Large scale anomalies in the CMB and non-Gaussianity in bouncing cosmologies

TL;DR

This paper suggests that a cosmic bounce before inflation could explain large-scale anomalies in the CMB, introducing non-Gaussian features and mode correlations that match observations.

Contribution

It introduces a phenomenological model linking a pre-inflationary bounce to CMB anomalies, emphasizing non-Gaussianity and mode correlations without relying on specific bouncing theories.

Findings

Explains power suppression and dipolar asymmetry in CMB

Accounts for odd-parity correlations in large-scale modes

Alleviates lensing amplitude tension

Abstract

We propose that several of the anomalies that have been observed at large angular scales in the CMB have a common origin in a cosmic bounce that took place before the inflationary era. The bounce introduces a new physical scale in the problem, which breaks the almost scale invariance of inflation. As a result, the state of scalar perturbations at the onset of inflation is no longer the Bunch-Davies vacuum, but it rather contains excitations and non-Gaussianity, which are larger for infrared modes. We argue that the combined effect of these excitations and the correlations between CMB modes and longer wavelength perturbations, can account for the observed power suppression, for the dipolar asymmetry, and it can also produce a preference for odd-parity correlations. The model can also alleviate the tension in the lensing amplitude $A_L$. We adopt a phenomenological viewpoint by characterizing the model with a few free parameters, rather than restricting to specific bouncing theories. We identify the minimum set of ingredients needed for our ideas to hold, and point out examples of theories in the literature where these conditions are met.