  The existence of a shadow world (or mirror universe) with matter and forces
identical to that of the visible world but interacting with the latter only via
gravity can be motivated by superstring theories as well as by recent attempts
to understand the nature of a sterile neutrino needed if all known neutrino
data are to be consistent with each other. A simple way to reconcile the
constraints of big bang nucleosynthesis in such a theory is to postulate that
the reheating temperature after inflation in the mirror universe is lower than
that in the visible one. We have constructed explicit models that realize this
proposal and have shown that the asymmetric reheating can be related to a
difference of the electroweak symmetry breaking scales in the two sectors,
which is needed for a solution of the neutrino puzzles in this picture.
Cosmological implications of the mirror matter are also discussed.
