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
It is well known that (static) regular black hole spacetimes can be sourced by appropriately chosen theories of non-linear electrodynamics. More recently, it was shown that many such models can also be obtained as solutions of vacuum gravity equations, upon considering an infinite series of quasi-topological higher-curvature corrections. After reviewing both these approaches, I will show that the latter construction can be upgraded to yield regular black holes with vanishing inner horizon surface gravity -- a necessary condition for the absence of classical instabilities associated with mass inflation on the inner horizon. I will also briefly discuss what happens when non-linear electrodynamics is combined with quasitopological gravity.
The equivalence principle played a pivotal role in the development of general relativity. Nowadays, it represents a useful tool for understanding and classifying various candidate theories of gravity. I first review the different formulations of the equivalence principle and the constraints they impose on the kinematics and dynamics of gravity. I then show that the equivalence principle for gravitational test physics singles out two theories – general relativity and Weyl transverse gravity.
The study investigates orbital motion of test particles near compact objects described by solutions involving massless scalar fields, electromagnetic fields, and nonlinear electrodynamics. Specifically, we analyze orbital dynamics in the Janis-Newman-Winicour, Janis-Newman-Winicour-Maxwell, Schwarzschild-Melvine, and Bonnor-Melvin spacetimes, comparing the results with those obtained for the Schwarzschild and Reissner-Nordström solutions. We examine the stability of circular orbits and the behavior of epicyclic frequencies under varying physical parameters. Our analysis shows that in certain cases the central object transitions into a naked singularity. Deviations from classical Schwarzschild and Reissner-Nordström solutions reveal conditions for the existence of multiple photon orbits or marginally stable orbits. In some instances, the geometry allows the presence of two photon orbits—one stable and one unstable—with an interesting connection to the region of stable orbits. We find that at lower intensities, the effects of the scalar field and electromagnetic fields are comparable and seemingly interchangeable. However, for a sufficiently strong scalar field, its influence becomes dominant, leading to the emergence of a distinct region of stable orbits near the naked singularity. These effects are illustrated within the framework of optical geometry using embedding diagrams. Based on arXiv:2501.13538.
The last few years in high-energy astrophysics were enriched by the advent of X-ray polarimetry, mostly thanks to the success of the IXPE mission (NASA/ASI, 2021) operating in 2-8 keV. New observational properties of tens of X-ray sources were deduced with IXPE, in simultaneous observations with other instruments, which put our knowledge of (not only) black-hole accretion significant steps further. In this talk, a brief overview of the IXPE results on accreting black holes will be given, together with examples of theoretical X-ray spectro-polarimetric models that are being developed at our institute as an illustration of the computation of the observable properties and of the intricate data interpretation. With these models, combined with the cutting-edge precision of IXPE gas pixel detectors, it is possible to constrain the black-hole spin in the Kerr spacetime assumption, the coronal geometry, the inclination of the accretion disc, the structure of the disc, and to derive geometrical properties of regions residing on parsec-scales away from the central black hole.
In this talk I will present a new family of Kerr-AdS like black holes in spacetimes of even dimensions. These have non-compact horizons which are negatively curved at large distances, and are obtained from the Myers-Perry family by "analytic continuation". I will discuss the solution and some global properties of the resulting black hole.
Geodesic motion in Schwarzschild spacetime is completely integrable, yet this property is typically broken if an additional source is present, such as a ring or a disc. We study how such a perturbation alters the geodesic dynamics, using standard tools (e.g. Poincaré maps or Lyapunov-type coefficients). More discussion will be focused on applicability of local, curvature-based criteria of predicting chaos, which may offer an additional insight into the global dynamics.
It is well-known that gravitational fields from an isolated system depend on the time varying quadrupole moment of the source. Quadrupole formula provides an estimate of the energy loss due to gravitational radiation. The first proposed quadrupole formula for gravitational waves in de Sitter was derived by Ashtekar, Bonga, Kesavan (ABK). We point out that a consistent quadrupolar truncation is needed to upgrade the ABK formula. We also compare our result to a recently obtained result of Bonga, Bunster, Perez. We write the quadrupole formula in two distinct ways which allow standard flat limit and negative definite energy flux in de Sitter.
Oldřich Semerák