Gerasimos Rigopoulos (Newcastle University, United Kingdom)
We show how the full quantum description of a scalar field with quartic self-interaction in de Sitter spacetime is equivalent to Brownian motion of a particle in a medium of de Sitter temperature T_DS = H/2π on large wavelengths. We then argue that the system exhibits a fluctuation-dissipation relation and its equilibrium distribution is Maxwell-Boltzmann, implying kinetic and potential energies of comparable magnitudes. The stochastic kinetic energy of the field causes de Sitter spacetime to cool down slowly with a corresponding decrease of the effective vacuum energy. The transition to equilibrium is a semiclassical process beyond the scope of perturbation theory for interacting fields.