  The recently published analytic probability density function for the mildly
non-linear cosmic density field within spherical cells is used to build a
simple but accurate maximum likelihood estimate for the redshift evolution of
the variance of the density, which, as expected, is shown to have smaller
relative error than the sample variance. This estimator provides a competitive
probe for the equation of state of dark energy, reaching a few percent accuracy
on wp and wa for a Euclid-like survey. The corresponding likelihood function
can take into account the configuration of the cells via their relative
separations. A code to compute one-cell density probability density functions
for arbitrary initial power spectrum, top-hat smoothing and various spherical
collapse dynamics is made available online so as to provide straightforward
means of testing the effect of alternative dark energy models and initial
power-spectra on the low-redshift matter distribution.
