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

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.

10 pages, 2 figures. v2: Updated references. v3: Added figure, minor clarifications in Appendix. Matches published version

References in corpus (4)

• The Standard Model Higgs boson as the inflaton
• String Cosmology: A Review
• On Inflation with Non-minimal Coupling
• Revisiting Cosmic No-Hair Theorem for Inflationary Settings

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