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
Semiclassical gravity has the potential to address several interesting problems in foundational physics, such as the evaporation of black holes, but its defining equations are notoriously difficult to solve. In this talk, I will introduce braneworld holography which can solve the semiclassical problem exactly in certain three-dimensional settings. I will discuss how four-dimensional C-metrics allow one to obtain three-dimensional quantum black holes incorporating all orders of semiclassical backreaction. Time permitting, I will illustrate how this framework can be used to test ideas about cosmic censorship in semiclassical gravity.
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.
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.
Superradiance provides a mechanism whereby hypothetical particles would form clouds around astrophysical black holes, emitting gravitational waves as they dissipate. Searches for these gravitational waves allow us to constrain the parameter space of new particles. Here I will talk about our gravitational waveform model (the main references are https://arxiv.org/abs/2410.21442, https://arxiv.org/abs/2211.03845). This new and more accurate waveform model may allow us to place tighter constraints on the existence of ultralight bosons using current detectors.
In this talk I analyze the construction of quantum field theory in multiply-connected spacetimes (i.e. spacetimes that contain regions with closed timelike curves (CTCs)). Additionally, I explore how particle detectors can discern the presence of CTCs in causally disconnected regions and differentiate between various spacetime topologies.
In this talk, I will show that the general EFT extension of GR can be constrained with two fundamental properties of GR: non-birefringent gravitational waves and isospectral black hole quasinormal modes (QNMs). This leads to a novel class of EFTs which exhibit intriguing features that align with string theory predictions. I will then discuss how to compute the QNMs through a geometric correspondence between eikonal QNMs and unstable gravitational wave orbits, which I will apply to equatorial orbits. Beyond the equatorial plane, we solve the full wave equation using an effective scalar description of gravitational perturbations. These results provide the first computation of QNMs for Kerr black holes with arbitrary spin in an EFT extension of GR.
In this talk I shall discuss propagation of the high-frequency electromagnetic and gravitational waves in a curved spacetime. I shall describe a so-called spinoptics formalism which takes into account the interaction of the spin of the field with the curvature of the background metric. This is achieved by modifying the standard geometric optics equations by including the helicity sensitive terms of the order $1/\omega$ in the eikonal equation. I shall demonstrate that these spinoptics equations can be obtained from an effective action. This allows one to reduce the study of the propagation of the high-frequency waves to the study of classical dynamics of massless particles with internal discrete degrees of freedom (helicity). The formalism is covariant and it can be applied for arbitrary (vacuum) space-time background. I shall illustrate the application of the spinoptics by discussing circular light propagation near a stationary black hole, and explain how hidden symmetries of the Kerr metric make it possible to solve the spinoptics equations exactly. At the end of the talk, I shall briefly discuss possible applications of the developed formalist to astrophysical problems.
In this talk, I will show how an infinite tower of higher-curvature corrections to Einstein gravity gives rise to the formation of regular black holes in space-time dimensions larger than four. These theories consist of specific combinations of higher-curvature terms at each order, multiplied by free couplings. I will begin by introducing a class of higher-curvature gravities with second-order equations of motion on spherically symmetric backgrounds, which will play a central role in the mechanism leading to these regular black holes. I will then examine the static and spherically symmetric black holes of these theories, which become regular after the introduction of the infinite tower of corrections, and demonstrate that the spherical collapse of a thin shell indeed leads to the formation of such regular black holes.
Oldřich Semerák