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We study the backreaction on the mean field geometry due to non-conformal quantum fields in a Robertson-Walker background. For weakly non-conformal fields we compute perturbatively the expectation value of the stress tensor operator corresponding to a variety of vacuum states, and use it to obtain explicitly the semiclassical gravity solutions for isotropic perturbations around de Sitter spacetime, which is found to be stable. Our results show clearly the crucial role of the non-local terms that appear in the effective action: they cancel the contribution from local terms proportional to the logarithm of the scale factor which would otherwise become dominant at late times and prevent the existence of a stable self-consistent de Sitter solution. In the opposite regime, namely, the case of strongly non-conformal fields with a large mass, we obtain the effective action for a general spacetime geometry. The kind of semiclassical corrections that it generates as well as the implications for dark energy models based on the renormalization-group running of the cosmological constant due to massive fields in the decoupling regime are discussed.

Backreaction from weakly and strongly non-conformal fields in de Sitter spacetime