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.
BH and GW (astro)physics, and the entire area of strong gravity, are exponentially growing. The landmark discovery of GWs has finally given us access to the dynamics of compact objects, and promises to make strong gravity a precision discipline, similar to cosmology in the past decade.
Although this field has literally exploded in the last few years, there are just a handful of groups in the world with the necessary track record in strong gravity and cross-cutting expertise in astrophysics, GW modelling, and particle physics to achieve our ambitious goals.
With this multidisciplinary proposal, we wish to continue an innovative and cross-cutting research program which might have a dramatic impact on tests of gravity and in extreme conditions. This novel field of study can potentially lead research in fundamental physics in the next decades. In this scenario, a project like the one we propose can provide a solid theoretical framework for the newborn area of GW astronomy and it is therefore extremely timely and relevant to consolidate strong gravity in Italy, as well as Sapienza's position at the international level.
The outcome of this project will be relevant to:
(i) Test whether GR deviations are present in the strong-field regime of gravity. Our models of gravitational waveform in modified gravity theories and our data-analysis algorithms will allow to go beyond simple null tests, and to find whether tiny new physics effects are buried in the data from the next generation of GW detectors.
(ii) Test possible consequences of quantum effects at the horizon scale, ruling out (of finding evidence of) exotic compact objects or signatures of new physics in the vicinity of BHs. The study of BH mimickers will allow us to understand if some of the LIGO/Virgo GW events (e.g. the mass gap events that challenge the standard astrophysical scenarios) are really BHs as predicted by GR or could fit within some different scenario.
(iii) Understanding the nature of DM is a question of the utmost importance in cosmology, particle and astroparticle physics. In this respect, the possibility that DM consists of PBHs is a fascinating hypothesis. The aim of our project is to improve over the current theoretical foundations of this idea, and to explore observational consequences via GWs that could be crucial to confirm or dismiss it.
These outcomes will be of direct use for tests of GR routinely performed by the LIGO/Virgo Collaboration and will pave the way for precision tests achievable with the next-generation GW detectors such as the ET and the space mission LISA. In particular, LISA will detect the GW signals emitted by extreme mass ratio inspirals (EMRIs) for the first time, proving extremely accurate measurements of the quadrupole, of several high-order multipole moments, of the tidal deformability of the binary, as well as unprecedented tests of the dipolar radiation, and ringdown tests either by detecting several QNMs of the remnant or searching for post-merger echoes.