  We examine a cosmological model with a dark energy density of 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 $H^2(t)$. This form of $\rho_Z(t)$ follows from
the recent proposal that the contribution of zero-point fluctuations of quantum
fields to the total energy density should be computed by subtracting the
Minkowski-space result from that computed in the FRW space-time. We discuss
theoretical arguments that support this subtraction. By definition, this
eliminates the quartic divergence in the vacuum energy density responsible for
the cosmological constant problem. We show that the remaining quadratic
divergence can be reabsorbed into a redefinition of Newton's constant only
under the assumption that the energy-momentum tensor of vacuum fluctuations is
conserved in isolation. However, in the presence of an ultra-light scalar field
$X$ with $m_X<H_0$, as typical of some dark energy models, the gravity
effective action depends both on the gravitational field and on the $X$ field.
In this case general covariance only requires the conservation of the total
energy-momentum tensor, including both the classical term $T^X_{\mu\nu}$ and
the vacuum expectation value of T_{\mu\nu}. If there is an exchange of energy
between these two terms, there are potentially observable consequences. We
construct an explicit model with an interaction between $\rho_X$ and $\rho_Z$
and we show that the total dark energy density
$\rho_{DE}(t)=\rho_X(t)+\rho_Z(t)$ always remains a finite fraction of the
critical density at any time, providing a specific model of early dark energy.
We discuss the implication of this result for the coincidence problem and we
estimate the model parameters by means of a full likelihood analysis using
current CMB, SNe Ia and BAO data.
