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 antiparticle 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 the de Sitter and electric field backgrounds respectively 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.