Running of the scalar spectral index in bouncing cosmologies

TL;DR

This paper calculates the running of the scalar spectral index in bouncing cosmologies, revealing distinct signs for ekpyrotic and matter bounce models, with potential observational implications.

Contribution

It provides the first detailed calculation of the spectral index running in bouncing cosmologies, highlighting differences from inflationary predictions.

Findings

Ekpyrotic models typically have negative running.

Matter bounce models with multiple fields tend to have positive running.

Running magnitude ranges from 10^{-4} to 10^{-2}, affecting future observations.

Abstract

We calculate the running of the scalar index in the ekpyrotic and matter bounce cosmological scenarios, and find that it is typically negative for ekpyrotic models, while it is typically positive for realizations of the matter bounce where multiple fields are present. This can be compared to inflation, where the observationally preferred models typically predict a negative running. The magnitude of the running is expected to be between $10^{-4}$ and up to $10^{-2},$ leading in some cases to interesting expectations for near-future observations.