Inflation and the Scale Dependent Spectral Index: Prospects and Strategies

TL;DR

This paper explores how the running of the spectral index can serve as a probe of inflation and early universe evolution, highlighting its potential detectability and the impact of post-inflationary history on measurements.

Contribution

It analyzes the consistency of spectral index running across simple inflation models and discusses how future observations can constrain inflationary physics and reheating history.

Findings

Running of the spectral index is relatively consistent across models.

Future large scale structure and 21 cm observations could detect the running.

Post-inflationary expansion history significantly affects spectral index predictions.

Abstract

We consider the running of the spectral index as a probe of both inflation itself, and of the overall evolution of the very early universe. Surveying a collection of simple single field inflationary models, we confirm that the magnitude of the running is relatively consistent, unlike the tensor amplitude, which varies by orders of magnitude. Given this target, we confirm that the running is potentially detectable by future large scale structure or 21 cm observations, but that only the most futuristic measurements can distinguish between these models on the basis of their running. For any specified inflationary scenario, the combination of the running index and unknown post-inflationary expansion history induces a theoretical uncertainty in the predicted value of the spectral index. This effect can easily dominate the statistical uncertainty with which Planck and its successors are expected to measure the spectral index. More positively, upcoming cosmological experiments thus provide an intriguing probe of physics between TeV and GUT scales by constraining the reheating history associated with any specified inflationary model, opening a window into the "primordial dark age" that follows the end of inflation.