Inflation versus Cyclic Predictions for Spectral Tilt

TL;DR

A nearly model-independent estimate is presented that yields the predictions of a class of simple inflationary and ekpyrotic or cyclic models for the spectral tilt of the primordial density inhomogeneities that enables us to compare the two scenarios.

Abstract

We present a simple, nearly model-independent estimate that yields the predictions of the simplest inflationary and ekpyrotic/cyclic models for the spectral tilt of the primordial density inhomogeneities. Remarkably, we find that the simplest models yield an identical result: n_s is approximately 0.95. For inflation, the same estimate predicts a ratio of tensor to scalar contributions to the low multipoles of the microwave background anisotropy of T/S=20%; the tensor contribution is negligible for ekpyrotic/cyclic models.