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