Cosmology or Catastrophe? A non-minimally coupled scalar in an inhomogeneous universe

TL;DR

This paper investigates the behavior of a non-minimally coupled scalar field in cosmology, revealing that while homogeneous models are smooth, inhomogeneities lead to divergences and potential catastrophic outcomes when the effective Planck mass passes through zero.

Contribution

It demonstrates that inhomogeneities cause divergences in cosmological models with non-minimally coupled scalars at the zero crossing of the effective Planck mass, highlighting limitations of homogeneous assumptions.

Findings

Homogeneous cosmology remains smooth at zero effective Planck mass.

Inhomogeneities diverge and cause catastrophic effects at the zero crossing.

Scalar anisotropic stress drives the divergence in inhomogeneous cases.

Abstract

A non-minimally coupled scalar field can have, in principle, a negative effective Planck mass squared which depends on the scalar field. Surprisingly, an isotropic and homogeneous cosmological universe with a non-minimally coupled scalar field is perfectly smooth as the rolling scalar field causes the effective Planck mass to change sign and pass through zero. However, we show that any small deviations from homogeneity diverge as the effective Planck mass vanishes, with catastrophic consequences for the cosmology. The physical origin of the divergence is due to the presence of non-zero scalar anisotropic stress from the non-minimally coupled scalar field. Thus, while the homogeneous and isotropic cosmology appears surprisingly sensible when the effective Planck mass vanishes, inhomogeneities tell a different story.