Red Giant evolution in Modified Gravity

TL;DR

This study investigates how modified gravity, specifically chameleon fields, affects red giant star evolution, finding that screening mechanisms make their behavior similar to general relativity.

Contribution

It provides new insights into the chameleon screening effect in inhomogeneous stars and assesses the scalar field dynamics during stellar evolution.

Findings

Fifth force weaker near the surface in inhomogeneous density distributions.

Scalar field follows the potential minimum quickly, reducing fifth force.

Star evolution remains similar to standard GR due to adiabatic scalar field behavior.

Abstract

In this paper, we study the chameleon profile in inhomogeneous density distributions and find that the fifth force in thin shell near the surface is weaker from what expected in homogeneous density distributions. Also, we check the validity of quasi-static approximation for the chameleon scalar field in the astrophysical time scales. We have investigated the rolling down behaviour of the scalar field on its effective potential inside a one solar mass red giant star by using MESA code. We have found that the scalar field is fast enough to follow the minimum of the potential. This adiabatic behaviour reduces the fifth force and extends the screened regions to lower densities where the field has smaller mass and was expected to be unscreened. As a consequence, the star evolution is similar to what expected from standard general relativity. In addition, considering the stability of star, an approximate constraint on the coupling constant $\beta$ is found.