An effective action model of dynamically scalarizing binary neutron stars

TL;DR

This paper develops an analytic effective action model for dynamical scalarization in binary neutron star systems, enabling better understanding and prediction of gravitational wave signals related to scalar-tensor gravity theories.

Contribution

It introduces the first Newtonian-order Hamiltonian model for dynamical scalarization, capturing the phase transition and connecting related phenomena within a unified framework.

Findings

Accurately predicts the scalarization onset frequency.

Establishes scalarization as a second-order phase transition.

Unifies phenomena like spontaneous and induced scalarization.

Abstract

Gravitational waves can be used to test general relativity (GR) in the highly dynamical strong-field regime. Scalar-tensor theories of gravity are natural alternatives to GR that can manifest nonperturbative phenomena in neutron stars (NSs). One such phenomenon, known as dynamical scalarization, occurs in coalescing binary NS systems. Ground-based gravitational-wave detectors may be sensitive to this effect, and thus could potentially further constrain scalar-tensor theories. This type of analysis requires waveform models of dynamically scalarizing systems; in this work we devise an analytic model of dynamical scalarization using an effective action approach. For the first time, we compute the Newtonian-order Hamiltonian describing the dynamics of a dynamically scalarizing binary in a self-consistent manner. Despite only working to leading order, the model accurately predicts the frequency at which dynamical scalarization occurs. In conjunction with Landau theory, our model allows one to definitively establish dynamical scalarization as a second-order phase transition. We also connect dynamical scalarization to the related phenomena of spontaneous scalarization and induced scalarization; these phenomena are naturally encompassed into our effective action approach.