Uncertainty relations for cosmological particle creation and existence of large fluctuations in reheating

TL;DR

This paper derives an uncertainty relation for quantum fields in cosmology, revealing large fluctuations that can significantly impact the reheating process after inflation, potentially disrupting thermalization.

Contribution

It introduces a new uncertainty relation for energy density and pressure in quantum fields during cosmological evolution, highlighting the impact of large fluctuations on reheating dynamics.

Findings

Large fluctuations exist in energy density and pressure during reheating.

Inflaton coherence can be lost rapidly due to these fluctuations.

Reheating may not lead to smooth thermal equilibrium as previously assumed.

Abstract

We derive an uncertainty relation for the energy density and pressure of a quantum scalar field in a time-dependent, homogeneous and isotropic, classical background, which implies the existence of large fluctuations comparable to their vacuum expectation values. A similar uncertainty relation is known to hold for the field square since the field can be viewed as a Gaussian random variable. We discuss possible implications of these results for the reheating process in scalar field driven inflationary models, where reheating is achieved by the decay of the coherently oscillating inflaton field. Specifically we argue that the evolution after backreaction can seriously be altered by the existence of these fluctuations. For example, in one model the coherence of the inflaton oscillations is found to be completely lost in a very short time after backreaction starts. Therefore we argue that entering a smooth phase in thermal equilibrium is questionable in such models and reheating might destroy the smoothness attained by inflation.