symmetric quantities. We show that under a suitable choice of an invariant weight factor the averaged scalars identically satisfy FLRW dynamics, so that inhomogeneity becomes encoded in their fluctuations. The evolution equations for these averaged scalars and their fluctuations leads to a fully consistent and complete 3-dimensional dynamical system that can be studied with standard techniques. These evolution equations lack the "back-reaction" terms that characterize Buchert's formalism (the average with unit weight factor), and lead in a natural way to define a rigorous perturbation formalism. The main curvature and kinematic invariant scalars are directly related to the variance and covariance momenta constructed with the fluctuations, which leads to a potentially useful and invariant definition of gravitational entropy.

Presentation:

Back to list of abstracts
Jiří Svoboda: On steep radial emissivity in relativistic iron lines

Jiří Svoboda (European Space Astronomy Centre of ESA)

On steep radial emissivity in relativistic iron lines

Authors: J. Svoboda, M. Dovčiak, R. W. Goosmann, M. Guainazzi, P. Jethwa, V. Karas, G. Miniutti

Abstract: X-ray spectroscopy of active galaxies and black hole binaries provides an opportunity to explore the innermost regions of black hole accretion discs. Some of the recent measurements have revealed a very steep radial decrease of the disc reflection emissivity, especially in the central region, suggesting the disc-irradiating corona to be compact and very centrally localised. We will discuss whether the special conditions on the corona properties are indeed required, and/or whether the steep radial emissivity could be an artifact of model assumptions. We will present two different effects which might account for the steep radial emissivities, the angular directionality of the reflected radiation properly calculated in the fully relativistic regime and the radial dependence of the accretion disc ionisation. We will show that these effects may also influence the measurements of the black hole angular momentum.

Presentation:

Back to list of abstracts
Laszlo Szabados: Mass, gauge conditions and spectral properties of the Sen-Witten operator in closed universes

Mass, gauge conditions and spectral properties of the Sen-Witten operator in closed universes

Abstract: A non-negative expression, built from the norm of the 3-surface twistor operator and the energy-momentum tensor of the matter fields on a spacelike hypersurface, is found which in the asymptotically flat/hyperboloidal case povides a lower bound for the ADM/Bondi-Sachs mass, while on closed hypersurfaces gives the first eigenvalue of the Sen-Witten operator. Also in the closed case, its vanishing is equivalent to the existence of non-trivial solutions of Witten's gauge condition. Moreover, it is vanishing if and only if the closed data set is in a flat spacetime with spatial topology $S^1\times S^1\times S^1$. Thus, it provides a positive definite measure of the strength of the gravitational field (with physical dimension mass) on closed hypersurfaces, i.e. some sort of the total mass of closed universes. Reference: Class. Quantum Grav. 29 (2012) 095001, or arXiv:1112.2966v2[gr-qc]

Presentation:

Back to list of abstracts
Juan Valiente Kroon: A class of conformal curves for spherically symmetric spacetimes

Juan Valiente Kroon (School of Mathematical Sciences, Queen Mary, University of London)

A class of conformal curves for spherically symmetric spacetimes

Authors: J. A. Valiente Kroon

Abstract: I will discuss a class of conformally privileged curves on spherically symmetric spacetimes. The class of curves under consideration provides a natural generalization of the notion of conformal geodesics for non-vacuum spacetimes. Like conformal geodesics in vacuum spacetimes, these curves can be arranged so that they provide a canonical conformal factor which can be read from the data of the curve. Of particular interest for our analysis are spacetimes containing black hole regions. A natural question in this context is whether the congruence of curves can cover the whole of the outer domain of communication of these spacetimes without forming conjugate points. When this is the case, the congruence can be used to construct “generalized Gaussian coordinates” by means of which one can evaluate (numerically) a conformal representation of the spherically symmetric spacetime. Special attention will be given, in this analysis, to the Reissner-Nordström (non-extremal and extremal) , Schwarschild-de Sitter, Schwarzschild-anti de Sitter and Vaydia spacetimes.

Presentation:

Back to list of abstracts
David Wiltshire: What is dust? Coarse graining, cosmic variance and cosmic expansion

David Wiltshire (University of Canterbury)

What is dust? Coarse graining, cosmic variance and cosmic expansion

Authors: D. L. Wiltshire

Abstract: When Einstein first applied his field equations to cosmology he imagined a universe of stellar density, in which the energy-momentum tensor averages only over nongravitational forces. Nearly 100 years later we observe a universe which is only homogeneous in a statistical sense on scales larger than 100/h Mpc. To coarse-grain dust on these scales requires us to coarse-grain the gravitational degrees of freedom themselves. This necessitates a re-examination of foundational questions relating to the nonlocalizability of gravitational energy; issues which vexed Einstein as he struggled towards general relativity 100 years ago. I argue that a re-examination of these issues has immediate observable consequences. Indeed, a detailed study of 4,534 redshifts and distances in a model-independent manner (arXiv:1201.5371) has led us to the suggestion that the CMB dipole is partly due to a 0.6% anisotropy in the distance-redshift relation due to foreground structures on scales up to 65/h Mpc, which can only be understood as the differential expansion of space rather than as Newtonian velocity perturbations on a fixed homogeneous background. This result, if true, will have profound consequences for cosmology.

Presentation:

Back to list of abstracts
Andrei Zelnikov: Self-energy of a scalar charge near higher-dimensional black holes

Andrei Zelnikov (University of Alberta)

Self-energy of a scalar charge near higher-dimensional black holes

Authors: A. Zelnikov, V. P. Frolov

Abstract: We study the problem of self-energy of charges in higher dimensional static spacetimes. Application of regularization methods of quantum field theory to calculation of the classical self-energy of charges leads to model-independent results. The correction to the self-energy of a scalar charge due to the gravitational field of black holes of the higher dimensional Majumdar-Papapetrou spacetime is calculated exactly. It proves to be zero in even dimensions, but it acquires non-zero value in odd dimensional spacetimes.

Presentation:

Back to list of abstracts
Miguel Zilhão: Dynamics of black holes in de Sitter spacetimes

Miguel Zilhão (Centro de Fisica do Porto, Universidade do Porto)

Dynamics of black holes in de Sitter spacetimes

Authors: M. Zilhão, V. Cardoso, L. Gualtieri, C. Herdeiro, U. Sperhake, and H. Witek

Abstract: We report on the first dynamical evolutions of black holes in asymptotically de Sitter spacetimes. We focus on the head-on collision of equal mass binaries and compare analytical and perturbative methods with full blown nonlinear simulations. Our results include an accurate determination of the merger/scatter transition (consequence of an expanding background) for small mass binaries and a test of the Cosmic Censorship conjecture, for large mass binaries. We observe that, even starting from small separations, black holes in large mass binaries eventually lose causal contact, in agreement with the conjecture.

Presentation:

Back to list of abstracts

### Posters

Giovanni Acquaviva: The temperature issue in curved spacetime

Giovanni Acquaviva (Università degli studi di Trento)

The temperature issue in curved spacetime

Authors: G. Acquaviva

Abstract: The longstanding study of quantum effects in curved spacetimes has led to stimulant connections between General Relativity, Quantum Field Theory and Thermodynamics. This connection is epitomized in the renowned effects named after Unruh and Hawking, as well as in the relevance of quantum fields in the cosmological evolution. The possibility to interpret some quantities in a thermodynamical frame is made explicit in two contexts: the semi-classical tunnelling methods (in which use of Kodama-Hayward's theory is made); and the field-theoretical response of detectors endowed with particular trajectories in curved backgrounds. We propose to compare the results of these approaches in different scenarios, both stationary and dynamical. The concordance of results in stationary cases is challenged when more general dynamical situations are considered, giving some insights for further developments and interpretations.

