  We discuss the problem of defining a preferred vacuum state at a given time
for a quantized scalar field in Friedmann, Lema\^itre, Robertson Walker (FLRW)
space-time. Among the infinitely many homogeneous, isotropic vacua available in
the theory, we show that there exists at most one for which every Fourier mode
makes vanishing contribution to the adiabatically renormalized energy-momentum
tensor at any given instant. For massive fields such a state exists in the most
commonly used backgrounds in cosmology, and provides a natural candidate for
the ground state at that instant of time. The extension to the massless and the
conformally coupled case are also discussed.
