We compute the leading order contribution to the stress-energy tensor corresponding to the modes of a quantum scalar field propagating in a Friedmann-Robertson-Walker universe with arbitrary coupling to the scalar curvature, whose exact mode functions can be expanded as an infinite adiabatic series. While for a massive field this is a good approximation for all modes when the mass of the field m is larger than the Hubble parameter H, for a massless field only the subhorizon modes with comoving wave-numbers larger than some fixed k* obeying k*/a>H can be analyzed in this way. As infinities coming from adiabatic zero, second and fourth order expressions are removed by adiabatic regularization, the leading order finite contribution to the stress-energy tensor is given by the adiabatic order six terms, which we determine explicitly. For massive and massless modes these have the magnitudes H^6/m^2 and H^6a^2/k*^2, respectively, and higher order corrections are suppressed by additional powers of (H/m)^2 and (Ha/k*)^2. When the scale factor in the conformal time \eta is a simple power a(\eta)=(1/\eta)^n, the stress-energy tensor obeys P=w\rho with w=(n-2)/n for massive and w=(n-6)/(3n) for massless modes. In that case, the adiabaticity is eventually lost when 0<n<1 for massive and when 0<n<3/2 for massless fields since in time H/m and Ha/k* become order one. We discuss the implications of these results for de Sitter and other cosmologically relevant spaces.Comment: 15 pages, revtex
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