Zero-point quantum fluctuations in cosmology

TL;DR

This paper revisits the renormalization of zero-point quantum fluctuations in cosmology, clarifies the vacuum energy's equation of state, and explores how ultra-light scalar fields influence dark energy models consistent with observational data.

Contribution

It introduces a covariant renormalization approach, clarifies the vacuum equation of state, and develops models where ultra-light scalar fields contribute to dark energy with testable predictions.

Findings

Covariant regularization yields consistent vacuum energy-momentum tensor.

Ultra-light scalar fields induce additional terms in the effective action.

Models with H^2(t) contributions fit cosmological data well.

Abstract

We re-examine the classic problem of the renormalization of zero-point quantum fluctuations in a Friedmann-Robertson-Walker background. We discuss a number of issues that arise when regularizing the theory with a momentum-space cutoff, and show explicitly how introducing non-covariant counter-terms allows to obtain covariant results for the renormalized vacuum energy-momentum tensor. We clarify some confusion in the literature concerning the equation of state of vacuum fluctuations. Further, we point out that the general structure of the effective action becomes richer if the theory contains a scalar field phi with mass m smaller than the Hubble parameter H(t). Such an ultra-light particle cannot be integrated out completely to get the effective action. Apart from the volume term and the Einstein-Hilbert term, that are reabsorbed into renormalizations of the cosmological constant and Newton's constant, the effective action in general also has a term proportional to F(phi)R, for some function F(phi). As a result, vacuum fluctuations of ultra-light scalar fields naturally lead to models where the dark energy density has the form rho_{DE}(t)=rho_X(t)+rho_Z(t), where rho_X is the component that accelerates the Hubble expansion at late times and rho_Z(t) is an extra contribution proportional to H^2(t). We perform a detailed comparison of such models with CMB, SNIa and BAO data.