Untangling features in the primordial spectra

TL;DR

This paper analyzes potential features in the primordial spectra from single field inflation, identifying two main sources—sound speed variations and expansion rate changes—and their observational signatures in power spectrum and bispectrum.

Contribution

It introduces a relation linking features in the bispectrum to those in the power spectrum, highlighting how different sources produce distinctive observational patterns.

Findings

Features can originate from sound speed variations or expansion rate changes.

Each feature source leaves a unique imprint in the primordial perturbation distribution.

Joint analysis of power spectrum and bispectrum can identify the dominant feature source.

Abstract

We discuss the possible existence of features in both the primordial power spectrum and bispectrum generated during a stage of single field cosmic inflation. We argue that there are two main classes of features: those produced by a sudden time variation of the sound speed of curvature perturbations, and those produced by a sudden change in the expansion rate during inflation. The former are known to be produced by heavy fields, when the inflationary background trajectory in field space undergoes a bend, whereas the latter are known to be produced by features in the inflaton potential encountered as the inflaton field descends its slope. In general, features are expected to be the result of these two sources combined, however, it is possible that one source dominated over the other, resulting in a distinctive pattern that may be observationally tested. We deduce a relation that gives us the shape of features in the bispectrum provided that we know the shape of features in the power spectrum, and show that each one of these two classes of features leaves a particular footprint in the distribution of perturbations that could be uncovered by a joint analysis of the primordial power spectrum and bispectrum.