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Stability of Geodesically Complete Cosmologies
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Stability of Geodesically Complete Cosmologies
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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.
Forward citations
Cited by 4 Pith papers
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Geodesically Complete Curvature-Bounce Inflation
Constructs an explicit closed FRW curvature-supported bounce-inflation model with a canonical scalar field that remains geodesically complete, NEC-compliant, and yields standard slow-roll predictions.
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Does Eternal Inflation Violate the Smeared Null Energy Condition?
In canonical single-field eternal inflation, stochastic upward fluctuations do not violate the SNEC within the semiclassical slow-roll regime due to parametrically bounded drift and a strong timescale hierarchy N_SNEC ≫ N_BR.
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Geodesically Complete Curvature-Bounce Inflation
A closed k=+1 FRW universe with curvature-driven bounce and canonical scalar inflation remains sub-Planckian, satisfies the null energy condition, and produces ns=0.9617-0.9650 and r=0.0037-0.0045 consistent with data.
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Geodesic completion of big bangs from emergent geometry
Phantom Chaplygin gas forces the Einstein-frame lapse to change sign smoothly while the causal-frame lapse stays positive, yielding a robust non-singular bounce even with extra matter.
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