Cosmological Perturbations From Inhomogeneous Reheating, Freeze-Out, and Mass Domination

TL;DR

This paper extends the origin of primordial metric perturbations by allowing light-field fluctuations during inflation to induce spatial variations in particle masses and decay rates, making reheating, freeze-out, and mass domination inhomogeneous. It derives how these inhomogeneities translate into curvature perturbations and computes the resulting non-Gaussianity. A key finding is that a mass-domination realization yields a universal non-Gaussianity f_NL = 5, independent of microphysics, with other scenarios producing different f_NL values that can still be observable. The work highlights a broad, potentially dominant post-inflation mechanism for perturbations and emphasizes the observational significance of non-Gaussianity in this framework.

Abstract

We generalize a recently proposed mechanism for the origin of primordial metric perturbations in inflationary models. Quantum fluctuations of light scalar fields during inflation give rise to super-horizon fluctuations of masses and reaction rates of various particles. Reheating, freeze-out, and matter-domination processes become inhomogeneous and generate super-horizon metric perturbations. We also calculate the degree of non-Gaussianity $f_{nl}$ for this new model of cosmological perturbations. The precise value of $f_{nl}$ depends on the specific models, but $|f_{nl}|\sim$few is a natural lower bound for our mechanisms. This is much larger than the currently assumed theoretical value $f_{nl}\sim tilt \lesssim 0.05$, and is thought to be observable. In a particularly attractive model of inhomogeneous mass-domination, the non-Gaussianity of perturbations generated by our mechanism is simply $f_{nl}=5$, irrespective of the detailed structure of the underlying field theory.