Is a step in the primordial spectral index favored by CMB data ?

TL;DR

The paper investigates a specific step-like feature in the primordial spectral index caused by a phase transition during inflation, showing it fits WMAP data better and suggesting a possible physical origin.

Contribution

It introduces a universal local feature in the primordial spectrum due to a potential step, and demonstrates its compatibility with CMB data, proposing a physical model involving a phase transition.

Findings

Better fit to WMAP data with the step feature

Feature localized at large scales (l < 40)

Sign of the effect linked to negative running of n_s

Abstract

A sudden small change in the second derivative of the inflaton potential can result in a universal local feature in the spectrum of primordial perturbations generated during inflation. The exact solution describing this feature \cite{minu} is characterized by a step in the spectral index modulated by characteristic oscillations and results in a large running of the spectral index localized over a few e-folds of scale. In this paper we confront this step-like feature with the 5 year WMAP results and demonstrate that it provides a better fit to this data than a featureless initial spectrum. If such a feature exists at all, then it should lie at sufficiently large scales $k_0 \lesssim 0.003 {\rm Mpc}^{-1}$ corresponding to $l \lesssim 40$. The sign of the effect is shown to correspond to the negative running of $n_s$ localized near this scale. This feature could arise as a result of a `mini-waterfall'-type fast second order phase transition experienced by an auxiliary heavy field during inflation, in a model similar to hybrid inflation (though for a different choice of parameters). If this is the case, then the auxiliary field should be positively coupled to the inflaton.