Nonlocal gravity and gravitational-wave observations

Abstract

We discuss a modified gravity model which fits cosmological observations at a level statistically indistinguishable from $\\Lambda$CDM and at the same time predicts very large deviations from General Relativity (GR) in the propagation of gravitational waves (GWs) across cosmological distances. The model is a variant of the RT nonlocal model proposed and developed by our group, with initial conditions set during inflation, and predicts a GW luminosity distance that, at the redshifts accessible to LISA or at a third-generation GW detectors such as the Einstein Telescope (ET), can differ from that in GR by as much as $60\\%$. An effect of this size could be detected with just a single standard siren with counterpart by LISA or ET. At the redshifts accessible to the LIGO/Virgo/Kagra network the effect is smaller but still potentially detectable.