Ústavu teoretické fyziky

Seminář se koná v úterý ve 13:10 v posluchárně ÚTF MFF UK

v 10. patře katedrové budovy v Tróji, V Holešovičkách 2, Praha 8

- 15. října 2024
- ZAČÁTEK JIŽ V 13:00
- Classical Double Copy: Solutions, Horizons, and Penrose Limits
*Prof. Cynthia Keeler*- University of Arizona
- (seminář v angličtině)
We will review the classical double copy, which maps exact solutions of classical gauge theories like electromagnetism, to solutions of general relativity. We will cover both the Kerr-Schild and Weyl formulations of this map. Following a survey of known exact solutions, we will review why a position-space classical solution double copy is even feasible. We will briefly discuss some perturbative approaches, and then close by relating several gravitational objects (including horizons and Penrose limits) to their gauge theory analogues.

- 15. října 2024
- OD 14:00
- Wavization, Gaussian beams and caustic avoidance
*Dr. Nezihe Uzun*- Center for Theoretical Physics, PAS, Warsaw
- (seminář v angličtině)
Geometric optics limit is known to be a good-enough approximation for the calculation of distances and image distortions in the Universe. In cosmology, we usually assume a point source that emits spherical waves and only a small section of the wavefront is accessible to the observer. In the geometric optics limit, this section is represented by a thin bundle of rays. Accordingly, the intensity profile on the transverse, observational screen is homogeneous. In this talk, I will outline the analogies between the paraxial ray optics of the Newtonian theory and the thin bundles in general relativity. I then propose a method adopted from the paraxial wave optics in order to study the wave-like effects of light propagation in general relativity. This method is sometimes referred to as ``wavization'' due to its resemblance to the semi-classical quantization techniques. The idea is to use certain phase space methods and symplectic symmetries to superpose two bundles initiated from a small yet finite size source. With this, we explore the possibility of obtaining inhomogeneous intensity profiles on the transverse plane associated with the fundamental Gaussian mode. The Gaussianity of the intensity profiles of such light beams is sourced by the spacetime curvature and not necessarily by random processes. We observe that their form is preserved throughout the propagation in curved spacetime on account of the symplectic symmetries of the underlying phase space. Finally, we show that the caustics can be avoided with this method. In addition, the extended source effect can be studied on account of Gaussian beam decomposition techniques.

- 22. října 2024
- Symmetry reduction of gravitational Lagrangians: A complete cookbook
*Dr. Ivan Kolář*- ITP, Charles University
- (seminář v angličtině)
The reduction of gravitational Lagrangians by a specified symmetry group can sometimes result in field equations that are fully equivalent to the reduced ones—if this holds for all Lagrangians, it is known as the principle of symmetric criticality (PSC), which is a property of the infinitesimal group action only. In this talk, I will explain the rigorous process of symmetry reduction of Lagrangians and present all infinitesimal group actions that satisfy PSC. I will identify the corresponding invariant metrics (and l-chains), discuss their interrelations, and analyze the simplifications allowed by the residual diffeomorphism group and Noether identities, both before and after the variation of the reduced Lagrangian.

- 29. října 2024
- ZAČÁTEK JIŽ V 13:00
- Gravitational radiation at infinity I: the case with vanishing cosmological constant
*Dr. Francisco Fernández-Álvarez*- Department of Physics, University of País Vasco, Bilbao
- (seminář v angličtině)
This is the first of two seminars on gravitational radiation at conformal infinity. In this one, the case with vanishing cosmological constant is addressed. After giving some brief historical context, the framework and tools are presented, namely: conformal completions à la Penrose and the Bel-Robinson tensor. The asymptotic version of the latter is used to characterise gravitational radiation in a new covariant, geometric and invariant way. This result is compared with the traditional condition based on the news tensor and showed to be equivalent to it. The advantages of the new criterium are stressed: in particular, its tidal nature allows one to apply it when the cosmological constant is positive - this matter is treated in the second talk of this series.

- 29. října 2024
- OD 14:00
- Gravitational radiation at infinity II: the case with positive cosmological constant
*Prof. José M.M. Senovilla*- Department of Physics, University of País Vasco, Bilbao
- (seminář v angličtině)
In this second presentation the case with a positive cosmological constant will be presented and the main differences with the case without cosmological constant will be enumerated. The entire idea is based on a couple of vector fields, defined at infinity, that describe the energy-momentum properties of the tidal gravitational field: the asymptotic super-momentum and the asymptotic super-Poynting vectors. They will be shown to lead to a criterion on the existence/absence of gravitational radiation at infinity. The necessity of going to the tidal level will be explained. The problem of incoming radiation will be analysed under the viewpoint of the so-called asymptotic radiant super-momenta. Application to specific spacetimes -and in particular to the general Einstein-Maxwell solution with type-D black holes- will be given showing the validity of the criterion. Time permitting, some discussion on conserved charges and symmetries will also be presented.

- 5. listopadu 2024
- TBA
*Dr. Alex Vikman*- CEICO, Czech Academy of Sciences
- (seminář v angličtině)

*Jiří Bičák*
*Oldřich Semerák*