Ricerc@Sapienza

With approximately 50 black-hole (BH) and a few neutron-star mergers detected by LIGO and Virgo to date and many more expected in the next few years, gravitational-wave (GW) astronomy is in full blossom. In addition to their enormous impact for astrophysics, GWs are unique probes to address open problems of fundamental physics in extreme gravitational settings. We propose to explore three big themes:

- The nature of gravity
GWs make it possible to probe gravity in the (mostly untested) regime of strong gravitational field and large curvature, where corrections to general relativity (GR) may be detectable. This will require theoretical models of GW sources with no prior assumption of GR as the underlying theory of gravity, and novel data-analysis algorithms to extract meaningful results from the impressive amount of data which will be delivered.

- The nature of compact objects
Several arguments suggest that quantum corrections may drastically change the nature of BHs. We recently started to employ semi-analytical and fully numerical techniques developed in GR to explore some "smoking guns" that can be used to search for "new physics" at the horizon scale with GWs, including a richer multipolar structure, nonvanishing tidal Love numbers, different quasinormal-mode spectrum, and "GW echoes".

- Primordial BHs as dark matter candidates
Primordial black holes (PBHs) are hypothetical BHs formed in the very early Universe and may constitute a sizable fraction of dark matter (DM) depending on their mass. Theoretical models describing PBH formation are still quite unsatisfactory in many respects, and testing observationally the nature of DM in the form of PBHs is an open challenge.

Our ultimate goal is to probe fundamental physics in the most extreme gravitational settings and devise new approaches for current and future GW interferometers. This proposal will also support the ongoing activity of some of the team members within the Einstein Telescope and LISA Consortia.