Tidal properties of neutron stars in scalar-tensor theories of gravity

TL;DR

This paper develops a framework to characterize neutron stars in scalar-tensor gravity theories using three tidal deformabilities, aiding future gravitational-wave tests of gravity modifications.

Contribution

It introduces an effective description of scalarized neutron stars with three key tidal parameters and provides a methodology for their computation in scalar-tensor theories.

Findings

Three independent tidal deformabilities are necessary to characterize scalarized neutron stars.

The methodology enables mapping between different calculation frames.

Results facilitate gravitational-wave tests of gravity and account for uncertainties like the equation of state.

Abstract

A major science goal of gravitational-wave (GW) observations is to probe the nature of gravity and constrain modifications to General Relativity. An established class of modified gravity theories are scalar-tensor models, which introduce an extra scalar degree of freedom. This affects the internal structure of neutron stars (NSs), as well as their dynamics and GWs in binary systems, where distinct novel features can arise from the appearance of scalar condensates in parts of the parameter space. To improve the robustness of the analyses of such GW events requires advances in modeling internal-structure-dependent phenomena in scalar-tensor theories. We develop an effective description of potentially scalarized NSs on large scales, where information about the interior is encoded in characteristic Love numbers or equivalently tidal deformabilities. We demonstrate that three independent tidal deformabilities are needed to characterize the configurations: a scalar, tensor, and a novel 'mixed' parameter, and develop the general methodology to compute these quantities. We also present case studies for different NS equations of state and scalar properties and provide the mapping between the deformabilities in different frames often used for calculations. Our results have direct applications for future GW tests of gravity and studies of potential degeneracies with other uncertain physics such as the equation of state or presence of dark matter in NS binary systems.