Stellar Structure and Tests of Modified Gravity

TL;DR

This paper explores how modified gravity theories with chameleon screening affect stellar structure, especially red giants, offering a new way to test these theories through precise stellar observations.

Contribution

It demonstrates that stellar evolution, particularly in red giants, can reveal signatures of environment-dependent gravity modifications, providing stronger constraints than existing tests.

Findings

Red giants would be smaller by tens of percent.

Stars would be hotter by hundreds of Kelvin.

Constraints on chameleon theories could improve by four orders of magnitude.

Abstract

Theories that attempt to explain cosmic acceleration by modifying gravity typically introduces a long-range scalar force that needs to be screened on small scales. One common screening mechanism is the chameleon, where the scalar force is screened in environments with a sufficiently deep gravitational potential, but acts unimpeded in regions with a shallow gravitational potential. This leads to a variation in the overall gravitational G with environment. We show such a variation can occur within a star itself, significantly affecting its evolution and structure, provided that the host galaxy is unscreened. The effect is most pronounced for red giants, which would be smaller by a factor of tens of percent and thus hotter by 100's of K, depending on the parameters of the underlying scalar-tensor theory. Careful measurements of these stars in suitable environments (nearby dwarf galaxies not associated with groups or clusters) would provide constraints on the chameleon mechanism that are four orders of magnitude better than current large scale structure limits, and two orders of magnitude better than present solar system tests.