De Sitter Stability and Coarse Graining
  We present a 4-dimensional back reaction analysis of de Sitter space for a
conformally coupled scalar field in the presence of vacuum energy initialized
in the Bunch-Davies vacuum. In contrast to the usual semi-classical
prescription, as the source term in the Friedmann equations we use expectation
values where the unobservable information hidden by the cosmological event
horizon has been neglected i.e. coarse grained over. It is shown that in this
approach the energy-momentum is precisely thermal with constant temperature
despite the dilution from the expansion of space due to a flux of energy
radiated from the horizon. This leads to a self-consistent solution for the
Hubble rate, which is gradually evolving and at late times deviates
significantly from de Sitter. Our results hence imply de Sitter space to be
unstable in this prescription. The solution also suggests dynamical vacuum
energy: the continuous flux of energy is balanced by the generation of negative
vacuum energy, which accumulatively decreases the overall contribution.
Finally, we show that our results admit a thermodynamic interpretation which
provides a simple alternate derivation of the mechanism. For very long times
the solutions coincide with flat space.
