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Gravitational Lensing Statistics in Universes Dominated by Dark Energy

The distribution of image separations in multiply imaged gravitational lens systems can simultaneously constrain the core structure of dark matter halos and cosmological parameters. We study lens statistics in flat, low-density universes with different equations of state w = pQ/ρQ for the dark energy component. The fact that dark energy modifies the distance-redshift relation and the mass function of dark matter halos leads to changes in the lensing optical depth as a function of image separation Δθ. Those effects must, however, be distinguished from effects associated with the structure of dark matter halos. Baryonic cooling causes galaxy-mass halos to have different central density profiles than group- and cluster-mass halos, which causes the distribution of normal arcsecond-scale lenses to differ from the distribution of "wide-separation" (Δθ ≳ 4′′) lenses. Fortunately, the various parameters related to cosmology and halo structure have very different effects on the overall image separation distribution: (1) the abundance of wide-separation lenses is extremely sensitive (by orders of magnitude) to the distribution of "concentration" parameters for massive halos modeled with the Navarro-Frenk-White profile, (2) the transition between normal and wide-separation lenses depends mainly on the mass scale where baryonic cooling ceases to be efficient, and (3) dark energy has effects at all image separation scales. While current lens samples cannot usefully constrain all of the parameters, ongoing and future imaging surveys should discover hundreds or thousands of lenses and make it possible to disentangle the various effects and constrain all of the parameters simultaneously. Incidentally, we mention that for the sake of discovering lensed quasars, survey area is more valuable than depth.