By comparing the results from numerical microlensing simulations to the observed long-term variability of quasars , strong upper limits on the cosmological density of compact objects in the 10 ^ { -4 } M _ { \odot } – 1 M _ { \odot } range may in principle be imposed .
Here , this method is generalized from the Einstein-de Sitter universe to the currently favored \Omega _ { \mathrm { M } } = 0.3 , \Omega _ { \Lambda } = 0.7 cosmology and applied to the latest observational samples .
We show that the use of high-redshift quasars from variability-selected samples has the potential to substantially improve current constraints on compact objects in this mass range .
We also investigate to what extent the upper limits on such hypothetical dark matter populations are affected by assumptions concerning the size of the optical continuum-emitting region of quasars and the velocity dispersion of compact objects .
We find that mainly due to uncertainties in the typical value of the source size , cosmologically significant populations of compact objects can not safely be ruled out with this method at the present time .