Kinetic screening in nonlinear stellar oscillations and gravitational collapse

TL;DR

This paper investigates the behavior of k-essence scalar-tensor theories in stellar oscillations and gravitational collapse, demonstrating the screening mechanism's effectiveness and challenges in dynamic scenarios, with potential detectability of scalar signals by space-based detectors.

Contribution

It constructs nonlinear stellar solutions in k-essence theories, analyzes the screening mechanism during oscillations and collapse, and introduces a method to evolve collapsing stars past characteristic speed divergences.

Findings

Screening suppresses scalar emission in stellar oscillations.

Collapse can lead to diverging scalar characteristic speeds.

Potential detection of scalar signals by space-borne interferometers like LISA.

Abstract

We consider k-essence, a scalar-tensor theory with first-order derivative self-interactions that can screen local scales from scalar fifth forces, while allowing for sizeable deviations from General Relativity on cosmological scales. We construct fully nonlinear static stellar solutions that show the presence of this screening mechanism, and we use them as initial data for simulations of stellar oscillations and gravitational collapse in spherical symmetry. We find that for k-essence theories of relevance for cosmology, the screening mechanism works in the case of stellar oscillation and suppresses the monopole scalar emission to undetectable levels. In collapsing stars, we find that the Cauchy problem, although locally well posed, can lead to diverging characteristic speeds for the scalar field. By introducing a ''fixing equation'' in the spirit of J. Cayuso, N. Ortiz, and L. Lehner [Phys. Rev. D 96, 084043 (2017)], inspired in turn by dissipative relativistic hydrodynamics, we manage to evolve collapsing neutron stars past the divergence of the characteristic speeds. We show that, in these systems, the screening mechanism is less efficient than for oscillating and static stars, because the collapsing star must shed away all of its scalar hair before forming a black hole. For k-essence theories of relevance for cosmology, the characteristic frequency of the resulting scalar monopole signal is too low for terrestrial detectors, but we conjecture that space-borne interferometers such as LISA might detect it if a supernova explodes in the Galaxy.