Dynamics around black holes: Radiation Emission and Tidal Effects

TL;DR

This thesis investigates black hole dynamics, focusing on radiation emission, superradiance, and tidal effects in various dimensions, revealing universal behaviors and energy transfer mechanisms that depend on dimensionality.

Contribution

It provides new insights into scalar radiation, superradiance, and tidal phenomena around black holes, especially in higher dimensions, with novel conjectures and interpretations.

Findings

Massive scalar perturbations exhibit a universal oscillatory tail.

Superradiance can be characterized by ergoregion volume in higher dimensions.

Tidal acceleration and floating orbits are prevalent in higher-dimensional black hole systems.

Abstract

In this thesis we study several dynamical processes involving black holes in four and higher dimensions. First, using perturbative techniques, we compare the massless and massive scalar radiation emitted by a particle radially infalling into a Schwarzchild black hole. We show that the late-time waveform of massive scalar perturbations is dominated by a universal oscillatory decaying tail, which appears due to curvature effects. We also show that the energy spectrum is in perfect agreement with a ZFL calculation once no-hair properties of black holes are taken into account. In the second part, we study the phenomenon of superradiance in higher dimensions and conjecture that the maximum energy extracted from a rotating black hole can be understood in terms of the ergoregion proper volume. We then study some consequences of superradiance in the dynamics of moons orbiting around higher-dimensional rotating black holes. In four-dimensional spacetime, moons around black holes generate low-amplitude tides, and the energy extracted from the hole's rotation is always smaller than the gravitational radiation lost to infinity. We show that in dimensions larger than five the energy extracted from the black hole through superradiance is larger than the energy carried out to infinity. Our results lend strong support to the conjecture that tidal acceleration is the rule, rather than the exception, in higher dimensions. Superradiance dominates the energy budget and moons "outspiral"; for some particular orbital frequency, the energy extracted at the horizon equals the energy emitted to infinity and "floating orbits" generically occur. We give an interpretation of this phenomenon in terms of the membrane paradigm and of tidal acceleration due to energy dissipation across the horizon.