Presentation:

Back to list of abstracts
Peter Aichelburg: Classical and Quantum Scattering by Impulsive Fields

Peter Aichelburg (University of Vienna)

Classical and Quantum Scattering by Impulsive Fields

Authors: P.C. Aichelburg, H. Balasin

Abstract: We discuss the scattering behaviour of both classical (point) particles as well as their quantum (wave) analogues by impulsive wave fronts. Examples cover Yang-Mills pulses with corresponding (generalized) Lorentz-trajectories and Yang-Mills-Dirac-fields. We also consider the dynamics of Mathiesson-Papapetrou-spinning particles and of Dirac-fields in impulsive gravitational pp-waves.

Presentation:

Back to list of abstracts
Marco Astorino: Charging axisymmetric space-times with cosmological constant

Marco Astorino (Centro de Estudios Cientificos, Valdivia)

Charging axisymmetric space-times with cosmological constant

Authors: Marco Astorino

Abstract: Ernst's solution generating technique for adding electromagnetic charge to axisymmetric space-times in general relativity is generalised in presence of the cosmological constant. Ernst equations for complex potentials are found and they are traced back to an affective dual complex dynamical system, whose symmetries are studied. In particular this method is able to generate charged, asymptotically (A)dS black holes from their uncharged version: as an example, it is shown explicitly how to pass from the Kerr-(A)dS to the Kerr-Newman-(A)dS metric. A new solution describing a magnetic universe in presence of the cosmological constant is also generated.

Presentation:

Back to list of abstracts
Tanwi Bandyopadhyay: A Study of Generalized Second Law of Thermodynamics in Magnetic Universe in the light of Non-Linear Electrodynamics

A Study of Generalized Second Law of Thermodynamics in Magnetic Universe in the light of Non-Linear Electrodynamics

Authors: T. Bandyopadhyay and U. Debnath

Abstract: In this work, we have considered the magnetic universe in non-linear electrodynamics. The Einstein's field equations for non-flat FRW model have been considered when the universe is filled with the matter and magnetic field only. We have discussed the validity of the generalized second law of thermodynamics of the magntic universe bounded by Hubble, apparent, particle and event horizons using Gibb's law and the first law of thermodynamics for interacting and non-interacting scenarios. It has been shown that the GSL is always satisfied for Hubble, apparent and particle horizons but for event horizon, the GSL is violated initially and satisfied at late stage of the universe.

Presentation:

Back to list of abstracts
Pavel Čížek: Ring perturbation of the Schwarzschild black hole

Pavel Čížek (Charles University in Prague, Institute of Theoretical Physics)

Ring perturbation of the Schwarzschild black hole

Authors: P. Čížek, O. Semerák

Abstract: Almost four decades ago Will considered a stationary and axially symmetric perturbation of a Schwarzschild black hole due to a light and slowly rotating ring. He solved the perturbation equations by Legendre-type expansions and discussed basic properties of the result up to a second order. Here we revisit the scheme and derive the Green functions for linear perturbations in a closed form (without using infinite sums). We also comment on the procedure leading to higher-order contributions.

Presentation:

Back to list of abstracts
Ovidiu Cupsa: STEMIONICS - A Grand Unified Theory Pattern

Ovidiu Cupsa (Constanta Maritime University)

STEMIONICS - A Grand Unified Theory Pattern

Authors: LPhDEng Ovidiu Sorin CUPSA

Abstract: THE STANDARD STEMIONIC PATTERN OF GRAND UNIFIED THEORY Stemionics proposes a complex model of Grand Unified Theory disambiguation which show that: 1.There is a Unique Universal Force that manifests itself as a continuous interraction between all the energy quanta. 2. The Unique Universal Force is the Primordial Cause for the existence of an unlimited number of Universes located on a Field of Potential Universes. 3. All the phenomena from any Universe are a consequence of this force. These Universes “flow” constantly one into the other, each Universe being an energy transporter for the next Universe. 4. Each Universe represents a potential dump located on a void amplitude core of a linear circular tensor on which is defined this transport function, and the energy transfer between the universes takes place on the loops of this tensor. 5. The total balances of the energy and of the impulse of a Universe are void and the total balance of the energies and impulses transfers between all the potential Universes is void. 6. All the subsystems of a Universe evolve in the sense of balancing the total potential, but also the relative one. Stemionics asserts that no matter which level the observer is situated in the interior of a Universe, he participates in the physical unsteady phenomena, being transported with them, “captive” by the gravity (the transport energy) toward the next loop. Meanwhile, the observer from the loop is not affected by the physical unsteady phenomena from the interior of the Universe or by its gravitational field. He sees the total balance: its energy “quantum” of transfer, which represents a state electromagnetic energy. The Unique Universal Force defines, a unique pattern, repeatable at every scale, for the Micro Universe, Macro Universe or Mega Universe. Unlike the classical physical patterns, the stemionic pattern is a pattern whose approach surpasses the spatial-temporary limits of the Universe we live in, which is considered only a particular case from the Field of Potential Universes. The EON (Energy Original Node) - energy elementary particle of the Universe has a multiple energetic potential in energetic fields: 1. Is born from an universal inergetic field (DARK ENERGY FIELD) formed out of identical origins called STEMION (Space, Time, Energy, Mass, Information Original Node). 2. It suffers a transformation from inergy to energy on an universal prenergetic pre-energetic field (DARK ENERGY FIELD), being a transitory particle – LERON (Low Energy Radiation Original Node), 3. It evolves on the universal energy field (BARYONIC ENERGY FIELD) as EON, 4. It suffers a transformation from energy to inergy on an universal prenergetic post-energetic field (DARK ENERGY FIELD. The stemionic pattern can coexist mainly with other physical patterns, having its own relativistic approach and a quantum interpretation which offer a disambiguation of certain approaches of the classical quantum mechanics.

Presentation:

Back to list of abstracts
Kamil Daněk: Critical-Curve Topologies of Triple Gravitational Lenses

Kamil Daněk (Charles University in Prague, Institute of Theoretical Physics)

Critical-Curve Topologies of Triple Gravitational Lenses

Authors: K. Daněk, D. Heyrovský

Abstract: An extrasolar analog of the Sun - Jupiter - Saturn system has been discovered recently by detecting its gravitational microlensing action on the flux from a background star (Gaudi et al. 2008). More generally, however, gravitational lensing by a system of three bodies has not yet been satisfactorily analyzed theoretically. Correct interpretation of microlensing light curves requires an understanding of the geometry of the underlying lens caustic and critical curves. These curves correspond to source positions and image positions, respectively, with infinite point-source-flux amplification. Following the pioneering Erdl \& Schneider (1993) analysis of the parameter dependence of binary lensing, we extend their approach to special cases of the triple lens. While the binary lens is characterized by two parameters, three more parameters are needed to describe the triple lens. We present here an example of two-dimensional and three-dimensional cuts through the five-dimensional parameter space, identifying the boundaries of regions with different critical-curve topology.

