Relativity Seminar
of the Institute of Theoretical Physics

fall 2015


Given seminars:

October 13, 2015
Separability of test fields / perturbations on the C-metric background
David Kofroň
ITP

October 20, 2015
Quasilocal horizons
Eliška Klozová
ITP
(seminar in Czech language)

Since the event horizon has some drawbacks because of its global definition, a quasilocal horizon has been introduced. It is a hypersurface foliated by marginally trapped surfaces on which expansion of the outer null normal congruence vanishes. Different types of quasilocal horizons will be listed, i.e. apparent horizon, trapping horizon and isolated and dynamical horizons. Analysis of quasilocal horizons in two dynamical spacetimes used as inhomogeneous cosmological models will also be presented. We discovered future and past horizon in spherically symmetric Lemaître spacetime. Both horizons are null and have locally the same geometry as the horizons in the LTB spacetime. Then we studied Szekeres-Szafron spacetime with no symmetries, particularly its subfamily with non-zero beta,z, and we derived the equation of the horizon. However, because of the lack of symmetries the space-time is not adapted to double-null foliation, therefore our attempts to estimate the equation´s solution were unsuccessful.

October 27, 2015
Cosmology beyond the linear and Newtonian approximations
dr. Eleonora Villa
Institute of Cosmology and Gravitation, University of Portsmouth, UK

Roughly speaking, cosmologists study large, linear scales and small, non-linear scales in two different ways: relativistic perturbation theory around a homogeneous and isotropic background describes scales where the growth of structures is at an early stage. At smaller scales, well inside the Hubble horizon, the relativistic effects are supposed to be completely negligible and General Relativity is replaced by Newtonian gravity. A post-Newtonian type approximation is a crucial improvement of this simple paradigm as it bridges the gap between relativistic perturbation theory and Newtonian structure formation. I focus on the relativistic corrections corrections for both the Eulerian and the Lagrangian approaches to gravitational dynamics in standard perturbation theory and in the post-Newtonian approximation. I finally present the post-Newtonian extension of the Zel'dovich solution for the plane parallel dynamics and its application in the context of the cosmological back-reaction proposal.

November 3, 2015
Thermodynamics of the gravitational field
dr. Giovanni Acquaviva
Department of Mathematical Sciences, University of Zululand, Kwa-Dlangezwa, South Africa

In the task of describing the history of our Universe - considered as a solution of the field equations of General Relativity - we analyse the issue of including the spacetime geometry as a thermodynamic actor along with the matter degrees of freedom. We show how it is possible to associate thermodynamic features to the free gravitational field in a purely geometrical way, following a proposal conceptualized by Penrose and recently formalized by different authors. We apply the resulting framework in the context of structure formation under the effect of gravity: the chain of processes leading from small matter inhomogeneities to black holes is shown to satisfy a "second law" of non-decreasing entropy. An interesting link to the thermodynamics of horizons is also presented. Eventually we contemplate the possibility of using the second law as a guiding principle for selecting thermodynamically favourable gravitational configurations: in this regard, an application to the Cosmic Censorship of naked singularities is briefly discussed.

November 10, 2015
Disks as sources of the Tomimatsu-Sato spacetimes
Tomáš Ledvinka
ITP

We describe the properties of disk sources of the gravitational field matching the first three members of the Tomimatsu-Sato family of stationary axisymmetric, asymptotically flat vacuum spacetimes (among them is the well-known Kerr solution). We cut out the central region containing singularities and by a proper identification we get a disk-like distribution of stress-energy tensor. If the cut-out region is large enough, realistic models of disk material can be found, e.g. in the form of counter-rotating streams of particles. We discuss how energy conditions become violated and possibility to find realistic models of disk material is lost once the cut-out region becomes small enough and how it relates to the cosmic censorship conjecture. We show that this prevents the construction of 'realistic' disks surrounded by regions of closed time-like curves which appear in the the Tomimatsu-Sato spacetimes.

