of the Institute of Theoretical Physics

- 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*- ITP
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 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
- 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*- ITP
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.

- 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.

- July 9, 2019
- from 13:10
- Lovelock-massive gravity: Thermodynamics
*Prof. Seyed Hossein Hendi*- Physics Department and Biruni Observatory, Shiraz University, Iran
After introducing the Lagrangian and exact solutions of Lovelock massive gravity, we focus on thermodynamic properties. We explain the generalized form of Smarr relation and then, discuss the role of massive/Lovelock gravity on the phase transition.

- July 9, 2019
- from 14:20
- Homogeneous AdS black strings in GR and Lovelock gravity
*Dr. Adolfo René Cisterna Roa*- Faculty of Physical and Mathematical Sciences, University of Concepción, Chile
A new method for the construction of homogenous black strings is shown. The method, which is based on a particular scalar-dressing of the extra dimensions of the spacetime under consideration, allow us to construct the black string generalization of the AdS Schwarzschild black hole in any dimension in General Relativity. Furthermore the method can be generalized to provide the black string extension of the Boulware-Deser black hole, or the black string extension of any black hole contained in the Lovelock theory. It will be also discussed how to construct black string with non-trivial matter fields.

- July 9, 2019
- from 15:30
- Dark matter: reality or a relativistic illusion?
*Prof. Asher Yahalom*- Department of Electrical and Electronic Engineering, Ariel University, Israel
We show that "dark matter" and "MOND" effects are explained in the framework of standard GR as effects due to retardation without assuming any exotic matter or modifications of the theory of gravity.

- July 15, 2019
- !!! MONDAY from 13:10 !!!
- On classical & quantum cosmology and on relativity research in Kolkata
*Prof. Narayan Banerjee*- Indian Institute of Science Education and Research, Kolkata, India
- July 23, 2019
- !!! From 15:00 !!!
- Seminar/Discussion
*Prof. Charles Hellaby*- Department of Mathematics and Applied Mathematics, University of Cape Town
- August 9, 2019
- !!! FRIDAY from 13:10 !!!
- On compactness limit of a charged object in general relativity
*Prof. Naresh Dadhich*- Inter-University Center for Astronomy & Astrophysics, Pune, India
A very general and novel prescription for compactness of a static object/star would be proposed. The compactness limit is defined when gravitational field energy exterior to object is less than or equal to half of its non-gravitational energy for a charged object described by the unique Reissner-Nordström electrovac solution of the Einstein equation. This definition makes no reference to the interior at all, may what that be. (arxiv:1903.03436)

- August 19, 2019
- !!! skype talk, Monday at 13:00 !!!
- Characterizing black holes with curvature invariants
*Dr. David McNutt*- Department of Mathematics and Physics, University of Stavanger, Norway
Black holes serve an important role in gravitational physics. While early solutions to Einstein's field equations provided models of black holes in equilibrium, black holes in our Universe are dynamical: they form at some point in time, interact with their environment, and ultimately evaporate. Due to the physical properties associated to each stage of a black hole's history it is possible to broadly distinguish them. From a mathematical perspective, any solution can be uniquely characterized by its associated curvature invariants. In this talk I will show that curvature invariants can be related to physical properties of black holes. In particular, I will focus on the (quasi-)local boundaries of black holes and introduce a new hyper-surface in terms of the zero-set of curvature invariants called a geometric horizon.

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