Perturbations of stars and black holes have been one of the main topics of relativistic
astrophysics for the last few decades. They are of particular importance today, because of …

Gravitational waves emitted by perturbed black holes or relativistic stars are dominated
byquasinormal ringing', damped oscillations at single frequencies which are characteristic of …

Black hole spectroscopy is the program to measure the complex gravitational wave
frequencies of merger remnants, and to quantify their agreement with the characteristic …

Newly formed black holes are expected to emit characteristic radiation in the form of
quasinormal modes, called ringdown waves, with discrete frequencies. LISA should be able …

We investigate the instability of the massive scalar field in the vicinity of a rotating black hole.
The instability arises from amplification caused by the classical superradiance effect. The …

Black hole spectroscopy with gravitational waves is an important tool to measure the mass
and spin of astrophysical black holes and to test their Kerr nature. Next-generation ground …

Scalarization is a very interesting phenomenon allowing a compact object to be endowed
with scalar hair while leaving all the predictions in the weak field limit unaltered. In Gauss …

The intricacies of black hole ringdown analysis are amplified by the absence of a complete
set of orthogonal basis functions for quasinormal modes. Although damped sinusoids …

The purpose of this chapter is to provide an overview of the exciting field of black hole quasi-
normal modes and its capabilities to test general relativity in the 21st century. After …

Inspirals of stellar-mass compact objects into massive black holes are an important source
for future gravitational wave detectors such as Advanced LIGO and LISA. The detection and …