  We discuss the creation of massless particles in a Universe, which transits
from a radiation-dominated era to any other expansion law. We calculate in
detail the generation of gravitons during the transition to a matter dominated
era. We show that the resulting gravitons generated in the standard
radiation/matter transition are negligible. We use our result to constrain one
or more previous matter dominated era, or any other expansion law, which may
have taken place in the early Universe. We also derive a general formula for
the modification of a generic initial graviton spectrum by an early matter
dominated era.
