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 …

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 …

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 …

A generic feature of nearly out-of-equilibrium dissipative systems is that they resonate
through a set of quasinormal modes. Black holes—the absorbing objects par excellence …

Recent detections of merging black holes allow observational tests of the nature of these
objects. In some proposed models, nontrivial structure at or near the black hole horizon …

Gravitational-wave echoes in the postmerger signal of a binary coalescence are predicted in
various scenarios, including near-horizon quantum structures, exotic states of matter in …

Gravitational-wave astronomy has the potential to substantially advance our knowledge of
the cosmos, from the most powerful astrophysical engines to the initial stages of our …

Gravitational-wave astronomy can give us access to the structure of black holes, potentially
probing microscopic or even Planckian corrections at the horizon scale, as those predicted …

Gravitational-wave echoes in the postmerger ringdown phase are under intense scrutiny as
probes of near-horizon quantum structures and as signatures of exotic states of matter in …