Search for Milli-lenses to discriminate between dark matter models

One of the most compelling mysteries in both cosmology and particle physics is the nature of Dark Matter (DM). We propose to investigate this problem using strong gravitational lensing of active galaxies on the key but poorly-explored milliarcsecond scales. Gravitational lensed images with angular separation on milliarcsecond scales probe gravitational lens systems where the lens is a compact object with mass in the range 10^6 - 10^9 solar masses. This mass range is particularly critical for the widely accepted Lambda-CDM cosmological model, which predicts many more DM sub-halos, i.e., DM halos on sub-galactic scales (masses below ~ 10^11 solar masses), than currently observed. The most direct way to explore these small angular scales is through the high-resolution of radio Very Long Baseline Interferometry (VLBI). We propose to use VLBI data on a complete and large sample of active galaxies (~ 5000 sources) to search for gravitational lens systems on milliarcsecond scales. Given that no gravitational lenses on milliarsecond scales have yet been found, if any of the gravitational lens candidates that this search will produce is indeed confirmed as a true gravitational lens system, this would be a first and a major discovery. A null result instead will allow us to infer a new constraint on the abundance of compact objects in the mass range of interest, with over an order of magnitude better precision than in previous studies, and tighter than the number of 10^6 - 10^9 solar masses subhalos predicted by Lambda-CDM. Such a constraint could help discriminate between DM models that predict different numbers of sub-halos in this mass range. It could also help to constrain a possible contribution of primordial black holes as a DM component.