Relativistic stars in f(R) gravity, and absence thereof

TL;DR

This paper argues that relativistic stars cannot exist in certain f(R) gravity models due to the dynamics of the scalar degree of freedom, challenging their viability despite successful cosmological and solar system tests.

Contribution

It demonstrates that relativistic stellar configurations are incompatible with some f(R) gravity theories, highlighting a fundamental limitation of these models.

Findings

Relativistic stars cannot form in specific f(R) models.

Nonrelativistic stellar configurations are possible in these theories.

The scalar degree of freedom's dynamics hinder relativistic star formation.

Abstract

Several f(R) modified gravity models have been proposed which realize the correct cosmological evolution and satisfy solar system and laboratory tests. Although nonrelativistic stellar configurations can be constructed, we argue that relativistic stars cannot be present in such f(R) theories. This problem appears due to the dynamics of the effective scalar degree of freedom in the strong gravity regime. Our claim thus raises doubts on the viability of f(R) models.