On the Instability of Global de Sitter Space to Particle Creation
  We show that global de Sitter space is unstable to particle creation, even
for a massive free field theory with no self-interactions. The O(4,1) de Sitter
invariant state is a definite phase coherent superposition of particle and
anti-particle solutions in both the asymptotic past and future, and therefore
is not a true vacuum state. In the closely related case of particle creation by
a constant, uniform electric field, a time symmetric state analogous to the de
Sitter invariant one is constructed, which is also not a stable vacuum state.
We provide the general framework necessary to describe the particle creation
process, the mean particle number, and dynamical quantities such as the
energy-momentum tensor and current of the created particles in both the de
Sitter and electric field backgrounds in real time, establishing the connection
to kinetic theory. We compute the energy-momentum tensor for adiabatic vacuum
states in de Sitter space initialized at early times in global S^3 sections,
and show that particle creation in the contracting phase results in
exponentially large energy densities at later times, necessitating an inclusion
of their backreaction effects, and leading to large deviation of the spacetime
from global de Sitter space before the expanding phase can begin.
