Spectator fields and their imprints on the Cosmic Microwave Background

TL;DR

This paper explores how a late-decaying scalar spectator field during inflation can influence the Cosmic Microwave Background, providing predictions consistent with observations and explaining features like power asymmetry.

Contribution

It introduces a detailed analysis of spectator fields' imprints on the CMB and derives their observable predictions, including explanations for power asymmetry.

Findings

Spectator fields can produce observable effects on the CMB power spectrum and non-Gaussianities.

The scenario is consistent with CMB data for specific potential forms.

A brief tachyonic fast-roll phase can account for power asymmetry.

Abstract

When a subdominant light scalar field ends slow roll during inflation, but well after the Hubble exit of the pivot scales, it may determine the cosmological perturbations. This thesis investigates how such a scalar field, the spectator, may leave its impact on the Cosmic Microwave Background (CMB) radiation and be consequently constrained. We first introduce the observables of the CMB, namely the power spectrum $P_\zeta$, spectral index $n_s$ and its running $dn_s/d\ln k$, the non-Gaussianities $f_{NL}$, $g_{NL}$ and $\tau_{NL}$, and the lack of isocurvature and polarization modes. Based on these studies, we derive the cosmological predictions for the spectator scenario, revealing its consistency with the CMB for inflection point potentials, hyperbolic tangent potentials, and those with a sudden phase transition. In the end, we utilize the spectator scenario to explain the CMB power asymmetry, with a brief tachyonic fast-roll phase.