Relativity Seminar
of the Department of Theoretical Physics

Seminar is held on Tuesdays at 13:10 pm in the lecture room of the Institute
on the 10th floor of the department building at Trója, V Holešovičkách 2, Prague 8

Next seminars:

April 23, 2019
!!! In the T1 lecture room !!!
Black hole in M87: from geometry to data processing
prof. Vladimír Karas, dr. Michal Bursa, dr. Miroslav Bárta, dr. Tomáš Ledvinka, dr. Anabella Araudo, et al.
Astronomical Institute of the Czech Academy od Sciences; Institute of Theoretical Physics of the MFF UK; ELI Beamlines, Institute of Physics of the Czech Academy of Sciences

Exactly 7x7 days before the centennial celebration of the Eddington-Dyson light-bending measurement, the Event Horizon Telescope Collaboration published the image resolving, for the first time, a photon sphere around a black hole. It is a supermassive black hole in the nucleus of the M87 galaxy. This galaxy is active and very interesting, and the measurement is a technological breakthrough. We will try to cover both these aspects in this discussion. In the meantime, light will continue orbiting that black hole -- just once in 7 days.

April 30, 2019
Pitfalls and loopholes of (collisional) Penrose process
Mgr. Filip Hejda
Center for Astrophysics and Gravitation, Instituto Superior Técnico, University of Lisbon
May 7, 2019
Symmetries and conservation laws for black hole backgrounds
Dr. Steffen Aksteiner, Prof. Lars Andersson
Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Potsdam/Golm
May 14, 2019
No seminar (Prof. Kip Thorne's visit in Prague)
(seminar in Czech language)
May 21, 2019
Almost universal spacetimes in higher-order gravity theories
Mgr. Martin Kuchynka
Department of Algebra, Geometry and Mathematical Physics, Institute of Mathematics, Czech Academy of Sciences

We present the so-called almost universal spacetimes. Key feature of this class of metrics is that the field equations of any generalized theory of gravity with Lagrangian constructed from the metric, the Riemann tensor and its covariant derivatives of arbitrary order reduce to only one algebraic and one differential equation (in any dimension). We prove that all Kundt spacetimes of Weyl type III and traceless Ricci type N or more special are almost universal. Explicit examples of Weyl type II almost universal metrics are also given. The considerable simplification of the field equations of higher-order gravities for almost universal spacetimes is then employed to obtain new Weyl type II, III, and N vacuum solutions to quadratic gravity and 6D conformal gravity. Necessary conditions for almost universal metrics are also studied.

Previous seminars:

February 26, 2019
Gravitational lensing in a plasma
PD Dr. Volker Perlick
ZARM, University of Bremen

I consider light propagation in a plasma on a general relativistic spacetime. In the first part of my talk I review a Hamiltonian formalism for light rays in a pressure-less non-magnetised plasma and I present, on the basis of this formalism, the resulting equations for the deformation of light bundles (Sachs equations). In the second part I discuss the shadows of black holes in the presence of a plasma. The latter is of relevance in view of the forthcoming observations by the Event Horizon Telescope.

March 5, 2019
Quantum field theory on simplicial lattice
Mgr. Jakub Káninský

An existing canonical formulation of dynamics for a discrete system will be briefly reviewed and used to define a version of quantum field theory on a fixed simplicial lattice. Such a model may serve as an intermediate step for incorporating matter or gauge fields into lattice-based quantum gravity theories. For start, we shall consider Euclidean lattice and real scalar field. We will discuss the relation to the algebraic approach used in quantum field theory in curved spacetime and to path integral.

March 12, 2019
Strong Cosmic Censorship in charged black-hole spacetimes
Mr. Kyriakos Destounis
Multidisciplinary Centre for Astrophysics, Technical University of Lisbon

The fate of Cauchy horizons, such as those found inside charged black holes, is intrinsically connected to the decay of small perturbations exterior to the event horizon. As such, the validity of the Strong Cosmic Censorship conjecture is tied to how effectively the exterior damps fluctuations. By studying scalar and fermionic fields in the exterior of Reissner-Nordstrom-de Sitter black holes we identify three families of modes: one directly linked to the photon sphere, another family whose existence and timescale is closely related to the de Sitter horizon and, finally, a third family which dominates for near-extremally-charged black holes. We give a detailed description of scalar and fermionic perturbations of such black holes, and conjecture that Strong Cosmic Censorship is violated in the near extremal regime.

March 19, 2019
Spacetime and dark matter from spontaneous breaking of Lorentz symmetry
Dr. Federico R. Urban
Central European Institute for Cosmology and Fundamental Physics, Institute of Physics, Czech Academy of Sciences

I will review the recent idea that, within the formalism of Cartan geometry, a spontaneously-broken gauge theory of the Lorentz group contains Ashtekar's chiral formulation of general relativity accompanied by dust, which could play the role of dark matter. The model is "pre-geometric" in the sense that the spacetime metric may be constructed only in the symmetry-broken regime; however, in principle the two phases can be smoothly connected, and spacetime be realised dynamically as a symmetry-breaking process.

March 26, 2019
Static, spherically symmetric vacuum solutions to quadratic gravity
Dr. Alena Pravdová, Dr. Vojtěch Pravda
Department of Algebra, Geometry and Mathematical Physics, Institute of Mathematics, Czech Academy of Sciences

We present several classes of static, spherically symmetric vacuum solutions to quadratic gravity. Notably, besides the Schwarzschild black hole, these solutions also contain a static, spherically symmetric black hole with non-trivial Ricci tensor.

April 2, 2019
Systematic tests of dark matter and modified gravity
Dr. Ivan de Martino
Donostia International Physics Center, Donostia-San Sebastian, Spain

The standard cosmological model has been constrained with unprecedented accuracy. Nevertheless, we are facing off new challenges. The lack of detection of Dark Matter and Dark Energy have opened to new paths. On one side, we are entering the "no-WIMP" era. On the other side, explaining the accelerated expansion of the Universe may require an extension of General Relativity. I will review the state of art of the standard cosmology while introducing new tests of both Dark Matter and Modified Gravity. I will introduce a relatively new paradigm for Dark Matter, named Ultra-light axions, and explain how to probe it with the current and forthcoming dataset. Then, I will also show some tests of the standard cosmological model and of modified gravity that near in future may be helpful to constrain/rule out models.

April 9, 2019
Gravitational multipole moments from Noether charges
Dr. Roberto Oliveri
Central European Institute for Cosmology and Fundamental Physics, Institute of Physics, Czech Academy of Sciences

We define multipole moments for an arbitrary theory of gravity in terms of canonical Noether charges associated with specific residual transformations in canonical harmonic gauge, which we call multipole symmetries. We show that our definition exactly matches Thorne's mass and current multipole moments in Einstein gravity. For radiative configurations, the total multipole charges -- including the contributions from the source and the radiation -- are given by surface charges at spatial infinity. The conservation of total multipole charges is used to derive the variation of source multipole moments in terms of the radiative multipole fluxes.

April 16, 2019
No seminar
No name
No affiliation
(seminar in Czech language)

Previous semesters:

Jiří Bičák                                                                                                  Oldřich Semerák

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