Constraints on dark energy and quintessence with a comoving standard ruler applied to 2dF quasars

Structures on very large scales (> 100 Mpc) have negligible peculiar motions, and are thus roughly fixed in comoving space. We looked for significant peaks at very large separation in the two-point correlation function -- corrected for redshift selection effects -- of a well convered subsample of 2378 quasars of the recently released 10k sample of the 2dF quasar survey. Dividing our sample in three redshift intervals, we find a peak at 244/h Mpc, which is perfectly comoving for a restricted set of cosmological parameters, namely Omega_m = 0.25+/-0.15 and Omega_Lambda=0.65+/-0.35 (both at 95% confidence). Assuming a flat Universe, we constrain the quintessence parameter w_Q < -0.35 (95% confidence). We discuss the compatibility of our analysis with possible peaks in the power spectrum.