The effect of varying sound velocity on primordial curvature perturbations

TL;DR

This paper investigates how sudden changes in sound velocity during inflation can produce oscillating features in the primordial power spectrum, potentially improving fits to CMB data.

Contribution

It provides an analytic expression for the oscillations in the power spectrum caused by sound speed transitions during inflation.

Findings

Oscillating features depend on sound speed ratio and transition scale.

Certain parameters yield better fit to CMB data than simple power-law spectrum.

Introduction of sound speed change as a free parameter is not statistically justified.

Abstract

We study the effects of sudden change in the sound velocity on primordial curvature perturbation spectrum in inflationary cosmology, assuming that the background evolution satisfies the slow-roll condition throughout. It is found that the power spectrum acquires oscillating features which are determined by the ratio of the sound speed before and after the transition and the wavenumeber which crosses the sound horizon at the transition, and their analytic expression is given. In some values of those parameters, the oscillating primordial power spectrum can better fit the observed Cosmic Microwave Background temperature anisotropy power spectrum than the simple power-law power spectrum, although introduction of such a new degree of freedom is not justified in the context of Akaike's Information Criterion.