November 17, 2015
svátek
(seminar in Czech language)
November 24, 2015
Spacetimes of Weyl and Ricci type N in higher dimensions
Martin Kuchynka
Matematický ústav, MFF UK
(seminar in Czech language)

We study geometrical properties of null congruences generated by an aligned null direction of the Weyl tensor (WAND) in spacetimes of the Weyl and Ricci type N in an arbitrary dimension. We prove that non-aligned Ricci type N spacetimes of the genuine Weyl type III or N do not exist and for aligned Weyl type III and N, Ricci type N spacetimes, the multiple WAND is geodetic. For aligned Weyl and Ricci type N spacetimes, the canonical form of the optical matrix in the twisting and non-twisting case is derived and the dependence of the Weyl tensor and the Ricci tensor on an affine parameter of the geodetic null congruences generated by the WAND is obtained. Finally, examples of higher-dimensional Weyl and Ricci type N spacetimes with various possible geometries of their multiple WANDs are constructed as direct products of four-dimensional pure radiational metrics with the Euclidean space.

December 1, 2015
Gamma Ray Bursts. The death throes of massive stars
Prof. Agnieszka Janiuk
Center for Theoretical Physics Polish Academy of Sciences, Warsaw
Gamma Ray Bursts are short, transient events, observed isotropically on the sky at a rate of up to a few times per day. The energetics of the bursts indicates that they must be associated with a cosmic explosion of an enormous power, when a new black hole is born and accretes material from its surrounding at a very high rate. This process leads to the formation of an ultra-relativistic jets moving with Lorentz factors up to ~ 200-1000. Observations show that long GRBs are associated with the collapse of massive stars, which are often accompanied by energetic supernovae. As the jet breaks out of the star and freely expands in the environment, dissipation of kinetic energy in internal shocks and/or at the jet photosphere produces the gamma-ray emission (the "prompt emission"). When the jet decelerates, it produces a multi-wavelength afterglow which can be observed in radio during several years. Hidden from direct observations is the "central engine" which is sustaining the production of relativistic jet. In this talk, the particular focus will be given to describe the GRB central engine that is powered by the collapse of a massive rotating star. The engine is a hot and dense accretion disk, formed around a rotating black hole, and composed of very dense matter, where the free nucleons, electron-positron pairs, and helium are present, and cooling is due to neutrino emission. A significant number density of neutrons in the disk provides conditions for neutron rich plasma in the GRB outflows or jets. Also, the heavy nuclei are formed in the outer disk as well as in the outflowing material. I will discuss the MHD and HD models of the GRB engine and the results of nuclear reaction network computations. I will also describe a speculative scenario for the longest duration GRBs, resulting from the collapse of a massive rotating star in a close binary system with a companion black hole. After the infall and spiral-in toward the primary, the two BHs merge inside the circumbinary disk. The second episode of mass accretion and high final spin of the postmerger BH prolongs the GRB central engine activity. The observed events should have two distinct peaks in the electromagnetic signal, separated by the gravitational wave emission. The gravitational recoil of the burst engine is also possible.
December 1, 2015
!!! Mimořádně v 15:30 !!!
Many faces of constraints
Prof. István Rácz
Wigner Research Center for Physics, Budapest
December 8, 2015
Meissner effect for astrophysically relevant black holes
Martin Scholtz
ITP
(seminar in Czech language)

Extremal black holes tend to expel magnetic field lines from the horizon. Since the temperature of extremal black hole vanishes, this effect is analogous to the well-known Meissner effect for superconductors. The presence of the black hole Meissner effect has been demonstrated in many particular situations, especially (but not only) in the cases when the electromagnetic field is treated as a test field. In this talk, we present an exact result that EM field is expelled from the horizon of arbitrary stationary, axially symmetric black hole, provided that the field shares the symmetries of the spacetime.

December 15, 2015
Summary of the anniversary Berlin conference "A Century of GR"
Jiří Podolský, Pavel Krtouš
ITP
(seminar in Czech language)
January 12, 2016
Asymptotická bezpečnost jako cesta ke kvantové teorii gravitace
dr. Martin Zdráhal
Ústav částicové a jaderné fyziky, MFF UK
(seminar in Czech language)


Other semesters:


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