Presentation:

Back to list of abstracts
Roldao da Rocha: Exotic (dark) eigenspinors of the charge conjugation operator and cosmological applications

Roldao da Rocha (Mathematics Institute, ABC Federal University)

Exotic (dark) eigenspinors of the charge conjugation operator and cosmological applications

Authors: R. da Rocha

Abstract: We report about some achievements and developments provided by the (dark) eigenspinors of the charge conjugation operator. Exotic dark spinor fields has been investigated in the context of inequivalent spin structures on arbitrary curved spacetimes, which induces an additional term on the associated Dirac operator, related to a Cech cohomology class. Exotic terms operating on standard model Dirac spinor fields are usually absorbed by gauge transformations encoded as a shift of some vector potential representing an element of the cohomology group $H^1(M,\mathbb{Z}_2)$. That is not the case of the dark spinor fields, once they cannot carry gauge charge. As a consequence, this program requires a complete evaluation of topological analysis. Since exotic dark spinor fields also satisfy Klein-Gordon propagators, the dynamical constraints related to the exotic term in the Dirac equation can be explicitly computed. It is possible for cosmological applications to assume that the dark spinor fields depend only on the time variable via a matter field $\kappa(t)$ compatible with homogeneity and isotropy, and acts as the only dynamical cosmological variable, The matter field $\kappa(t)$ satisfies a first order ordinary differen- tial equation in time derivative, involving the time component of the total energy-momentum tensor, the Planck mass, and the Hubble constant. It forthwith implies that the non-trivial topology associated to the spacetime can drastically engender — from the dynamics of dark spinor fields — constraints on the spacetime metric structure. Besides being candidates to the dark matter problem, dark spinor fields are shown to be poten- tial candidates to probe non-trivial topologies in spacetime, as well as to explain the spacetime metric structure. (R. da Rocha, A. E. Bernardini and J. M. Hoff da Silva, JHEP 1104, 110 (2011).)

Presentation:

Back to list of abstracts
Marek Dwornik: Modified gravity theories and dark matter models tested by galactic rotation curves

Marek Dwornik (University of Szeged)

Modified gravity theories and dark matter models tested by galactic rotation curves

Authors: Marek Dwornik, Zoltán Keresztes, Tiberiu Harko, László Á. Gergely

Abstract: The rotation curves of galaxies provide a tool for studying the distribution and fundamental properties of gravitating matter. Rotation curve data shows that either gravity should be modified on the galactic scale or baryonic matter should be supplemented by a dark matter halo. As a first possibility we investigated higher-dimensional modifications of general relativity. In the brane-world scenario the four dimensional effective Einstein equation has an additional source term originating in the higher dimensional curvature, behaving as a fluid. We have shown that a linear equation of state of this Weyl fluid is compatible with the rotation curves. As a second attempt, we assumed a cold dark matter distribution in the form of a Bose-Einstein condensate, and tested this assumption on a selected sample of different type of galaxies. This second model is suitable to explain the rotation curves of the high surface brightness and dwarf galaxies, and a subset of the low surface brightness galaxies.

Presentation:

Back to list of abstracts
Felipe Falciano: Probing the Spacetime structure through dynamics

Felipe Falciano (CBPF - Brazilian Center for Physics Research)

Probing the Spacetime structure through dynamics

Authors: F. T. Falciano, E. Goulart, M. Novello and J. D. Toniato

Abstract: We propose to review the meaning of geometrization in view of the analogue program where the emergence of a metric appears as a consequence of linear perturbation. We shall show that the self-interacting of a field can be geometrized together with its perturbations in the sense that both dynamics are controlled by the same metric. In the attempt to disentangle the dynamics from the spacetime structure, we have run into a new symmetry of the Klein-Gordon equation that is related to redefinitions of the metric tensor which implement a map between non-equivalent manifolds.

Presentation:

Back to list of abstracts
Toshifumi Futamase: A New approach to calculate the evolution of the non-linear matter power spectrum

Toshifumi Futamase (Astronomical Institute, Tohoku University)

A New approach to calculate the evolution of the non-linear matter power spectrum

Authors: T. Futamase

Abstract: New approximation method(Wiener-Hermite expansion) is applied for the non-linear evolution of Large-Scale Structure, and an approximated matter power spectrum in full order of expansion is obtained. The power spectrum agrees with N-body result within 2% accuracy in a wide range of the wavenumber k=0.1~0.5 h/Mpc at z=0.5-3. A merit of our method is that the computaional time is very short of the order of a few second.

Presentation:

Back to list of abstracts
Maria Gabach-Clement: The conical singularity in axisymmetric multiple black hole configurations

Maria Gabach-Clement (Max Planck Institute, Albert Einstein Institute)

The conical singularity in axisymmetric multiple black hole configurations

Authors: Maria E. Gabach-Clement

Abstract: We study the problem of N interacting, axisymmetric black holes and analyse the presence of a conical singularity on the bounded components of the symmetry axis. We obtain and discuss relations among physical parameters of the system (such as angular momentum and horizon area) and the deficit angle at the axis.

Presentation:

Back to list of abstracts
Lisa Glaser: New multicritical matrix models and multicritical 2d CDT

Lisa Glaser (Niels Bohr Institute)

New multicritical matrix models and multicritical 2d CDT

Authors: Jan Ambjorn, Lisa Glaser, Andrzej Gorlich, Yuki Sato

Abstract: We define multicritical CDT models of 2d quantum gravity and show that they are a special case of multicritical generalized CDT models obtained from the new scaling limit, the so-called "classical" scaling limit, of matrix models. The multicritical behavior agrees with the multicritical behavior of the so-called branched polymers.

Presentation:

Back to list of abstracts
Graeme Gossel: Quantum particles around near-black hole objects: resonant particle capture, spectrum collapse, and the smooth transition to black hole absorption

Graeme Gossel (School of Physics, University of New South Wales, Sydney)

Quantum particles around near-black hole objects: resonant particle capture, spectrum collapse, and the smooth transition to black hole absorption

Authors: J. C. Berengut, V. V. Flambaum, G. H. Gossel, G. F. Gribakin, A. F. Spencer-Smith and Y. Stadnik

Abstract: We investigate quantum properties of particles in the gravitational fields of "near black-hole objects". These bodies have radius R that slightly exceeds the Schwarzschild radius, and we describe them using interior and exterior metrics that allow us to examine the black-hole limit, $R \rightarrow r_s$. We find that massless quantum particles scattered by the gravitational field of such an object possess a dense spectrum of narrow resonances: a set of long lived meta-stable states whose lifetimes and density tend to infinity in the black-hole limit. We have confirmed the behavior for particles of spin-0, 1/2 and 1. The cross section of particle capture into these resonances is equal to the absorption cross section for a Schwarzschild black hole; thus, a non-singular static metric acquires black-hole properties before the actual formation of a black hole [1]. Bound states exist for massive particles in these near black-hole metrics, and we examine their energies in the limit $R \rightarrow r_s$. In this limit all bound states tend to zero energy, where the binding energy is equal to the rest mass of the particle. However until there is a singularity in the metric, there are no zero-energy states, and hence no pair production occurs in these metrics. This contrasts with the Coulomb case, where a large non-singular field can produce pairs from the vacuum. We have also shown that the energy spectrum becomes quasi-continuous as the metric becomes singular [2]. Additionally we examine the Schwarzschild interior metric which develops a pressure singularity before $R = r_s$ (at $r_s = 8R/9$), hence we consider particle properties as the metric approaches this limit. Sharp resonances still occur in the scattering problem, but there is no absorption for zero-energy scattering (in contrast to the black hole case). The absorption cross-section is finite for non-zero energy particles. Once again the bound state spectrum of massive particles collapses to zero [2]. [1] V. V. Flambaum, G. H. Gossel, and G. F. Gribakin, accepted to Phys. Rev. D (2012), arXiv:1012.2134 [2] G. H. Gossel, J. C. Berengut and V. V. Flambaum, Gen. Rel. Gravit., 43, 2673 (2011), arXiv: 1006:5541

