  We search for the most suitable set of cosmological parameters that describes
the observable universe. The search includes the possibility of quintessential
flat universes, i.e., the analysis is restricted to the determination of the
dimensionless matter density and the quintessential parameters, $\Omega_{\rm
M}$ and $w_{\rm Q}$, respectively. Our study is focused on comparing the
position of features at large scales in the density fluctuation field at
different redshifts by analysing the evolution of the quasar two-point
correlation function. We trace the density field fluctuations at large scales
using a large and homogeneous sample of quasars ($\sim$ 38000 objects with 0.3
$\lesssim$ z $\le$ 2.4 and a median $z=1.45$) drawn from the Sloan Digital Sky
Survey Data Release Six. The analysis relies on the assumption that, in the
linear regime, the length scale of a particular feature should remain fixed at
different times of the universe for the proper cosmological model. Our study
does not assume any particular comoving length scale at which a feature should
be found, but intends to perform a comparison for a wide range of scales
instead. This is done by quantifying the amount of overlap among the quasar
correlation functions at different times using a cross-correlation technique.
The most likely cosmological model is $\Omega_{\rm M}=0.21\pm 0.02$ and $w_{\rm
Q}=-0.93\pm0.04$, in agreement with previous studies. These constraints are the
result of a good overall agreement of the correlation function at different
redshifts over scales $\sim 100-300\mpc$. Under the assumption of a flat
cosmological model, our results indicate that we are living in a low density
universe with a quintessential parameter greater than the one corresponding to
a cosmological constant.
