On 14 September 2015, the LIGO interferometers made the first direct detection of gravitational waves (GWs), providing also the strongest evidence that black holes (BHs) exist and merge. This landmark discovery marks the dawn of GW astronomy and has opened a new window onto Einstein's General Relativity (GR) in extreme gravitational settings.
In addition to their impact for astrophysics, GWs are unique probes of fundamental interactions. The aim of this multidisciplinary project is to investigate novel effects related to strong gravitational sources -such as black holes (BHs) and neutron stars (NSs)- where matter in extreme conditions, fundamental physics, and the very foundations of GR can be put to the test. We propose to:
i) Test the nature of BHs with GWs
Theoretical arguments suggest that extensions of GR, new fundamental fields, or quantum effects might modify dramatically the formation of BHs. We have recently identified some smoking guns that can be used to search for these phenomena with GWs. These include "GW echoes" in the post-merger signal of the coalescence, as well as the tidal deformability of exotic objects in the late-time inspiral. We will study these new effects in full detail, by building precise waveforms to be implemented in GW data analysis.
ii) Constrain the equation of state (EoS) of NSs with GWs
The coalescence of two NSs provides unique information on the behavior of matter in the inner core of the stars, in a regime that is inaccessible by laboratory experiments. Constraining the EoS of NSs from GW observations requires state-of-the-art modelling of the signal and appropriate post-processing strategies which we plan to devise. In addition, we wish to improve current analytical waveforms by including spin effects and higher-order post-Newtonian terms.
Our ultimate goal is to probe fundamental physics in the most extreme gravitational settings and to devise new approaches for detection with current and future GW interferometers.
BH and GW (astro)physics are blooming and the entire area of strong gravity is 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, similarly to cosmology in the past decade.
Although this field has literally exploded in the last months, 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 of matter in extreme conditions. This novel research area can potentially lead fundamental physics in the next decades. In this flourishing 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 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 what we have observed in the first GW detections are really BHs or other exotic sources.
Furthermore, it is reasonable to expect the first GW detection of a NS-NS or BH-NS binary any time soon. Similarly to the first LIGO detections of BH mergers, there is a concrete possibility that the detection of NS mergers will revolutionize current paradigms in astrophysics and in nuclear physics. However, such a revolution needs to be supported by a solid theoretical framework and its actual potential will lie in our ability to extract reliable astrophysical information about the sources and their strong-gravity dynamics. This is why our proposal to improve current gravitational waveforms by including spin effects, spin-tidal couplings, and higher-order post-Newtonian terms is particularly relevant and timely.
Historically, each time a new window on the universe has been opened, unexpected discoveries were lurking behind. It is therefore reasonable to expect that GW astronomy -if supported by innovative theoretical GR models as those proposed here- can soon lead to paradigm shifts in our understanding of the fundamental laws of nature.