Abstract The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders
about the fundamental theory of nature at play in the extreme gravity regime, where the …

Very compact objects probe extreme gravitational fields and may be the key to understand
outstanding puzzles in fundamental physics. These include the nature of dark matter, the …

The existence of black holes and spacetime singularities is a fundamental issue in science.
Despite this, observations supporting their existence are scarce, and their interpretation is …

General Relativity has shown an outstanding observational success in the scales where it
has been directly tested. However, modifications have been intensively explored in the …

While singularities are inevitable in the classical theory of general relativity, it is commonly
believed that they will not be present when quantum gravity effects are taken into account in …

We perform the first nonlinear and self-consistent study of the merger and ringdown of a
black hole mimicking object with stable light rings. To that end, we numerically solve the full …

Structure at the horizon scale of black holes would give rise to echoes of the gravitational
wave signal associated with the postmerger ringdown phase in binary coalescences. We …

Gravitational wave astronomy provides an unprecedented opportunity to test the nature of
black holes and search for exotic, compact alternatives. Recent studies have shown that …

KAGRA is a newly build gravitational wave observatory, a laser interferometer with 3 km arm
length, located in Kamioka, Gifu, Japan. In this paper, one of a series of articles featuring …

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 …