Astrophysical Probes of the Vainshtein Mechanism: Stars and Galaxies

TL;DR

This paper investigates how certain modified gravity theories affect astrophysical objects, revealing weaker gravity inside stars and galaxies, and proposes strong lensing as a key observational test.

Contribution

It introduces new astrophysical probes of ghost-free beyond-Horndeski theories by analyzing their effects on stars and galaxy dynamics.

Findings

Main-sequence stars are dimmer and cooler than in GR.

Galaxy rotation curves are reduced compared to GR predictions.

Lensing mass exceeds dynamical mass in galaxy models.

Abstract

Ghost-free theories beyond the Horndeski class exhibit a partial breaking of the Vainshtein mechanism inside non-relativistic sources of finite extent. We exploit this breaking to identify new and novel astrophysical probes of these theories. Non-relativistic objects feel a gravitational force that is weaker than that predicted by general relativity. The new equation of hydrostatic equilibrium equation is derived and solved to predict the modified behaviour of stars. It is found that main-sequence stars are dimmer and cooler than their general relativity counterparts but the red giant phase is largely indistinguishable. The rotation curves and lensing potential of Milky Way-like galaxies are calculated. The circular velocities are smaller than predicted by general relativity at fixed radius and the lensing mass is larger than the dynamical mass. We discuss potential astrophysical probes of these theories and identify strong lensing as a particularly promising candidate.