On the importance of radiation-era initial conditions for tensor perturbations

TL;DR

This paper investigates how the initial conditions of radiation during reheating affect the conservation of super-horizon tensor fluctuations, reconciling conflicting results in the literature.

Contribution

It demonstrates that the physical initial state during reheating preserves tensor conservation, resolving previous discrepancies caused by different assumptions about radiation states.

Findings

Super-horizon tensor fluctuations are conserved when initial conditions are physically realistic.

Global thermal equilibrium assumptions lead to suppressed tensor amplitudes.

The study clarifies the impact of initial radiation states on tensor perturbation evolution.

Abstract

Conservation of super-horizon tensor fluctuations is crucial for connecting inflation to observations. Starting from first principles, recent works have found violations of this conservation if free-streaming radiation is produced during reheating. We show that the non-conservation is sensitive to the radiation initial state, and argue that the physical state should be affected by tensor perturbations that are already present during reheating. The deviation from super-horizon conservation is then negligible, recovering the standard result from kinetic theory. In contrast, a globally homogeneous and isotropic plasma state leads to a large suppression of tensor amplitudes. This difference between the local (physical) and global thermal equilibrium settles the discrepancy between the older and recent literature.