Unavoidable shear from quantum fluctuations in contracting cosmologies

Abstract

Contracting cosmologies are known to be flawed with a shear instability, where the contribution from the anisotropic stress to the overall energy density grows as $a^{-6}$, with $a$ the scale factor. Classically, whether or not this contribution becomes important before the bounce depends on its initial value, which can always be sufficiently fine tuned to make it irrelevant. However, vacuum quantum fluctuations inevitably provide a non-vanishing source of anisotropic stress. In this work, we compute the minimum amount of shear that is obtained if one assumes that it vanishes initially, but lets quantum fluctuations build it up. In practice, we consider a massless test scalar field, and describe its quantum fluctuations by means of the stochastic "inflation" (though here applied to a contracting phase) formalism. We find that, if the equation-of-state parameter of the contraction satisfies $w>-1/9$, regardless of when the contracting phase is initiated, the time at which the shear becomes sizeable is always when the Hubble scale approaches the Planck mass (which is also where the bounce is expected to take place). However, if $w<-1/9$, the shear backreaction becomes important much earlier, at a point that depends on the overall amount of contraction.

arXiv commentmatches the published version in EPJC

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• Small-scale clustering of Primordial Black Holes: cloud-in-cloud and exclusion effects
• Anisotropic separate universe and Weinberg's adiabatic mode
• Is Bianchi I a Bouncing Cosmology in the Wheeler-DeWitt picture?
• Entanglement negativity in de Sitter biverse from Stringy Axionic Bell pair: An analysis using Bunch-Davies vacuum
• Spatial Curvature from Super-Hubble Cosmological Fluctuations
• Hamiltonian formalism for cosmological perturbations: fixing the gauge
• Exact path integrals on half-line in quantum cosmology with a fluid clock and aspects of operator ordering ambiguity
• Constraining matter-bounce scenario from scalar-induced vector perturbations
• Time-reversed Stochastic Inflation