A solution to the anisotropy problem in bouncing cosmologies

Abstract

Bouncing cosmologies are often proposed as alternatives to standard inflation for the explanation of the homogeneity and flatness of the universe. In such scenarios, the present cosmological expansion is preceded by a contraction phase. However, during the contraction, in general the anisotropy of the universe grows and eventually leads to a chaotic mixmaster behavior. This would either be hard to reconcile with observations or even lead to a singularity instead of the bounce. In order to preserve a smooth and isotropic bounce, the source for the contraction must have a super-stiff equation of state with $P/\rho =w>1$. In this letter we propose a new mechanism to solve the anisotropy problem for any low-energy value of $w$ by arguing that high energy physics leads to a modification of the equation of state, with the introduction of non-linear terms. In such a scenario, the anisotropy is strongly suppressed during the high energy phase, allowing for a graceful isotropic bounce, even when the low-energy value of $w$ is smaller than unity.

9 pages, accepted for publication in JCAP

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• Particle Creation in Bouncing Cosmologies
• Cuscuton gravity as a classically stable limiting curvature theory
• Reconstruction of a Nonminimal Coupling Theory with Scale-invariant Power Spectrum
• Microphysical manifestations of viscosity and consequences for anisotropies in the very early universe
• Reconstruction method of $f(R)$ gravity for isotropic and anisotropic spacetimes
• Fine-tuning challenges for the matter bounce scenario
• Analysis of big bounce in Einstein--Cartan cosmology
• Nonsingular Cosmology from an Interacting Vacuum
• Primordial power spectrum from a matter-Ekpyrotic bounce scenario in loop quantum cosmology
• Massive gravity and the suppression of anisotropies and gravitational waves in a matter-dominated contracting universe
• Can $f(R)$ gravity isotropize a pre-bounce contracting universe?
• Quasi-isotropic cycles and non-singular bounces in a Mixmaster cosmology
• Comments on "Growth of Covariant Perturbations in the Contracting Phase of a Bouncing Universe" by A. Kumar
• Bouncing universe with modified dispersion relation
• Observational constraints on anisotropies for bouncing alternatives to inflation
• Nonlinear anisotropy growth in Bianchi-I spacetime in metric $f(R)$ cosmology
• Exponential correction to Friedmann equations
• Bouncing solutions from generalized EoS
• Smoking-gun signatures of bounce cosmology from echoes of relic gravitational waves
• Bouncing cosmology without anisotropy