Testing dark matter with high-redshift supernovae

Dark matter in the Universe consisting of macroscopic objects such as primordial black holes may cause gravitational lensing of distant objects. The magnification associated with lensing will lead to additional scatter in the received flux from standard candles, and too small an observed scatter could rule out compact dark matter entirely. In this paper, we show how the scatter in fluxes of distant Type la supernovae could be used to distinguish between models with and without lensing by macroscopic dark matter. The proposed Supernova/Acceleration Probe (SNAP) project, with similar to2400 supernovae in the range 0.1 less than or similar to z less than or similar to 1.7, should be able to identify between our set of cosmological models at 99.9 per cent confidence, if systematic errors are controlled. Note that this test is independent of any evolution of the mean supernova luminosity with redshift. The variances of the current Supernova Cosmology Project sample do not rule out compact lenses as dark matter: formally they favour such a population, but the significance is low, and removal of a single faint supernova from the sample reverses the conclusion.