The Search for Cosmological Black Holes: A Surface Brightness   Variability Test
  Recently it has been suggested that the majority of dark matter in the universe resides in the form of Jupiter mass black holes distributed cosmologically. This population makes itself apparent by microlensing high redshift quasars and introducing pronounced variability into their observed light curves. While several arguments dismissing this hypothesis have been presented, a conclusive observational test is, alas, sadly lacking. In this paper we investigate the effect of a cosmologically distributed population of microlensing masses on galaxies at low to intermediate redshift. The magnification of bright stars in these galaxies leads to small, but observable, fluctuations in their surface brightness. The variability time scale for Jupiter-mass lensing objects is of the order of a few months and this population may be detected through a future space-based monitoring campaign of a field containing $z \sim 0.5$ galaxies. The monitoring of galactic surface brightness will provide an effective test of the nature of dark matter on cosmological scales. 
Geraint F. Lewis & Rodrigo A. Ibata