Presentation:

Back to list of abstracts
Alfonso Guillen: On the absolute motion in the inertial systems

Alfonso Guillen (Independent scientific researcher)

On the absolute motion in the inertial systems

Authors: Alfonso Leon Guillen Gomez

Abstract: Mechanics declares character relative of motion. But, vacuum is frame of motion of bodies. Thus, motion is absolute like Newton said, since an observer in rest, inside an inertial system (according Minkowski´s metric), does not drag vacuum, and fully insulated from outside, may determine whether his inertial system is in motion and measure its speed, in absolute terms, by generate inside his system, two perpendicular electromagnetic waves (laser rays) for detect Doppler effect. In its absence he proves that the inertial system is in absolute rest. The first longitudinal laser ray has component in the direction of movement, it generates inside his system, for reach a interior point (allocated to observer), with the object of know the detected wavelength with Doppler. The second perpendicular laser ray without the Doppler since it has no component in the direction of movement, with the object of know the emitted wavelength $\lambda_{d}$. In the address and sense of the motion, longitudinal wave will suffer red shifted due to decrease of photons arriving, since observer it moves away of ray laser. Or in the same address and opposite sense, blue shifted due to increase of photons arriving, since observer it approaches to ray laser. The two laser rays are generated in different time with same device, therefore with same wavelength. Absolute speed of inertial system is measured in relation to interior point coordinates at rest of observer. Resulting speed will be: $(v=c(\lambda_{d}/\lambda_{e}-1)$ or $v=c[(\lambda_{d} / \lambda_{e})^{2}-1]/[(\lambda_{d}/\lambda_{e})^{2}+1]$ in relativistic case). In a time is measured $\lambda_{e}$ and in other $\lambda_{d}$. The wavelengths $\lambda_{e}$ and $\lambda_{d}$ are obtained with high accuracy interferometry (detector instead of observer), where the yard-stick, is the wavelength of light itself. Between laser ray and detector will be used a decelerator of photons (closed to stop the photons), according scheme: laser-vacuum-decelerator- vacuum-detector, that allows inertial system can travel with a speed greater than speed of electromagnetic wave. Before that this, in each period, reaches one length of wave, the source radiates one new wave, in a advanced position. On the observer, these waves produce the Doppler.

Presentation:

Back to list of abstracts
Shahen Hacyan: Stokes parameters in the wake of a gravitational wave

Shahen Hacyan (Instituto de Fisica, Universidad Nacional Autonoma de Mexico)

Stokes parameters in the wake of a gravitational wave

Authors: S. Hacyan

Abstract: Explicit expressions are given for the Stokes parameters of an electromagnetic wave in the presence of a plane gravitational wave described by the Ehlers-Kundt metric. In particular, it is shown that the axis of the polarization ellipse oscillates, with its ellipticity remaining constant.

Presentation:

Back to list of abstracts
Jakub Haláček: Analytical conformal compactification of Schwarzschild spacetime

Jakub Haláček (Institute of Theoretical Physics; Charles University in Prague)

Analytical conformal compactification of Schwarzschild spacetime

Authors: J. Halá\v{c}ek, T. Ledvinka

Abstract: Almost all the time the causal structure of the Schwarzschild manifold is illustrated, the diagram introduced in the classical textbook by Misner, Thorne and Wheeler is used. It is known that coordinate transformations giving rise to this diagram are not angle-preserving (conformal) at infinity. Although several transformations were proposed to make the compactified Schwarzschild spacetime look similarly to the compactified Minkowski spacetime near infinities, these transformations do not cover smoothly $\cal J^\pm$. We explain how to find the coordinates covering the complete Schwarzschild manifold as well as its extension beyond $\cal J^\pm$. We also show, that having such analytic compactification can improve convergence in numerical problems including both horizon and ${\cal J}^\pm$.

Presentation:

Back to list of abstracts
Tina Harriott: Solutions in the 2+1 null surface formulation

Tina Harriott (Mount Saint Vincent University)

Solutions in the 2+1 null surface formulation

Authors: Tina A. Harriott, J.G. Williams

Abstract: The null surface formulation of general relativity (NSF) differs from the standard approach by featuring a function Z, describing families of null surfaces, as the prominent variable, rather than the metric tensor. It is possible to reproduce the metric, to within a conformal factor, by using Z (entering through its third derivative, which is denoted by $\Lambda$) and an auxiliary function $\Omega$. The functions $\Lambda$ and $\Omega$ depend upon the spacetime coordinates, which are usually introduced in a manner that is convenient for the null surfaces, and also upon an additional angular variable. A brief summary of the (2+1)-dimensional null surface formulation is presented, together with the NSF field equations for $\Lambda$ and $\Omega$. A few special solutions are found and their properties explored. One such solution describes a spacetime for a perfect fluid source with variable mass-energy density.

Presentation:

Back to list of abstracts
Hideo Iguchi: Phase structure of five-dimensional black di-ring

Hideo Iguchi (Nihon University)

Phase structure of five-dimensional black di-ring

Authors: Hideo Iguchi

Abstract: We investigate the phase structure of black di-ring in five-dimensional asymptotically flat vacuum gravity. We numerically plot the points of black di-rings in the phase diagram to study the region covered by black di-ring. The distribution of black di-ring shows that the area of black di-ring is always less that the maximum value of black ring. The plot indicates that there are black di-ring configurations whose area parameters are arbitrarily close to zero. In addition we analyze the zero area black di-ring solution constructed from $S^2$ rotating black ring.

Presentation:

Back to list of abstracts
Asieh Karami Majoomerd: Semiclassical Solutions of Vacuum Bianchi IX Loop Quantum Cosmology

Asieh Karami Majoomerd (Instituto de Física y Matemáticas, Universidad Michoacana de San Nicolás de Hidalgo; Instituto de Matemáticas, UNAM )

Semiclassical Solutions of Vacuum Bianchi IX Loop Quantum Cosmology

Authors: A. Corichi, A. Karami, E. Montoya

Abstract: In classical general relativity, the chaotic behavior of Bianchi IX model can display the generic local evolution near to a singularity. But it is expected that the Quantum effects can change it. In this work we show that the modifications which come from Loop Quantum Gravity, cause to have a non-chaotic effective behavior.

Presentation:

Back to list of abstracts
Petr Kašpar: Averaging inside the LRS family

Petr Kašpar (Charles University in Prague, Institute of Theoretical Physics)

Averaging inside the LRS family

Authors: P. Kašpar, D. Vrba, O. Svítek

Abstract: Averaging problem in GR and cosmology is of fundamental importance. It is still not clear how to unambiguously average Einstein equations and the metric tensor. One of the most promising attempts how to deal with averaging in GR are the Buchert equations. However, only scalar part of the Einstein equations is averaged and the system is not closed. Here we will present LRS (locally rotationally symmetric) spacetimes, where one can find preferred spatial direction and the evolution and the constraint equations are described only by scalars. By averaging these scalars we will obtain generalized Buchert equations (for LRS spacetimes), which form the closed system and constraints are preserved in time.

