Cluster Lensing of QSOs as a Probe of LCDM and Dark Energy Cosmologies

Abstract

Wide-separation lensed QSOs measure the mass function and evolution of massive galaxy clusters, in a similar way to the cluster mass function deduced from X-ray-selected samples or statistical measurements of the Sunyaev-Zeldovich effect. We compute probabilities of strong lensing of QSOs by galaxy clusters in dark energy cosmologies using semianalytical modelling and explore the sensitivity of the method to various input parameters and assumptions. We highlight the importance of considering both the variation of halo properties with mass, redshift and cosmology and the effect of cosmic scatter in halo concentration. We then investigate the extent to which observational surveys for wide-separation lensed QSOs may be used to measure cosmological parameters such as the fractional matter density Omega_M, the rms linear density fluctuation in spheres of 8 Mpc/h, sigma_8, and the dark energy equation of state parameter w. We find that wide-separation lensed QSOs can measure sigma_8 and Omega_M in an equivalent manner to other methods such as cluster abundance studies and cosmic shear measurements. In assessing whether lensing statistics can distinguish between values of w, we conclude that at present the uncertainty in the calibration of sigma_8 in quintessence models dominates the conclusions reached. Nonetheless, lensing searches based on current QSO surveys such as the Two-degree Field and the Sloan Digital Sky Survey with 10^4-10^5 QSOs should detect systems with angular separations greater than 5'' and hence can provide an important test of the standard cosmological model that is complementary to measurements of cosmic microwave background anisotropies.