  Gravitational particle production in time variable metric of an expanding
universe is efficient only when the Hubble parameter $H$ is not too small in
comparison with the particle mass. In standard cosmology, the huge value of the
Planck mass $M_{Pl}$ makes the mechanism phenomenologically irrelevant. On the
other hand, in braneworld cosmology the expansion rate of the early universe
can be much faster and many weakly interacting particles can be abundantly
created. Cosmological implications are discussed.