Presentation:

Back to list of abstracts
David Kofroň: Variations on spacetimes with boost-rotation symmetry

David Kofroň (Institute for Theoretical Physics; Charles University in Prague)

Variations on spacetimes with boost-rotation symmetry

Authors: D. Kofro\v{n}, J. Bi\v{c}\'ak

Abstract: Various aspects of boost-rotation symmetric spacetimes representing, in general, two rotating charged objects accelerated in opposite directions as, for example, the C-metric describing two accelerating black holes, are summarized and their limits are considered. A particular attention is paid (a) to the special-relativistic limit in which the electromagnetic field becomes the “magic field” of two oppositely accelerated, rotating charged relativistic discs (Gen. Rel. Grav. {\bf 41} (2009), 1981); (b) to the Newtonian limit which is analyzed using the Ehlers frame theory. In contrast to some previous discussions, our results are physically plausible in the sense that the Newtonian limit corresponds to the fields of classical point masses accelerated uniformly in classical mechanics. This corroborates the physical significance of the boost-rotation symmetric spacetimes (Gen. Rel. Grav. {\bf 41} (2009), 153). The Ernst method of removing nodal singularities from the charged C-metric representing a uniformly accelerated black hole with mass $m$, charge $q$ and acceleration $A$ by ‘‘adding’’ an electric field $E$ is generalized. Utilizing the new form of the C-metric found recently, Ernst’s simple ‘‘equilibrium condition’’ $mA = qE$ valid for small accelerations is generalized for arbitrary $A$ (Phys. Rev. D {\bf 82} (2010), 024006). The electromagnetic and gravitational radiation of the charged C-metric is also analyzed (work in progress).

Presentation:

Back to list of abstracts
Sanved Kolekar: Aspects of horizon entropy in Lanczos-Lovelock gravity and Action principle for the fluid/gravity correspondence

Sanved Kolekar (The Inter-University Centre for Astronomy and Astrophysics)

Aspects of horizon entropy in Lanczos-Lovelock gravity and Action principle for the fluid/gravity correspondence

Authors: Sanved Kolekar

Abstract: In this talk, we explore the thermodynamic nature of horizons in two parts: We compare the expressions of entropy obtained in two different approaches. In the extrinsic' approach, we evaluate the entropy of a configuration of densely packed gravitating shells on the verge of forming a black hole in Lanczos-Lovelock theories of gravity. We find that this matter entropy is not equal to (it is less than) Wald entropy, except in GR, where they are equal. In the `intrinsic' approach, we consider the Euclidean action of Lanczos-Lovelock models for a class of spherically symmetric metrics off-shell. We show that one can interpret it as the free energy and read off both the entropy and energy of a black hole spacetime to be exactly equal to the Wald entropy and the quasi-local energy of the spacetime in Lanczos-Lovelock models obtained by other methods. We know that Einstein's field equations when projected onto any null surface look very similar to a Navier-Stokes equation which is reminiscent of the Membrane paradigm for black holes. We develop an action principle, the extremization of which leads to the above result, in an arbitrary spacetime. The degrees of freedom varied in the action principle are the null vectors in the spacetime and not the metric tensor. We further show that the action can be given a thermodynamic interpretation of describing an on-shell local entropy density of the spacetime. The null surface is found to obey an equation of state of the form PA = TS which continues to hold even in higher curvature theories such as Lanczos-Lovelock theories. References: 1) S. Kolekar, D. Kothawala \& T. Padmanabhan (2011), Phys.Rev.D 85, 064031. arXiv:1111.0973]. 2) S. Kolekar \& T. Padmanabhan (2011), Phys.Rev.D 85, 024004. [arXiv:1109.5353]. 3) S. Kolekar \& D. Kothawala (2012), Membrane Paradigm and Horizon Thermodynamics in Lanczos-Lovelock gravity, JHEP 1202, 006 [arXiv:1111.1242].

Presentation:

Back to list of abstracts
Alicja Konieczny: The null geodesics in the Black Saturn spacetime

Alicja Konieczny (Jagiellonian University)

The null geodesics in the Black Saturn spacetime

Authors: A. Konieczny

Abstract: We study the null geodesics in the Black Saturn spacetime. The Black Saturn is the solution to the $4+1$D Einstein's equations that has been found by Henriette Elvang and Pau Figueras. It describes a black hole with spherical topology surrounded by a black ring.

Presentation:

Back to list of abstracts
Ivan Krasnyy: Gravitational lensing of gravity: The Big Zoom theory

Ivan Krasnyy (Strate Research Navigation-Hydrographic Iinstitute)

Gravitational lensing of gravity: The Big Zoom theory

Authors: I. V. Krasnyy

Abstract: Generalization of classical mechanics at cosmological distances, based on geometric interpretation of the gravitational mass is suggested. Relativistic corrections due to geodesics warping in gravitational field are proposed in order to adjust the Newton’s law of universal gravitation. The notion of an observable gravitational mass is introduced, which is back deduced from the magnified Schwarzschild radius value, depending on its angular size and distance to celestial body. The corrections are defined by the following factors: 1. The gravitational self-lensing, manifesting itself at a distance of more than 1 pc, and increasing as $\sqrt{D}$. Together with other corrections it may explain mass deficit in the universe, rotation curves of the galaxies and provide reasoning for choice of the static closed universe model. 2. Projective zooming in the closed universe, an analogy of scaling in the azimuthal equidistant map projection of the Earth's surface. The scale increases from negligibly small values at near zone to 1.57 at half of the maximal distance, and has asymptotical growth in the antipodal zone. The zooming is accompanied by the gravitational redshift, for the observed masses. It allows us to offer an alternative explanation for the photometric paradox and the CMBR origin. 3. Repulsion, in fact, corresponds to attraction in the opposite direction for a closed universe. This correction also accounts for asymptotic scaling in the antipodal and near zones. 4. Lensing of the gravitational background at antipodal zone. Manifesting inside the minimal focal length of point gravitational lens, it also accounts for possible overestimation of the proper masses of celestial bodies. Adoption of the model will require a significant adjustment of astronomical distance scale, would provide alternative for the dark matter and energy concepts, and explanations for various anomalies.

Presentation:

Back to list of abstracts
Devaky Kunneriath: High angular resolution radio interferometry observations of Galactic Centre

Devaky Kunneriath (Astronomical Institute, Prague)

High angular resolution radio interferometry observations of Galactic Centre

Authors: D. Kunneriath

Abstract: High angular resolution radio interferometry observations at high frequencies enable us to study the emission mechanisms of the central region of our Galactic Centre, which harbours the supermassive black hole source Sgr A*, in great detail. This talk will focus on recent results from GC observations with radio interferometer arrays such as CARMA and ATCA. I will present results from the flare modelling of Sgr A* based on our global coordinated multiwavelength observing campaigns in 2007 and 2008, and high resolution maps along with a spectral index analysis of the extended emission of the central minispiral region at multiple wavelengths.

