  In this paper we study a massless, minimally coupled scalar field in a FLRW
spacetime with periods of different constant deceleration parameter. We assume
the Bunch-Davies vacuum during inflation and then use a sudden matching
approximation to match it onto radiation era and subsequently onto matter era.
We then proceed to calculate the one-loop energy-momentum tensor from the
inflationary quantum vacuum fluctuations in different eras. The energy-momentum
tensor has the form of an ideal (quantum) fluid, characterized by an equation
of state. When compared with the background, far away from the matching the
quantum energy density in radiation era exhibits a contribution that grows
logarithmically with the scale factor. In matter era the ratio of the quantum
to classical fluid settles eventually to a tiny constant, (rho_q) / (rho) =
10^(-13) for a grand unified scale inflation. Curiously, the late time scaling
of quantum fluctuations suggests that they contribute a little to the dark
matter of the Universe, provided that it clusters as cold dark matter, which
needs to be checked.
