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We extend the method of adiabatic regularization by introducing an arbitraryparameter $\mu$ for a scalar field with quartic self-coupling in aFriedmann-Lema\^itre-Robertson-Walker (FLRW) spacetime at one-loop order. Thesubtraction terms constructed from this extended version allow us to define apreferred vacuum state at a fixed time $\eta = \eta_0$ for this theory. Wecompute this vacuum state for two commonly used background fields in cosmology.We also give a possible prescription for an adequate value for $\mu$.

Adiabatic regularization and preferred vacuum state for the $\lambda \phi^4$ field theory in cosmological spacetimes