Presentation:

Back to list of abstracts
Tomáš Ledvinka: On the effects of rotating gravitational waves

Tomáš Ledvinka (Charles University in Prague, Institute of Theoretical Physics )

On the effects of rotating gravitational waves

Authors: J. Bi\v c\'ak, J. Katz, T. Ledvinka, and D. Lynden-Bell

Abstract: We study gravitational waves to first and second order in amplitude in vacuum asymptotically flat spacetimes. The Einstein equations are solved to first order and these solutions are superposed to form a time-symmetric ingoing and then outgoing wave-pulse regular everywhere. The waves are assumed to have odd-parity and a non-vanishing angular momentum which keeps them away from the axis at all times. The averaged energy of the waves is evaluated. The relevant Einstein equation is then solved to second order in the amplitude. The influence of the angular momentum of the waves on the rotation of local inertial frames with respect to the frames at great distances is analyzed. The rotation of the frames occurs even in the region around the origin where spacetime is almost flat. The rotation is without time delay as it follows from the constraint equation (Phys. Rev. D {\bf 85}, 124003 (2012)).

Presentation:

Back to list of abstracts
Patryk Mach: Newtonian self-gravitating disks revisited

Patryk Mach (Jagiellonian University)

Newtonian self-gravitating disks revisited

Authors: P. Mach, E. Malec, W. Simon

Abstract: Recent analytic results on self-gravitating gaseous disks in Newtonian theory are discussed. We start with a Sobolev bound on the disk mass. Next, we give a theorem that forbids infinitely extended configurations of rotating self-gravitating fluids, depending on the assumed equation of state and the rotation law. This part extends former results valid for the static case that were obtained both in General Relativity and in Newtonian theory. Finally, we reformulate the virial theorem in order to allow for a singular point-mass potential of the central object and radiation transfer in the disk. Some applications of this new version are discussed.

Presentation:

Back to list of abstracts
Jianwei Mei: On the General Kerr/CFT Correspondence in Arbitrary Dimensions

Jianwei Mei (Max Planck Institute for Gravitational Physics (Albert Einstein Institute))

On the General Kerr/CFT Correspondence in Arbitrary Dimensions

Authors: Jianwei Mei

Abstract: We study conformal symmetries on the horizon of a general stationary and axisymmetric black hole. We find that there exist physically reasonable boundary conditions that uniquely determine a set of symmetry generators, which form one copy of the Virasoro algebra. For extremal black holes, Cardy's formula reproduces exactly the Bekenstein-Hawking entropy.

Presentation:

Back to list of abstracts
Hans-Peter Nollert: "Einstein-Wellen-mobil", a mobile exhibition about gravitational waves

Hans-Peter Nollert (SFB/TR 7 "Gravitational Wave Astronomy"; Theoretische Astrophysik; Universität Tübingen)

"Einstein-Wellen-mobil", a mobile exhibition about gravitational waves

Authors: B. Brügmann, K. Kokkotas, H.-P. Nollert

Abstract: The "Einstein-Wellen-mobil" is a mobile exhibition about gravitational waves. It is part of the outreach project in the Sonderforschungsbereich/Transregio 7 "Gravitational Wave Astronomy". It features interactive experiments, simulations, models, and a series of explanatory movies. It is directed at the general public with a special emphasis on high schools. While the participants of the conference will probably not learn anything about gravitational waves that they don't know already, they may enjoy visiting the exhibition and playing with the exhibits.

Presentation:

Back to list of abstracts
Marcello Ortaggio: Electric and magnetic Weyl tensors in higher dimensions

Marcello Ortaggio (Institute of Mathematics, Academy of Sciences of the Czech Republic)

Electric and magnetic Weyl tensors in higher dimensions

Authors: S. Hervik, M. Ortaggio, and L. Wylleman

Abstract: The splitting of the Weyl tensor into its electric and magnetic parts relative to an observer is considered in higher dimensions, and purely electric (PE) or magnetic (PM) spacetimes are studied. Several results previously known in four dimensions are generalized, and new features of higher dimensions discussed. For example, we show that the only permitted Weyl types are G, I${}_i$ and D, and provide other invariant conditions that characterize PE/PM metrics. In particular, we present conditions under which direct (or warped) products are PE/PM, and discuss various classes of PE spacetimes. PM solutions, however, prove to be more elusive: for instance, PM Einstein spacetimes of type D do not exist. Nevertheless, some examples of PM spacetimes in higher dimensions are also provided. Corresponding results for the electric/magnetic parts of the Riemann tensor will be briefly mentioned.

Presentation:

Back to list of abstracts
Christian Pfeifer: Finsler extension of Einstein gravity

Christian Pfeifer (II Institute for Theoretical Physics, University of Hamburg)

Finsler extension of Einstein gravity

Authors: C. Pfeifer, M. Wohlfarth

Abstract: I present an extension of Einstein gravity based on the use of Finsler geometry which generalizes Lorentzian metric geometry. The definition of Finsler spacetimes is presented in some detail and it will be discussed that these are nice generalizations of Lorentzian metric spacetimes, which admit all structures necessary for physics: causality, observers, their measurements, field theories and dynamics consistent with general relativity. The dynamical theory of Finsler spacetimes is constructed from a well defined action principle, the Field equation is obtained by variation and it reduce to the Einstein equations in the case the Finsler spacetime is a Lorentzian metric spacetime. A specific first order spherical symmetric solution of this Finsler gravity theory turns out to be a refinement of the well known Schwarzschild solution of general relativity and can in principle be compared with experimental data in the solar system.

Presentation:

Back to list of abstracts
Ignacio Ranea Sandoval: Effect of magnetic fields on equatorial circular orbits around Kerr spacetimes

Ignacio Ranea Sandoval (Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata)

Effect of magnetic fields on equatorial circular orbits around Kerr spacetimes

Authors: Ignacio F. Ranea-Sandoval and H. Vucetich

Abstract: In this work we analyze the effects of an external magnetic field on charged particles following equatorial circular orbits around a Kerr spacetime, both in the black hole and the naked singularity cases. Understanding these phenomena is of great importance because equatorial circular orbits are a key ingredient of (simple) accretion disc models. In particular we study two important magnetic field configurations: a) a uniform magnetic field aligned with the angular momentum and b) a dipolar magnetic field. We center our attention on the effect of these external fields on the marginally bound and marginally stable equatorial circular orbits because they are potentially observable quantities that could be useful to determine the nature of the central object. Using a perturbative approach we are able to give analytic results and compare (in the black hole case) with previous results.

Presentation:

Back to list of abstracts
Jorge Rocha: Inverse scattering construction of dipole black rings

Jorge Rocha (Instituto Superior Técnico - Lisbon)

Inverse scattering construction of dipole black rings

Authors: J. V. Rocha, M. J. Rodriguez, A. Virmani, and O. Varela

Abstract: It is well known that D-dimensional vacuum gravity with D-2 commuting Killing vectors is integrable. For such theories an efficient solution-generating technique has been available since it was first presented by Belinski and Zakharov in 1978. This method, which was later improved by Pomeransky, has been successfully used to discover new black hole solutions, most notably in five spacetime dimensions. I will first demonstrate how this technique can be employed in six dimensions to generate regular and asymptotically flat dipole black rings in the five-dimensional Einstein-Maxwell-dilaton theory obtained by Kaluza-Klein reduction. This approach allows us to systematically construct Emparan's dipole black ring in the theory under consideration. Secondly, I will exploit this method to generate more general black ring solutions than those previously known. These solutions, whose existence has been conjectured, display rotation along the two orthogonal planes and electric charge in addition to a non-conserved magnetic dipole charge.

