Transient weak lensing by cosmological dark matter microhaloes

We study the time variation of the apparent flux of cosmological point sources due to the transient weak lensing by dark matter microhaloes. Assuming a transverse motion of microhaloes with respect to our line of sight, we derive the correspondence between the temporal power spectrum of the weak lensing magnification and the spatial power spectrum of density on small scales. Considering different approximations for the small-scale structure of dark matter, we predict the apparent magnitude of cosmological point sources to vary by as much as 10(-4) -10(-3), due to this effect, within a period of a few months. This red photometric noise has an almost perfect Gaussian statistics, to one part in similar to 10(4). We also compare the transient weak lensing power spectrum with the background effects such as the stellar microlensing on cosmological scales. A quasar lensed by a galaxy or cluster like a SDSSJ1004 + 4112 strong lensing system, with multiple images, is a suitable system for this study because (i) using the time-delay method between different images, we can remove the intrinsic variations of the quasar, and (ii) strong lensing enhances signals from the transient weak lensing. We also require the images to form at large angular separations from the center of the lensing structure, in order to minimize contamination by the stellar microlensing. With long-term monitoring of quasar strong lensing systems with a 10-meter class telescope, we can examine the existence of dark microhaloes as the building blocks of dark matter structures. Failure to detect this signal may either be caused by a breakdown of cold dark matter (CDM) hierarchy on small scales, or rather interpreted as evidence against a CDM paradigm, e.g., in favor of modified gravity models.