Non-Gaussianities in Single Field Inflation and their Optimal Limits from the WMAP 5-year Data

TL;DR

This paper analyzes non-Gaussianities in single-field inflation using WMAP data, constraining parameters that describe the shape and size of these signals, and translating these into limits on inflationary model parameters.

Contribution

It introduces a novel analysis of the orthogonal non-Gaussianity shape and provides the first constraints on these parameters from WMAP data.

Findings

No evidence of non-Gaussianity was found.

Constraints on f_NL^equil and f_NL^orthog were established.

Limits on the inflaton's speed of sound and higher-derivative operators were derived.

Abstract

Using the recently developed effective field theory of inflation, we argue that the size and the shape of the non-Gaussianities generated by single-field inflation are generically well described by two parameters: f_NL^equil, which characterizes the size of the signal that is peaked on equilateral configurations, and f_NL^orthog, which instead characterizes the size of the signal which is peaked both on equilateral configurations and flat-triangle configurations (with opposite signs). The shape of non-Gaussianities associated with f_NL^orthog is orthogonal to the one associated to f_NL^equil, and former analysis have been mostly blind to it. We perform the optimal analysis of the WMAP 5-year data for both of these parameters. We find no evidence of non-Gaussianity, and we have the following constraints: -125 < f_NL^equil < 435, -369 < f_NL^orthog < 71 at 95% CL. We show that both of these constraints can be translated into limits on parameters of the Lagrangian of single-field inflation. For one of them, the speed of sound of the inflaton fluctuations, we find that it is either bounded to be c_s > 0.011 at 95% CL. or alternatively to be so small that the higher-derivative kinetic term dominate at horizon crossing. We are able to put similar constraints on the other operators of the inflaton Lagrangian.