Presentation:

Back to list of abstracts
Debraj Roy: Trivial symmetries in models of gravity

Debraj Roy (S.N. Bose National Centre for Basic Sciences, Kolkata)

Trivial symmetries in models of gravity

Authors: Debraj Roy

Abstract: We perform a systematic construction of off-shell gauge symmetries of the action in a total hamiltonian formalism. Since the systems analysed are models of gravity (in Riemann-Cartan spacetime with torsion), there is an underlying Poincare symmetry, also off-shell. Comparison is made between this underlying Poincare symmetry and the hamiltonian symmetries constructed systematically. Though both the symmetries are themselves off-shell, it has been noted in literature that the two symmetries become equivalent only on-shell. But does this imply that there are really two independent sets of symmetries? We show here that the hamiltonian and Poincare symmetries are actually equivalent to each other, modulo trivial gauge symmetries. [1] R. Banerjee, D. Roy, Phys. Rev. D 84, 124034 (2011) [2] R. Banerjee, S. Gangopadhyay, and D. Roy, JHEP 2011, 121 (2011) 121 [3] R. Banerjee, S. Gangopadhyay, P. Mukherjee and D. Roy, JHEP 1002, 075 (2010)

Presentation:

Back to list of abstracts
Martin Scholtz: On the existence and properties of helically symmetric systems

Martin Scholtz (Czech Technical University in Prague)

On the existence and properties of helically symmetric systems

Authors: J. Bičák, M. Scholtz, P. Tod

Abstract: In flat spacetime there is a clear definition of a helical Killing vector and exact helically symmetric solutions are known in electrodynamics (Schild, 1963), in scalar gravity (Beig, Heinzle, Schmidt, 2007) and in the Nordstrom gravity (Bruneton, 2006). In curved spacetime, several definitions of helical Killing vector have been suggested (Bonnazola, Gourgoulhon, 1997 and Friedman, Uryu, Shibata, 2002). However, no exact non-stationary solution to Einstein's equations with helical symmetry is known. There are "physical" arguments that a helically symmetric solution of Einstein's equations cannot be asymptotically flat: a source moving along a circular orbit is forced to produce gravitational waves and therefore lose the energy. Thus, helical symmetry requires the presence of ingoing gravitational radiation compensating the energy loss. Since the ingoing radiation must be present for all time, corresponding spacetime is not expected to be asymptoticaly flat. A more general question whether a periodic non-stationary solution can be asymptotically flat was attacked by Gibbons and Stewart (1985). They have shown that periodic, asymptotically flat vacuum solution of Einstein's equations is necessarily stationary. It turns out, however, that their proof suffers from some drawbacks: an inappropriate definition of periodicity from which the stationarity cannot be deduced; and the choice of gauge incompatible with the periodicity assumed. Here we summarize the work of Bicak, Scholtz, and Tod (2010a,b) in which the proof of Gibbons and Stewart was revisited and generalized to the cases when electromagnetic or scalar fields are also present. We proved the existence of a timelike Killing vector and thus deduced the stationarity of any periodic, asymptoticaly flat vacuum or electrovacuum spacetimes and spacetimes with scalar fields (standard massless or conformally invariant scalar fields). As an auxiliary result, new expressions for the Bondi mass of both kinds of scalar fields have been obtained. Moreover we present corollaries concerning the inheritance of stationarity in the case of electromagnetic and massless Klein-Gordon field. In the second part we present our recent results on helically symmetric systems in linearized Einstein's gravity. In particular, we concentrate on the conditions under which a two-particle system can stay in equilibrium, and represents thus a generalization of the classical system of two (opposite) charges in electrodynamics represented by the helically symmetric Schild's solution of Maxwell's equations.

Presentation:

Back to list of abstracts
Jozef Skakala: Black hole asymptotic quasi-normal modes and their conjectured relation to the black hole thermodynamics

Black hole asymptotic quasi-normal modes and their conjectured relation to the black hole thermodynamics

Authors: Jozef Skakala

Abstract: We summarize the basic conjectures about the link between the highly damped black hole quasi-normal modes and the black hole thermodynamics. We analyze basic common features of the asymptotic QNM frequencies of different black hole spacetimes and what these features tell us in terms of the conjectured connection with quantum black holes. We focus in more detail on the case of non-extremal Reissner-Nordstrom black hole and present some open questions.

Presentation:

Back to list of abstracts
Cristinel Stoica: Advances on the problem of singularities in General Relativity

Cristinel Stoica (Institute of Mathematics of the Romanian Academy)

Advances on the problem of singularities in General Relativity

Authors: O. C. Stoica

Abstract: Results concerning an extension of differential geometry and the Einstein equation to singularities are reported. These results are based mathematically on an extension of semi-Riemannian geometry which allows the definition of curvature invariants and covariant derivatives of differential forms, even when the metric becomes degenerate. It is shown that the singularities of this form allow a smooth extension of the Einstein equations, including matter fields. A cosmological model which extends the FLRW solution, turn out to have the Big-Bang singularity of this type. The metric of the stationary black hole solutions are shown to be of the type studied here (apparently their singularities were of another type, but it turns out that the standard coordinates are singular, masking the smoothness of the metric). Charged black holes, when expressed in the proposed coordinates, have the electromagnetic potential regular everywhere. Implications on the Weyl curvature hypothesis (Penrose) are presented. In addition, these singularities are accompanied by a (geo)metric dimensional reduction, which acts as a regulator for the quantum fields in the UV regime, including for quantum gravity.

Presentation:

Back to list of abstracts
Zdeněk Stuchlík: The role of the cosmological constant in the motion of Magellanic Clouds in the gravitational field of Milky Way

Zdeněk Stuchlík (Institute of Physics, Silesian University in Opava)

The role of the cosmological constant in the motion of Magellanic Clouds in the gravitational field of Milky Way

Authors: Z. Stuchlík and J. Schee

Abstract: Using the pseudo-Newtonian (PN) potential related to the Schwarzschild-de Sitter spacetimes, we estimate the influence of the repulsive cosmological constant $\lambda\sim 1.3\times 10^{-56}\,\mathrm{cm}^{-2}$ implied by recent cosmological tests onto the motion of both Small and Large Magellanic Clouds (SMC and LMC) in the gravitational field of the Milky Way. The role of the cosmological constant is on the $10%$ level and is most conspicuous when binding mass is estimated for the satellite galaxies. We have found a strong influence of cosmic repulsion on the total binding mass for both galaxies; in the case of LMC it grows to $50%$ level. We have found the effect of the cosmic repulsion to be comparable to the effect of the dynamical friction and to be exceeding the effect of the Andromeda Galaxy.

Presentation:

Back to list of abstracts
Petra Suková: Geodesic chaos in perturbed black-hole fields

Petra Suková (Charles University in Prague, Institute of Theoretical Physics)

Geodesic chaos in perturbed black-hole fields

Authors: P. Suková, O. Semerák

Abstract: Dynamics of time-like geodesics in the static and axially symmetric field of a black hole surrounded by a thin disc or ring is studied in several different ways: on Poincar\'e sections, on phase-variable behaviours and their power spectra, and by two recurrence methods. The geodesic motion turns chaotic if the disc/ring is sufficiently massive and/or if the particle has sufficiently large energy. The occurrence of chaos due to the presence of ambient matter may be important for the evolution and appearance of astrophysical black-hole systems.

