  The stability of the electroweak vacuum imposes important constraints on new
physics models. Such new physics models may introduce one or more new thermal
phases with a lower free energy than that of the electroweak vacuum. In this
case, the early universe may stay or have already evolved into one of these
deeper vacuum states. We investigate this possibility in detail in the singlet
extension of the Standard Model, and delineate the corresponding constraints in
its parameter space. We also discuss the situation in supersymmetry as another
example. To account for the possibility that the universe is trapped in a
non-electroweak vacuum, we propose a procedure of calculating the vacuum
stability limit efficiently.
