Flickering quasar images

Distant quasars tend to change their brightness, causing them to flicker. As the light which creates the different images of the quasar follows paths with slightly different lengths, the images do not flicker simultaneously but are delayed with respect to each other by several days. This delay in flickering can be used to measure the Hubble constant which describes the speed of expansion of our Universe.

While the relative time between two flickers is correctly represented in this animation, in reality the delays are in the range of days to two weeks.