# Nonlocal gravity and gravitational-wave observations

## TL;DR

This paper explores a nonlocal gravity model that aligns with cosmological data like $ ext{Lambda}$CDM but predicts significant deviations in gravitational wave propagation, potentially detectable by future GW observatories.

## Contribution

It introduces a variant of the RT nonlocal gravity model that predicts large deviations in GW luminosity distances at cosmological scales, distinguishable from GR predictions.

## Key findings

- GW luminosity distance can differ by up to 60% at high redshifts.
- Potential detection of deviations with a single standard siren by LISA or ET.
- For LIGO/Virgo NS-BH events, deviations could be as much as 7%.

## 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 to a third-generation GW detector 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 a LIGO/Virgo/Kagra network at target sensitivity the effect is smaller but still potentially detectable. Indeed, for the recently announced LIGO/Virgo NS-BH candidate S190814bv, the RT model predicts that, given the measured GW luminosity distance, the actual luminosity distance, and the redshift of an electromagnetic counterpart, would be smaller by as much as $7\%$ with respect to the value inferred from $\Lambda$CDM.