> Lewis explained that the brightening and fading of quasar light, known as their variability is like the stock mark, unpredictable on the surface but with stat

> Lewis explained that the brightening and fading of quasar light, known as their variability is like the stock mark, unpredictable on the surface but with statistical properties that allow it to be modelled over time. This variability can be used like the ticking of a cosmic clock. It was this understanding and the complex quasar firework display from 190 of these feeding supermassive black holes that allowed the team to spot time dilation when the universe was just 1.5 billion years old, less than 10% of its current age.