Abstract: On rare occasions, a galaxy acts as gravitational lens producing multiple images of a quasar directly behind it.  The stars within this galaxy then act as micro-lenses, breaking up the “macro-images” into
“micro-images”.  As the stars move, the macro-images twinkle — the gravitational analog of atmospheric scintillation. Counterintuitively, the amplitude of the twinkling does not increase monotonically with stellar density, and instead decreases at high optical depth.  A single strongly micro-lensed quasar can set a significant upper limit on the graininess of the gravitational potential.  The poster-child for such a limit is SDSS0924+0219 for which at least 50% of the lens’ surface mass density must be in a smooth component rather a than grainy one.  A sample of ten lensed quasars gives a 10% upper limit on  the contribution of
LIGO-mass primordial black holes to the cosmological dark matter density after discounting the graininess due to the observed stars.

Encore: If you’re tempted to think about the gravitational lensing of gravity wave events, you might as well start thinking as well about the effects of micro-lensing.  Of particular interest might be

http://www.arXiv.org/pdf/1711.07919

A Quadruply Lensed SN Ia: Gaining a Time-Delay…Losing a Standard Candle

Authors: Daniel A. Yahalomi, Paul L. Schechter, Joachim Wambsganss

If you include the effects of micro-lensing it is difficult to recover the unlensed brightness of the supernova to better than 0.5 mag.