On Primordial Density Perturbation and Decaying Speed of Sound

TL;DR

This paper explores how a decreasing speed of sound during cosmic expansion can generate primordial density perturbations, providing insights into a mechanism that produces scale-invariant spectra even in decelerated phases.

Contribution

It offers a detailed analysis of the decaying sound speed mechanism for primordial perturbation generation, expanding on recent proposals and clarifying its underlying physics.

Findings

Decaying sound speed can produce scale-invariant spectra in decelerated expansion.

The mechanism is applicable to various expanding phases, not limited to inflation.

Provides detailed insights into the seeding process of primordial density perturbations.

Abstract

The decaying speed of sound can lead to the emergence of the primordial density perturbation in any expanding phase, even if the expansion is decelerated. Recently, some proposals have been given to implement this mechanism, in which it was found that the primordial spectrum of scalar perturbation can be scale invariant. In this note, we will give more insights for the details of this seeding mechanism.