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
Spontaneous scalarization is a mechanism that allows black holes to develop a non-trivial profile of a scalar field “scalar hair” because of tachyonic instabilities, enabling tests of gravity beyond General Relativity. Motivated by stability and threshold issues in Gauss-Bonnet scalarization, we propose a new model characterized by two nonminimal couplings of the scalar field to both Gauss-Bonnet curvature and a U(1) gauge field (e.g. electromagnetic field). The presence of two distinct sources of tachyonic instability broadens the conditions for spontaneous scalarization. We track how the electric charge and the coupling constants govern the onset of the scalar field and derive new solution branches with nontrivial scalar profiles. Numerical integration shows multiple coexisting scalarized black hole solutions with adjustable thresholds, influenced by the relative strengths of curvature and matter couplings. We examine their scalar charge, horizon properties, and thermodynamic characteristics, demonstrating how the model can selectively activate or suppress scalarization phenomena. The matter source term modifies the scalarization onset and promotes stable solutions, as indicated by the evolution of the scalar charge and horizon quantities. These findings suggest an alternative approach to scalarization, may avoid the instabilities of curvature-only or matter-only models, and offer new ways to test strong-gravity effects in upcoming observations.
Extreme Mass Ratio Inspirals (EMRIs) are among the key gravitational-wave sources expected to be observed by the Laser Interferometer Space Antenna (LISA). Accurately modeling their waveforms requires accounting for various astrophysical effects, including the influence of surrounding matter, since black holes are rarely isolated. The presence of matter can break spacetime symmetries, making standard techniques inapplicable. However, by treating both the inspiraling body and the surrounding matter as independent perturbations, we can formulate the problem using a modified Teukolsky equation. In this presentation, we demonstrate how this equation can be solved in the spacetime of a Schwarzschild black hole surrounded by a ring, approximated by its leading multipoles. This calculation represents a first step toward the relativistic modeling of EMRIs in general spacetime backgrounds.
Due to spectacular advances in observational astronomy, strong evidence for the existence of dark massive compact objects has accumulated over the last few decades, thus gradually shifting our perception of black holes from purely mathematical entities to potentially real physical objects. However, event horizons are not physically observable, and therefore the question of whether the observed astrophysical black holes candidates are genuine black holes is still open. To alleviate the pathologies associated with event horizons, I will present dynamical black hole solutions built from the assumption that a regular quasilocal (e.g., apparent or trapping) horizon forms in finite asymptotic time (i.e., according to the clock of a distant observer). Taking this as the only requirement within the semiclassical framework, the spherically symmetric Einstein field equations admit only two distinct families of real-valued dynamical solutions, namely evaporating black holes and accreting white holes. Both of them violate the null energy condition near the outer horizon. I will derive their properties and present the physical implications.
David Kubizňák Oldřich Semerák