
We provide an explicit expression for the renormalized expectation value of the stressenergy tensor of a spin1/2 field in a spatially flat FriedmannLemaitreRobertsonWalker universe. Its computation is based on the extension of the adiabatic regularization method to fermion fields introduced recently in the literature. The tensor is given in terms of UVfinite integrals in momentum space, which involve the mode functions that define the quantum state. As illustrative examples of the method efficiency, we see how to compute the renormalized energy density and pressure in two interesting cosmological scenarios: a de Sitter spacetime and a radiationdominated universe. In the second case, we explicitly show that the latetime renormalized stressenergy tensor behaves as that of classical cold matter. We also check that, if we obtain the adiabatic expansion of the scalar field mode functions with a similar procedure to the one used for fermions, we recover the wellknown WKBtype expansion.
