Decay of the Cosmic Vacuum Energy

TL;DR

This paper explores the decay of vacuum energy in the universe, showing that coupling with radiation leads to a specific Hubble evolution, affects the universe's asymptotic states, and introduces early-time singularities.

Contribution

It demonstrates that vacuum decay coupled with radiation results in a unique Hubble evolution and alters the universe's late-time behavior, expanding on warm inflation models.

Findings

Hubble rate evolves as H∝ t^{-1/3} under vacuum-radiation coupling

Radiation-vacuum coupling can cause early-time singularities

Radiation can dominate over dust at late times in this model

Abstract

In his 2005 review, Gravity and the Thermodynamics of Horizons, Paddy suggested that a vacuum in thermal equilibrium with a bath of radiation should have a gradually diminishing energy. We work through the consequences of this scenario, and find that a coupling between the vacuum and a bath of black-body radiation at the temperature of the horizon requires the Hubble rate, $H$, to approach the same type of evolution as in the "intermediate inflation" scenario, with $H\propto t^{-1/3}$, rather than as a constant. We show that such behaviour does not conflict with observations when the vacuum energy is described by a slowly-rolling scalar field, and when the fluctuations in the scalar field are treated as in the "warm inflation" scenario. It does, however, change the asymptotic states of the universe. We find that the existence of the radiation introduces a curvature singularity at early times, where the energy densities in both the radiation and the vacuum diverge. Furthermore, we show that the introduction of an additional non-interacting perfect fluid into the space-time reveals that radiation can dominate over dust at late times, in contrast to what occurs in the standard cosmological model. Such a coupling can also lead to a negative vacuum energy becoming positive.