Scientists using quasars as cosmic clocks have observed that when the universe was just a billion years old, time ran five times slower, helping to confirm Einstein’s general theory of relativity and the expansion of the universe.
By studying distant supernovae, it’s been well established that the universe has been expanding since the Big Bang 13.8 billion years ago. As it does so, the galaxies have been spreading away from one another across four dimensions, allowing for no objective center to the expansion.
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As they spread, they move faster in relation to one another. One thing this does is cause the spectra of stars and other objects to shift red-ward due to the Doppler effect, which can be measured, so scientists can clock the spread. The other thing it does is that, eventually, the expansion becomes so fast at the edge of the universe, which we see as the distant past, that relativistic effects become apparent, causing shifts in how fast time passes compared to us.
Read more: New Atlas
