Effects of Nonlinear Dispersion Relations on Non-Gaussianities

TL;DR

This paper examines how nonlinear dispersion relations influence the primordial bispectrum, finding that corrections are small, configuration-dependent, and generally do not lead to significant enhancements, especially when early-time WKB conditions are satisfied.

Contribution

It provides a detailed analysis of the effects of nonlinear dispersion relations on non-Gaussianities, especially in the context of the Jacobson-Corley model, and compares different modeling approaches for trans-Planckian physics.

Findings

Corrections to the bispectrum are suppressed by H^2/p_c^2.

Equilateral configurations are more suppressed than local ones.

Particle creation effects are small and do not cause significant bispectrum enhancements.

Abstract

We investigate the effect of non-linear dispersion relations on the bispectrum. In particular, we study the case were the modified relations do not violate the WKB condition at early times, focusing on a particular example which is exactly solvable: the Jacobson-Corley dispersion relation with quartic correction with positive coefficient to the squared linear relation. We find that the corrections to the standard result for the bispectrum are suppressed by a factor $\frac{H^2}{p_c^2}$ where $p_c$ is the scale where the modification to the dispersion relation becomes relevant. The modification is {\it mildly} configuration-dependent and equilateral configurations are more suppressed with respect to the local ones, by a factor of one percent. There is no configuration leading to enhancements. We then analyze the results in the framework of particle creation using the approximate gluing method of Brandenberger and Martin, which relates more directly to the modeling of the trans-Planckian physics via modifications of the vacuum at a certain cutoff scale. We show that the gluing method overestimates the leading order correction to the spectrum and bispectrum by one and two orders, respectively, in $\frac{H}{p_c}$. We discuss the various approximation and conclude that for dispersion relations not violating WKB at early times the particle creation is small and does not lead to enhanced contributions to the bispectrum. We also show that in many cases enhancements do not occur when modeling the trans-Planckian physics via modifications of the vacuum at a certain cutoff scale. Most notably they are only of order O(1) when the Bogolyubov coefficients accounting for particle creation are determined by the Wronskian condition and the minimization of the uncertainty between the field and its conjugate momentum.