Presentation:

Back to list of abstracts
Nikodem Szpak: A sheet of graphene - quantum fields in a discrete curved space

Nikodem Szpak (University Duisburg-Essen)

A sheet of graphene - quantum fields in a discrete curved space

Authors: N. Szpak

Abstract: Optical lattices or crystalline materials like graphene offer a fascinating laboratory for studying and simulating the impact of non-trivial geometries (e.g. curved graphene sheets) on the quantum fields living in it. Despite existing analogies between effects like deformations and defects in the lattice systems and curvature and torsion in the differential geometry approach, the correspondence is still incomplete and the language allowing for effective calculations is still lacking. We report on progress in this direction and new experimental possibilities.

Presentation:

Back to list of abstracts
Márton Tápai: Gravitational waveforms for black hole binaries with unequal masses

Márton Tápai (University of Szeged)

Gravitational waveforms for black hole binaries with unequal masses

Authors: Márton Tápai, László Árpád Gergely, Zoltán Keresztes

Abstract: We derived a post-Newtonian (PN) inspiral only gravitational waveform for unequal mass, spinning black hole binaries. Towards the end of the inspiral the larger spin dominates over the orbital angular momentum (while the smaller spin is negligible), hence the name Spin-Dominated Waveforms (SDW). Such systems are common sources for future gravitational wave detectors and during the inspiral the largest amplitude waves are emitted exactly in its last part. The SDW waveforms emerge as a double expansion in the PN parameter and the ratio of the orbital angular momentum to the dominant spin.

Presentation:

Back to list of abstracts
Gabriel Török: High frequency quasi-periodic oscillations in low-mass binary systems with neutron stars

Gabriel Török (Institute of Physics, Silesian University in Opava)

High frequency quasi-periodic oscillations in low-mass binary systems with neutron stars

Authors: G. Torok, P. Bakala, E. Sramkova, Z. Stuchlik, M. Urbanec, and K. Goluchova

Abstract: Twin-peak quasiperiodic oscillations (QPOs) are observed in the X-ray power-density spectra of several accreting low-mass neutron star (NS) binaries. We consider various QPO models (e.g, the relativistic precession - RP model) and estimate the mass of NS in the sources 4U 1636-53 and Circinus X-1. We show that QPO estimates result in specific mass-angular-momentum (M-j) relations rather than single preferred combinations of M and j. We also discuss differences in the chi-square behaviour between high- and low- QPO frequency sources and the requirement of a correction to the QPO models. We demonstrate that the differences in the chi-square behaviour can be related to the variability of the model predictive power across the frequency plane and the same correction can be required for both classes of sources. We show that for a particular toy-modification of the RP model the data of 4U 1636-53 are well matched assuming the angular momentum inferred from the X-ray burst measurements.

Presentation:

Back to list of abstracts
Monica Valencia-S.: Relativistic effects on the synchrotron emission originated from hot spots in the vicinity of Sagittarius A*

Monica Valencia-S. (I Physikalisches Institut - University of Cologne. IMPRS for Astronomy and Astrophysics - MPIfR)

Relativistic effects on the synchrotron emission originated from hot spots in the vicinity of Sagittarius A*

Authors: M. Valencia-S., M. Bursa, G. Karssen, A. Eckart, M. Dovčiak and V. Karas

Abstract: The black hole at the galactic center, Sagittarius A*, is source of variable sub-millimeter, near-infrared (NIR) and X-ray emission. Simultaneous observations in these regimes show strong evidence for correlated flux variations in different wavelength-domains. The strong variability seems to be the result of synchrotron and synchrotron self-Compton radiation from overdensities -or hot spots- in the accretion flow or at the base of a faint jet. Using a numerical code we track the time evolution of the intensity and the detectable polarization properties (linear-polarization degree and angle) produced by such compact sources in the vicinity of the black hole. Here we present the theoretical polarimetric light curves expected in the NIR from different configurations. The polarized NIR emission of Sagittarius A* can be used to constrain the essential parameters of the system like the spin and the inclination by comparing the predicted light curves with the observed ones.

Presentation:

Back to list of abstracts
J.G. Williams: Solutions in the 2+1 null surface formulation

J.G. Williams (Brandon University)

Solutions in the 2+1 null surface formulation

Authors: Tina A. Harriott, and J. G. Williams

Abstract: The null surface formulation of general relativity (NSF) differs from the standard approach by featuring a function Z, describing families of null surfaces, as the prominent variable, rather than the metric tensor. It is possible to reproduce the metric, to within a conformal factor, by using Z (entering through its third derivative, which is denoted by $\Lambda$) and an auxiliary function $\Omega$. The functions $\Lambda$ and $\Omega$ depend upon the spacetime coordinates, which are usually introduced in a manner that is convenient for the null surfaces, and also an additional angular variable. A brief summary of the (2+1)-dimensional null surface formulation is presented, together with the NSF field equations for $\Lambda$ and $\Omega$. A few special solutions are found and their properties explored. One such solution describes a spacetime for a perfect fluid source with variable mass-energy density.

Presentation:

Back to list of abstracts
Steven Willison: Isometric embeddings and asymptotically AdS spacetimes

Steven Willison (Centro Multidisciplinar de Astrofísica, Instituto Superior Técnico)

Isometric embeddings and asymptotically AdS spacetimes

Authors: Steven Willison

Abstract: ... not yet given.

Presentation:

Back to list of abstracts
Christof Witte: A smart way to modify gravity

Christof Witte (Max-Planck-Institut für Gravitationsphysik, Albert-Einstein-Institut)

A smart way to modify gravity

Authors: C. Witte

Abstract: We show that the formidable task of constructing alternative tensorial theories of gravity including consistent matter couplings can be reformulated as a system of linear partial differential equations. This is achieved by incorporating the causal structure of a given system of matter field equations directly into the underlying gravitational dynamics described as an evolution of initial data on a suitable hypersurface.

Presentation:

Back to list of abstracts

Public lecture (Wednesday, June 27 at 20:00 in the Great Lecture Hall of the Faculty of Arts of Charles University)

Michael Kramer: Was Einstein right? How cosmic time keepers in space probe Einstein's strange world

Michael Kramer (Max-Planck Institute for Radioastronomy, Bonn)

Was Einstein right?
How cosmic time keepers in space probe Einstein's strange world

Abstract: When Albert Einstein presented his theory of general relativity, it was a revolution in our understanding of the Universe. Century-long held views on the physical laws of nature had to be revised and replaced. The theory was capable of resolving some astronomical mysteries and predicted a number of further strange effects that we are still pursuing to detect. Measuring such effects allows us to test Einstein's theory and eventually give a verdict on the validity of general relativity. This talk tells about the wonderful world of exotic, dead stars which turn out to be precise cosmic time keepers that are ideal for probing the Universe of Albert Einstein.

Back to list of abstracts

Friday, June 29 at 9:00 inthe Blue Lecture Hall

Andrew Hamilton: Interactive Black Hole Flight Simulation

Andrew Hamilton (JILA, University of Colorado, Boulder)

Interactive Black Hole Flight Simulation

Abstract: A real-time interactive Black Hole Flight Simulator will be used to demonstrate the appearance of a black hole as seen by observers on geodesics both outside and inside the horizon.

Back to list of abstracts