Ambiguity in running spectral index with an extra light field during inflation

TL;DR

This paper explores how extra light scalar fields during inflation can cause ambiguity in the running of the spectral index, potentially resolving tensions between BICEP2 and Planck data.

Contribution

It demonstrates that additional scalar fields can alter the spectral index running by about ±0.01 without significantly affecting the spectral index itself.

Findings

Extra scalar fields can modify the spectral index running by ±0.01.

Such modifications can help reconcile BICEP2 and Planck observations.

The corrections are relevant for the global spectrum behavior, not just at the pivot scale.

Abstract

At the beginning of inflation there could be extra dynamical scalar fields that will soon disappear (become static) before the end of inflation. In the light of multi-field inflation, those extra degrees of freedom may alter the time-dependence of the original spectrum of the curvature perturbation. It is possible to remove such fields introducing extra number of e-foldings prior to $N_e\sim 60$, however such extra e-foldings may make the trans-Planckian problem worse due to the Lyth bound. We show that such extra scalar fields can change the running of the spectral index to give correction of $\pm 0.01$ without adding significant contribution to the spectral index. The corrections to the spectral index (and the amplitude) could be important in considering global behavior of the corrected spectrum, although they can be neglected in the estimation of the spectrum and its spectral index at the pivot scale. The ambiguity in the running of the spectral index, which could be due to such fields, can be used to nullify tension between BICEP2 and Planck experiments.