Although controversial , the scenario of microlensing as the dominant mechanism for the long-term optical variability of quasars does provide a natural explanation for both the statistical symmetry , achromaticity and lack of cosmological time dilation in quasar light curves .
Here , we investigate to what extent dark matter populations of compact objects allowed in the currently favored \Omega _ { \mathrm { M } } = 0.3 , \Omega _ { \Lambda } = 0.7 cosmology really can explain the quantitative statistical features of the observed variability .
We find that microlensing reasonably well reproduces the average structure function of quasars , but fails to explain both the high fraction of objects with amplitudes higher than 0.35 magnitudes and the mean amplitudes observed at redshifts below one .
Even though microlensing may still contribute to the long-term optical variability at some level , another significant mechanism must also be involved .
This severely complicates the task of using light-curve statistics from quasars which are not multiply imaged to isolate properties of any cosmologically significant population of compact objects which may in fact be present .