[Paper Review] Large scale anomalies in the CMB and non-Gaussianity in bouncing cosmologies

Iván Agulló, Dimitrios Kranas+1 more|arXiv (Cornell University)|Jun 16, 2020

Cosmology and Gravitation Theories107 references46 citations

TL;DR

This paper proposes that large-scale anomalies in the CMB—power suppression, dipolar asymmetry, and odd-parity preference—arise from a cosmic bounce preceding inflation, which breaks scale invariance and generates infrared-enhanced non-Gaussianity. The bounce-induced initial state for inflation introduces excitations and non-Gaussian correlations that collectively explain multiple observed anomalies without requiring new physics beyond a bounce scenario.

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.

Motivation & Objective

• To explain multiple large-scale CMB anomalies—power suppression, dipolar asymmetry, and odd-parity preference—within a single unified framework.
• To investigate whether a pre-inflationary cosmic bounce can generate the necessary non-Gaussian initial conditions for scalar perturbations.
• To demonstrate that these anomalies arise from a common physical origin: a new scale introduced by the bounce, breaking the approximate scale invariance of inflation.
• To show that the resulting non-Gaussianity increases the probability of observing extreme CMB features, thereby explaining their low p-values.
• To validate the model phenomenologically using a minimal set of parameters, independent of specific bouncing theories.

Proposed method

• Uses a phenomenological model where a cosmic bounce precedes inflation, introducing a new physical scale that modifies the initial quantum state of perturbations.
• Models the primordial power spectrum and bispectrum using a spectator mode formalism to describe non-Gaussian modulation of the CMB covariance matrix.
• Applies the Bipolar Spherical Harmonic (BiSH) formalism to encode non-Gaussian effects on CMB statistics, enabling computation of angular correlation functions and parity asymmetries.
• Analyzes monopolar, dipolar, and quadrupolar modulations to connect non-Gaussianity to observed anomalies like power suppression and lensing amplitude tension.
• Performs a perturbative validity check to ensure the approximations used in computing non-Gaussian effects remain consistent.
• Identifies minimal theoretical requirements for the model to work and identifies existing bouncing theories in the literature that satisfy them.

Experimental results

Research questions

• RQ1 Can a cosmic bounce preceding inflation explain the observed large-scale CMB anomalies collectively, rather than as separate issues?
• RQ2 How does the bounce break the scale invariance of inflation and generate non-Gaussian correlations in the primordial perturbations?
• RQ3 To what extent can non-Gaussianity induced by super-horizon modes explain the observed power suppression and dipolar asymmetry in the CMB?
• RQ4 Can the model alleviate the tension in the lensing amplitude AL observed in Planck data?
• RQ5 What are the minimal physical conditions required for the bounce to produce these effects, and which existing bouncing models satisfy them?

Key findings

• The bounce introduces a new physical scale that breaks the approximate scale invariance of inflation, leading to a non-Bunch-Davies initial state for scalar perturbations.
• Non-Gaussianity generated by the bounce enhances the probability of observing extreme CMB features, increasing their p-values and explaining their statistical significance.
• The model accounts for power suppression, dipolar asymmetry, and a preference for odd-parity correlations through a single mechanism rooted in infrared-enhanced non-Gaussianity.
• The lensing amplitude tension (AL > 1) is alleviated due to modified primordial power spectra and non-Gaussian correlations.
• The model remains consistent under perturbative expansion, validating the analytical approach used to compute non-Gaussian effects.
• The framework is general and applicable to a wide class of bouncing cosmologies, provided they satisfy minimal conditions like a well-defined bounce and slow-roll inflation afterward.