Einstein's theory of relativity predicts that because space is expanding, "we should see the distant universe run in slow motion," said Geraint Lewis, an astrophysicist at the University of Sydney and the lead author of a new study.
Researchers had previously used observations of very bright exploding stars called supernovas as cosmic clocks to show that time ran twice as slowly back when the universe was half its current age.
The new study used even brighter quasars to peer further back into the history of the 13.8-billion-year-old universe.
Just over a billion years after the Big Bang, time appeared to flow five times slower, according to the study in the journal Nature Astronomy.
While "everything looks like it's slowed down" from here, Lewis emphasised that the experience of time in these distant places was not different.
"If I could magically transport you back 10 billion years and drop you next to one of these quasars, and you've got a stopwatch, time would just be normal," he told AFP.
"One second would be one second."
- Cosmic clocks -
Aiming to measure this phenomenon, which is called cosmological time dilation, Lewis and University of Auckland statistician Brendon Brewer analysed data from 190 quasars collected over two decades.
Quasars -- supermassive black holes at the centres of distant galaxies -- are thought to be the brightest and most powerful objects in the universe.
This makes them "useful beacons for charting the universe," Lewis said.
But they have proved more difficult to turn into cosmic clocks than supernovas, which provide a reliable single flash as a "tick".
Previous attempts to use quasars to measure time dilation had failed, leading to some "strange suggestions," Lewis said.
These included theories that perhaps quasars were not as distant as had been thought -- or even that "something fundamental was broken" in cosmology, he said.
But the new research "puts everything back in the right place," Lewis said.
It also confirmed that "Einstein is right again," he added.
The researchers were able to succeed where other attempts had fallen short because they had far more data on quasars, according to Lewis. Recent advances in the statistical understanding of randomness also helped.
To turn quasars into clocks with measurable ticks, the researchers had to make sense of the turbulent explosions that occurred as the black holes swallowed material.
Lewis compared it to watching a fireworks display, in which the great flashes seem random but different elements are "brightening and fading on their own kind of timescales".
"What we have done is unravel this firework display, showing that quasars, too, can be used as standard markers of time for the early universe."
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