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

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.