Stability of Geodesically Complete Cosmologies

Abstract

We study the stability of spatially flat FRW solutions which are geodesically complete, i.e. for which one can follow null (graviton) geodesics both in the past and in the future without ever encountering singularities. This is the case of NEC-violating cosmologies such as smooth bounces or solutions which approach Minkowski in the past. We study the EFT of linear perturbations around a solution of this kind, including the possibility of multiple fields and fluids. One generally faces a gradient instability which can be avoided only if the operator ${}^{(3)}R\delta N$ is present and its coefficient changes sign along the evolution. This operator (typical of beyond-Horndeski theories) does not lead to extra degrees of freedom, but cannot arise starting from any theory with second-order equations of motion. The change of sign of this operator prevents to set it to zero with a generalised disformal transformation.

18 pages, 2 figures. v2: minor changes; references added; version published in JCAP

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Cited by in corpus (8)

• The Effective Field Theory of nonsingular cosmology
• Matter bounce cosmology with a generalized single field: non-Gaussianity and an extended no-go theorem
• Higher order derivative coupling to gravity and its cosmological implications
• Scale-invariant perturbations from NEC violation: A new variant of Galilean Genesis
• Probing the primordial universe with gravitational waves detectors
• Spinor driven cosmic bounces and their cosmological perturbations
• Perturbations in generalized Galileon theories
• A class of regular bouncing cosmologies