Decay of the Cosmic Vacuum Energy

Understand

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.