Curvature perturbation from velocity modulation

TL;DR

This paper introduces a novel velocity-modulated reheating model where large-scale velocity fluctuations of particles induce curvature perturbations, especially significant for semi-relativistic particles at decay.

Contribution

It presents a new mechanism for curvature perturbation generation via velocity modulation, expanding the understanding of reheating processes in the early universe.

Findings

Velocity fluctuations induce curvature perturbations.

Non-linearity parameters are explicitly calculated.

Effect is significant for semi-relativistic particles at decay.

Abstract

We propose a new variant model of the modulated reheating. If particles have large scale fluctuations on their velocities, or equivalently their Lorentz factors, the decay rate also fluctuates and the curvature perturbation is induced via their decay processes in analogy with modulated reheating. For example, if they are produced nonthermally by the decay of another field with its mass fluctuating on large scales, such a situation is realized. We explicitly calculate the resulting curvature perturbation and non-linearity parameters and show that the effect of velocity-modulation is not negligible if the particles are semi-relativistic at the decay.