How delicate are the $f(R)$ gravity models with disappearing cosmological constant?

Abstract

We consider stability of spherically symmetric solutions in $f(R)$ gravity model proposed by Starobinsky. We find that the model suffers from a severe fine tuning problem when applied to compact objects like neutron stars. The problem can be remedied by introducing a cut off on the mass of the scalar degree of freedom present in the model. A new mass scale associated with neutron stars density is then required for the stabilities of $f(R)$ gravity solutions inside relativistic stars.

Comments: 5 pages and no figures, minor clarification added, final version to appear in PRD