GRavity from Astrophysical to Microscopic Scales

ended

General Relativity (GR) describes gravity on a huge range of scales, field strengths and velocities. However, despite its successes, GR has been showing its age. Cosmological data support the existence of a Dark Sector, but may also be interpreted as a breakdown of our understanding of gravity. Also, GR is intrinsically incompatible with quantum field theory, and should be replaced, at high energies, by a (still unknown) quantum theory of gravity. This deadlock may prelude to a paradigm change in our understanding of gravity, possibly triggered by the direct observations of neutron stars and black holes by gravitational-wave interferometers. The recent LIGO/Virgo observations, and in particular the coincident detection of electromagnetic and gravitational signals from neutron-star binaries, have already made a huge impact on our theoretical understanding of gravity, by severely constraining several extensions of GR. GRAMS is a high-risk/high-gain project seeking to push the implications of these observations even further, by exploring whether the existing LIGO/Virgo data, and in particular their absence of non-perturbative deviations from GR, are consistent with gravitational theories built to reproduce the large-scale behaviour of the Universe (i.e. the existence of Dark Energy and/or Dark Matter), while at the same time passing local tests of gravity thanks to non-perturbative screening mechanisms. I will prove that the very fact of screening local scales makes gravitational emission in these theories much more involved than in GR, and also intrinsically unlikely to yield results in agreement with existing (and future) gravitational-wave observations. This would be a huge step forward for our understanding of cosmology, as it would rule out a modified gravity origin for the Dark Sector. Even if this conjecture is incorrect, GRAMS will provide the first numerical-relativity simulations of compact binaries ever in gravitational theories of interest for cosmology.