Thermal radiation produced by the expansion of the Universe

I show here that the expansion of the Universe can create particles with a black-body distribution. I give a simple model in which the creation of particles in an initially empty space time leads at very early times to a Friedmann expansion dominated by hot relativistic particles (probably including gravitons) having a black-body spectrum, even before classical thermalising processes have had a chance to act. If the particle creation occurs near the Planck time, the entropy produced is consistent, in order of magnitude, with that required by the Einstein equations. The conditions in which one achieves consistency with the observed 3K black-body radiation are also discussed.