Based on the Markovian master equation for a localized system embedded in de Sitter (dS) space, we find that the dS vacuum acts as a thermal bath at the Gibbons-Hawking temperature h / 2 \pi, where h is the Hubble parameter. An observer in the "cosmic rest frame" measures a background energy density proportional to h^4 (Stefan-Boltzmann law). By extending this result adiabatically to non-constant h and including the bath's contribution in the Friedmann equations, we arrive at a picture of the irreversible relaxation of the cosmological constant (h -> 0) and a graceful exit to inflation. Thermal particle production during inflation gives adiabatic, Gaussian, and approximately scale-invariant cosmological perturbations. No separate reheating phase is needed and the observed matter-antimatter asymmetry can be obtained with fewer parameters than in previous models of gravitational baryogenesis. This offers a simple and viable cosmology for the very early Universe without requiring any scalar fields, not even an inflaton.
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