Can Neutron Star Tidal Effects Obscure Deviations from General Relativity?

TL;DR

This paper investigates whether uncertainties in nuclear physics and deviations from general relativity can bias measurements of neutron star properties from gravitational-wave data, finding current data cannot exclude such effects.

Contribution

It introduces a study of how scalar-tensor gravity theories impact neutron star property measurements and the potential bias in constraining the nuclear equation of state.

Findings

Parameter inference yields consistent masses and tidal deformabilities in both GR and ST theories.

Radius and EOS posteriors differ between theories but are not currently excluded by data.

Deviations from GR could remain undetected in current neutron star merger analyses.

Abstract

One of the main goals of gravitational-wave astrophysics is to study gravity in the strong-field regime and constrain deviations from general relativity (GR). Any such deviation affects not only binary dynamics and gravitational-wave emission but also the structure and tidal properties of compact objects. In the case of neutron stars, masses, radii, and tidal deformabilities can all differ significantly between different theories of gravity. Currently, the measurement uncertainties in neutron star radii and tidal deformabilities are quite large. However, much less is known about how the large uncertainty in the nuclear equation of state (EOS) might affect tests of GR using binary neutron star mergers. Conversely, using the wrong theory of gravity might lead to incorrect constraints on the nuclear EOS. Here, we study this problem within scalar-tensor (ST) theory. We apply the recently derived $\ell = 2$ tidal Love numbers in this theory to parameter estimation of GW170817. Correspondingly, we test if physics beyond GR could bias measurements of the nuclear EOS and neutron star radii. We find that parameter inference for both the GR and ST cases return consistent component masses and tidal deformabilities. The radius and the EOS posteriors, however, differ between the two theories, but neither is excluded by current observational limits. This indicates that measurements of the nuclear EOS may be biased and that deviations from GR could go undetected when analyzing current binary neutron star mergers.