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Vacuum and particles can be naturally defined in the adiabatic regime of anexpanding universe. In general, however, there is no preferred choice of avacuum state, unless the spacetime background possesses special symmetries. Inthe absence of symmetries the standard viewpoint is to construct distinguishedadiabatic states permitting renormalizability of the stress-energy tensor. Westudy a special non-adiabatic vacuum for a massive scalar field in aradiation-dominated universe defined by imposing early-times conformalsymmetry. We show that this state is consistent with renormalization, despiteits ultraviolet behaviour is not the conventional one due to the emergence ofoscillatory terms.

Renormalization and a non-adiabatic vacuum choice in a radiation-dominated universe