Quasar clocks - another test of Einstein's GR

 Over 100 years later, and we continuously seem to be testing Einstein's ideas about space, time and gravity over and over, in numerous different contexts. The latest is a test of how the passage of time has changed as the universe has expanded! 

In Einstein's general theory of relativity (GR), he predicts the 'strength' of what we call gravity, which is really the consequence of the warping of space AND time, can change how quickly time passes. For someone living near a black hole, time would be passing very slowly compared to those of us living on earth, simply due to the gravity being MUCH stronger near a black hole compared to earth's gravity. 

Well, when the universe began the mass-energy density (which was really entirely energy until the universe expanded and cooled a bit so that matter could form) was large, and we might expect that time passed more slowly then when compared to the passage of time now, after the universe has expanded for 13.7 billion years and the densities have decreased tremendously. Because quasars are very far away, they 'lived' 12, 13 billion or more years ago, when the universe was young and time should have been running more slowly. By observing and measuring the light emitted from quasars so long ago, scientists have analysis techniques where they can compute how much that light was affected by the early universe compared to now, and the result is time had been running 5 times slower than time today!! These measurements once again are in line with Einstein's predictions from his GR field equations. Remarkable!!