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
I will show a complete classification of D-dimensional Kundt spacetimes of Weyl type N and traceless Ricci type N in (anti-)de Sitter. Using a coordinate system with almost-Killing normalization, I will provide invariant subfamilies (generalzed Siklos, pp-, and Kundt waves) and present their canonical forms.
TBA
In this talk, I will present new rotating black hole solutions of the low-energy effective action of string theory in three and four dimensions. These geometries are asymptotically flat with a linear dilaton vacuum and exhibit unusual thermodynamic properties, including a mass-independent Hawking temperature. I will also discuss charged extensions, which lead to closed time-like curves inside the inner horizon. Finally, I will show that these solutions arise from the large-d limit of singly rotating Myers–Perry black holes.
TBA
A series of works by Belinskii, Khalatnikov and Lifshitz (BKL) conjectured that spacetime near a generic spacelike singularity locally behaves in a chaotic way. Since BKL conjecture concerns very strong gravity regimes, it is natural to ask how quantum gravity influences it. We tackle this question by looking at perturbative quantum corrections to Bianchi I and II metrics approximating the relevant features of BKL conjecture. We argue that even such perturbative effects can completely change the nature of BKL dynamics. While we work in the framework of thermodynamic gravity, our findings also apply to the effective dynamics of loop quantum cosmology.
I will describe a new construction of Ricci flat metrics using gauged linear sigma models satisfying the Calabi-Yau condition. The approach will be introductory using elementary examples to illustrate the main points and to underline the connection between physical and mathematical concepts.
The Vaidya spacetime is a spherically symmetric solution of the Einstein equations with a null dust source. This can be used to model the gravitational collapse of a thick shell of radiation: a flat interior region is matched at an inner boundary to the null dust filled region, which is then matched at an outer boundary to Schwazschild spacetime. A central singularity inevitably forms, and depending on the profile of the energy density of the null dust, this singularity can be globally naked. Motivated by the cosmic censorship hypothesis, we consider perturbations of this configuration. We review previous work, and describe recent work where the perturbation of the inner boundary - the past null cone of the central singularity - is analysed using a framework for studying perturbations of general hypersurfaces. This sets boundary conditions for perturbations at the past null cone, and we then consider the evolutionary problem, focussing on the question of the stability of the Cauchy horizon of the naked singularity.
This talk is based on our recent paper e-Print: 2511.04650. Scalar fields with a global U(1) symmetry often appear in cosmology and astrophysics. We study the spherically-symmetric, stationary accretion of such a classical field onto a Schwarzschild black hole in the test-field approximation. Thus, we consider the relativistic Bondi accretion beyond a simplified perfect-fluid setup. We focus on the complex scalar field with canonical kinetic term and with a generic quartic potential which either preserves the U(1) symmetry or exhibits spontaneous symmetry breaking. It is well known that in the lowest order in gradient expansion the dynamics of such a scalar field is well approximated by a perfect (super)fluid; we demonstrate that going beyond this approximation systematically reduces the accretion rate with respect to the perfect fluid case. Hence, black holes can provide a way to distinguish a perfect fluid from its ultraviolet completion in the form of the complex scalar field.
Taub–NUT spacetimes are related by double Wick rotations to the near-horizon extreme Kerr geometry and to the swirling universe. Similarly, the planar Reissner-Nordström solution can be mapped to the Melvin spacetime. We show that this is not a peculiarity of individual solutions in general relativity. Rather, double Wick rotations establish mappings between entire classes of spacetime symmetries, independently of the underlying geometric theory of gravity. This provides a systematic way of generating new solutions in a given theory from known ones by exploiting the correspondence between these symmetry classes.
Oldřich Semerák