The Cosmological Constant and Quintessence from a Correlation Function Comoving Fine Feature in the 2dF Quasar Redshift Survey

16 pages, 15 figures, accepted, Astronomy & Astrophysics, minor changes in revised versionDetections of local maxima in the density perturbation spectrum at characteristic comoving scales L~100-200h^{-1}Mpc have previously been claimed. Here, this cosmic standard ruler is sought in the ``10K\'\' release of the 2dF QSO Redshift Survey (2QZ-10K), by estimating the comoving, spatial correlation functions \\xi(r) of the three-dimensional distribution of the N=2378 quasars in the most completely observed and ``covered\'\' sky regions, over the three redshift ranges 0.6 < z < 1.1, 1.1 < z < 1.6 and 1.6 < z < 2.2. Because of the selection method of the survey and sparsity of the data, the analysis was done conservatively to avoid non-cosmological artefacts. (i) Avoiding á priori\' estimates of the length scales of features, local maxima in \\xi(r) are found in all three redshift ranges. The requirement that a local maximum be present in all three redshift ranges at a fixed comoving length scale implies strong, purely geometric constraints on the local cosmological parameters. The length scale of the local maximum common to the three redshift ranges is 2L= (244\\pm17)h^{-1}Mpc. (ii) For a standard FLRW model, the matter density \\Omm and cosmological constant \\Omega_\\Lambda are constrained to \\Omm= 0.25\\pm0.10, \\Omega_\\Lambda=0.65\\pm0.25 (68% confidence), \\Omm= 0.25\\pm0.15, \\Omega_\\Lambda=0.60\\pm0.35 (95%), respectively, *from the 2QZ-10K alone*. Independently of the SNe Ia data, the zero cosmological constant model (\\Omega_\\Lambda=0) is rejected at the 99.7% confidence level. (iii) For an effective quintessence (w_Q) model and zero curvature, w_Q<-0.5 (68%), w_Q<-0.35 (95%) are found, again *from the 2QZ-10K alone*