  Vacuum and particles can be naturally defined in the adiabatic regime of an
expanding universe. In general, however, there is no preferred choice of a
vacuum state, unless the spacetime background possesses special symmetries. In
the absence of symmetries the standard viewpoint is to construct distinguished
adiabatic states permitting renormalizability of the stress-energy tensor. We
study a special non-adiabatic vacuum for a massive scalar field in a
radiation-dominated universe defined by imposing early-times conformal
symmetry. We show that this state is consistent with renormalization, despite
its ultraviolet behaviour is not the conventional one due to the emergence of
oscillatory terms.
