Can higher curvature corrections cure the singularity problem in f(R) gravity?

TL;DR

This paper investigates whether higher curvature corrections can fix the nonexistence of neutron stars in certain $f(R)$ gravity models, revealing significant fine-tuning issues that challenge their viability.

Contribution

It demonstrates that higher curvature corrections do not naturally resolve the neutron star problem in $f(R)$ gravity without unnatural fine-tuning.

Findings

Higher curvature corrections require extreme fine-tuning of energy scales.

Additional fine-tuning is needed to avoid quadratic curvature terms.

Constructing viable $f(R)$ models without fine-tuning remains challenging.

Abstract

Although $f(R)$ modified gravity models can be made to satisfy solar system and cosmological constraints, it has been shown that they have the serious drawback of the nonexistence of stars with strong gravitational fields. In this paper, we discuss whether or not higher curvature corrections can remedy the nonexistence consistently. The following problems are shown to arise as the costs one must pay for the $f(R)$ models that allow for neutrons stars: (i) the leading correction must be fine-tuned to have the typical energy scale $\mu \lesssim 10^{-19}$ GeV, which essentially comes from the free fall time of a relativistic star; (ii) the leading correction must be further fine-tuned so that it is not given by the quadratic curvature term. The second problem is caused because there appears an intermediate curvature scale and laboratory experiments of gravity will be under the influence of higher curvature corrections. Our analysis thus implies that it is a challenge to construct viable $f(R)$ models without very careful and unnatural fine-tuning.