Non-locality in Quadrupolar Gravitational Radiation

TL;DR

This paper explores how non-local modifications to gravity, specifically through $Roxminus^{-1}R$ terms, alter gravitational wave emission, potentially providing observable signatures in astrophysical systems and tests for deviations from General Relativity.

Contribution

It derives a modified quadrupole formula in non-local gravity theories and discusses the potential detectability of scalar modes with future gravitational wave detectors.

Findings

Modified quadrupole formula due to non-local effects

Scalar gravitational wave mode could be detectable near model divergence

Potential for astrophysical tests of non-local gravity theories

Abstract

General Relativity suffers for two main problems which have not yet been overcome: it predicts spacetime singularities and cannot be formulated as a perturbative renormalizable theory. In particular, many attempts have been made for avoiding singularities, such as considering higher order or infinite derivative theories. The price to pay in both cases is to give up locality and therefore they are known altogether as non-local theories of gravity. In this paper, we investigate how to recognize the presence of non-local effects by exploiting the power emitted by gravitational waves in a binary system in presence of non-local corrections as $R\Box^{-1}R$ to the Hilbert-Einstein action. After solving the field equations in terms of the source stress-energy tensor $T_{\mu\nu}$ and obtaining the gravitational wave stress-energy pseudo-tensor, $\tau_{\mu\nu}$, we find that the General Relativity quadrupole formula is modified in a non-trivial way, making it feasible to find a possible signature of non-locality. Our final results on the gravitational wave stress-energy pseudo-tensor could also be applied to several astrophysical scenarios involving energy or momentum loss, potentially providing multiple tests for non-local deviations from General Relativity. We finally discuss the detectability of the massless transverse scalar mode, discovering that, although this radiation is extremely weak, in a small range around the model divergence, its amplitude could fall within the low-frequency Einstein Telescope sensitivity.