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:

July 2, 2019
Thermodynamics of Lorentzian Taub-NUT spacetimes
Dr. David Kubizňák
Perimeter Institute for Theoretical Physics, Waterloo, Ontario, Canada

I will show how to formulate consistent thermodynamics of the Lorentzian Taub-NUT spacetimes, maintaining (as recently shown relatively harmless) Misner strings. The obtained first law is of full cohomogeneity and allows for asymmetric distributions of Misner strings as well as their potential variable strengths -- encoded in the gravitational "Misner charges". Notably, the angular momentum is no longer given by the Noether charge over the sphere at infinity and picks up non-trivial contributions from Misner strings.

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)
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
Penrose process remains a textbook example of how to extract energy from a black hole, although its practical viability was contested soon after its discovery. Collisional Penrose process is a more general alternative, which attracted a lot of attention in the past decade especially due to various possibilities of highly energetic collisions under special conditions. However, the practical relevance of these hypothetical phenomena also faces many doubts. We would like to extend the understanding of whether the high collision energy can also lead to increased extraction of energy from the black hole. In particular, we are interested in different variants of the so-called Bañados-Silk-West (BSW) effect. In its original version, it requires corotating fine-tuned particles moving close to an extremally rotating black hole. Although the efficiency of collisional Penrose process can be increased via such an effect, it was found to be capped by unconditional bounds. On the other hand, for the electrostatic version of BSW effect, which is possible for radially moving charged particles close to extremally charged black holes, no such bounds on extracted energy or efficiency were found. We recently demonstrated that these two variants can be unified as subcases of a more general effect [Phys. Rev. D 95, 084055 (2017)], which makes the discrepancy even more puzzling. To sketch out the resolution of this conundrum is the main purpose of the present talk.
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
(seminar in Czech language)
Prague lectures of Prof. Thorne:
May 15, 16:00: Creating Gravitational-Wave Astronomy, Blue Hall of the Rectorate, Celetná 20 (registration)
May 16, 14:00: Geometrodynamics: The Nonlinear Dynamics of Curved Spacetime, M1 hall at Ke Karlovu 3
May 17, 17:00: The Warped Side of the Universe: from the big bang to black holes and gravitational waves, Faculty of Law, nám. Curieových 901/7
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.

May 28, 2019
Gravitational radiation and evolution of gravitational collapse in cylindrical symmetry
Dr. Alfonso García-Parrado Gómez-Lobo

Using the Sparling form and a geometric construction adapted to spacetimes with a 2-dimensional isometry group, we analyse a quasi-local measure of gravitational energy. We then study the gravitational radiation through spacetime junctions in cylindrically symmetric models of gravitational collapse to singularities. The models result from the matching of collapsing dust fluids interiors with gravitational wave exteriors, given by the Einstein-Rosen type solutions. For a given choice of a frame adapted to the symmetry of the matching hypersurface, we are able to compute the total gravitational energy radiated during the collapse and state whether the gravitational radiation is incoming or outgoing, in each case. This also enables us to distinguish whether a gravitational collapse is being enhanced by the gravitational radiation.

Work done in collaboration with Dr. Filipe Mena (University of Lisbon).

May 31, 2019
!!! ON FRIDAY FROM 10:40 !!!
What is the surface of a black hole?
Prof. José M.M. Senovilla
Theoretical Physics and Science History Department, University of País Vasco, Bilbao

Black holes are fundamental objects in equilibrium predicted by General Relativity. However, in reality, black holes form, evolve and eventually evaporate, thus they are dynamical. Do they have a boundary? If so, where it is? For dynamical black holes, the usual Event Horizon is global and teleological, thus not well defined. One can then resort to using the local concept of closed trapped surface to try and define the surface of black holes, leading to the concepts of dynamical and trapping horizons. We will show the fundamental problems inherent to dynamical or trapping horizons. The trapped region and its boundary will then be introduced, and the difficulties in finding them highlighted. Finally, the concept of core of a black hole will be briefly discussed.

Previous semesters:

Jiří Bičák                                                                                                  Oldřich Semerák

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