Observational constraints on anisotropies for bouncing alternatives to inflation

TL;DR

This paper investigates how primordial anisotropies in bouncing cosmological models influence perturbations, revealing a specific quadrupolar pattern in the CMB and deriving strong observational constraints on anisotropy levels.

Contribution

It provides a detailed calculation of anisotropy effects in bouncing scenarios and establishes new, more stringent observational bounds compared to previous estimates.

Findings

Anisotropies induce a scale-invariant quadrupolar pattern in the CMB.

Higher-order moments and scalar-tensor cross-correlations are generated.

Observational constraints significantly limit allowed anisotropies in bouncing models.

Abstract

We calculate how primordial anisotropies in the background space-time affect the evolution of cosmological perturbations for bouncing alternatives to inflation, like ekpyrosis and the matter bounce scenario. We find that the leading order effect of anisotropies in the contracting phase of the universe is to induce anisotropies in the cosmic microwave background with a very concrete form: a scale-invariant quadrupolar angular distribution. Sub-leading effects are the generation of higher-order moments in the angular distribution, as well as cross-correlations between scalar and tensor modes. We also find that observational constraints from the cosmic microwave background on the quadrupole moment provide strong bounds on allowed anisotropies for bouncing alternatives to inflation that are significantly more constraining than the bounds previously obtained using scaling arguments based on the conjectured Belinski-Khalatnikov-Lifshitz instability.