I re-examine the classic problem of the renormalization of zero-point quantum fluctuations in a Friedmann-Robertson-Walker background. I discuss a number of issues that arise when regularizing the theory with a momentum-space cutoff, and show that non-covariant counter-terms allow to obtain covariant results for the renormalized vacuum energy-momentum tensor. Furthermore, I explore the structure of the gravity effective action that becomes richer if the theory contains a scalar field with a mass smaller than the Hubble parameter. Such an ultra-light particle cannot be integrated out completely to get the effective action. As a result, vacuum fluctuations of ultra-light scalar fields naturally lead to models where the dark energy density is tracking the total energy density. A detailed comparison of such models with CMB, SNIa and BAO data shows that they are not ruled out yet, but strongly constrained.

IPhT