Cosmic Replay is one of the most spectacular examples of how fascinating our universe could possibly get: Provided with the right conditions, it operates in such a way that it allows us to see a cosmic event re-play. Let’s see how it works:
The phenomenon is a direct consequence of Einstein’s theory of general relativity in which he proposed that space-time is curved under the influence of mass and energy(1915). Soon after its formulation, scientists at the time embarked on a series of experiments aimed at the predictions of the general relativity. One of them was the expedition conducted by Eddington to observe the 1919’s solar eclipse off Africa. They reasoned, if space-time is curved by mass, then the paths light rays take in the gravitational field would also be curved and the effect would be noticeable in the light emitted by the background stars that are normally outshined by the Sun during the day but become visible in solar eclipses during which the sunshine is mostly blocked out by the Moon. They observed that the gravity of the Sun deflected the light rays from the background stars that happened to pass by, making them appear slightly different from where they actually are in great accordance with the theory.
Today this effect is known as the ‘’gravitational lensing’’ since the deflecting body behaves like a lens that focuses light.
To date, astronomers have caught a lot of weird pictures that are explained by gravitational lensing; however, the most interesting case is the ‘’Supernova Refsdal’’ that happened only once far behind MACS J1149.5+2223 galaxy cluster.
What’s interesting about it is that this massive intervening galaxy cluster acted as a lens and focused split images of the supernova to the Hubble. Since the paths the photons emanating from the supernova follow are all different, they arrived at the Earth at different times. With as large as a decade interval!
For instance, one of the images (the upper red circle) reached the Earth in around 1995. One (the lower circle) did in 2014 as four separate bright spots, forming an ‘’Einstein cross’’(This is also a result of lensing). Estimating and studying the amount of matter combined with dark matter and how they’re distributed across the cluster, astronomers predicted another path for the photons. And they started pointing the Hubble at the cluster’s surrounding periodically. Finally, on the 11th of December 2015, they captured the anticipated re-appearance of the Supernova Refsdal(in the middle circle), making it the first time ever to predict a supernova. They are essentially seeing the same explosion take place over and over again, which is nothing but a short of a Cosmic Replay.