The Search for Cosmological Black Holes: A Surface Brightness Variability Test

Recently it has been suggested that the majority of dark matter in theuniverse resides in the form of Jupiter mass black holes distributedcosmologically. This population makes itself apparent by microlensing highredshift quasars and introducing pronounced variability into their observedlight curves. While several arguments dismissing this hypothesis have beenpresented, a conclusive observational test is, alas, sadly lacking. In thispaper we investigate the effect of a cosmologically distributed population ofmicrolensing masses on galaxies at low to intermediate redshift. Themagnification of bright stars in these galaxies leads to small, but observable,fluctuations in their surface brightness. The variability time scale forJupiter-mass lensing objects is of the order of a few months and thispopulation may be detected through a future space-based monitoring campaign ofa field containing $z \sim 0.5$ galaxies. The monitoring of galactic surfacebrightness will provide an effective test of the nature of dark matter oncosmological scales.Comment: 19 pages, 9 figures, Accepted for publication in the Astrophysical Journa