A fundamental consequence of the relativistic picture of expanding space is cosmological time dilation, where events in the distant Universe appear to run slowly compared to those in the local cosmos. Whilst this time dilation has been unambiguously detected in extremely distant supernovae, the appearance of time dilation in other distant sources is less conclusive. In new research, a duo of astrophysicists from the University of Sydney and the University of Auckland identified the cosmic time dilation in a sample of 190 quasars located in the early Universe.
Lewis & Brewer present the detection of the cosmological dependence of the time dilation in a recent sample of 190 quasars; these extremely distant objects were monitored in multiple wavebands over a two-decade period, allowing the determination of a characteristic timescale by treating the observed quasar variability as a damped random walk. Image credit: M. Weiss / Harvard-Smithsonian Center for Astrophysics.
Albert Einstein’s general theory of relativity means that we should observe the early Universe running much slower than the present day.
However, peering back that far in time has proven elusive. Astrophysicists have now cracked that mystery by using quasars as ‘clocks.’
“Looking back to a time when the Universe was just over a billion years old, we see time appearing to flow five times slower,” said University of Sydney’s Professor Geraint Lewis.
“If you were there, in this infant Universe, one second would seem like one second — but from our position, more than 12 billion years into the future, that early time appears to drag.”
To analyze this time dilation, Professor Lewis and his colleague, University of Auckland’s Dr. Brendon Brewer, used data from a sample of 190 quasars observed over two decades.
Combining the observations taken at different colors (or wavelengths), they were able to standardize the ‘ticking’ of each quasar.
Through the application of Bayesian analysis, they found the expansion of the Universe imprinted on each quasar’s ticking.
“Thanks to Einstein, we know that time and space are intertwined and, since the dawn of time in the singularity of the Big Bang, the Universe has been expanding,” Professor Lewis said.
“This expansion of space means that our observations of the early Universe should appear to be much slower than time flows today.”
“In our paper, we have established that back to about a billion years after the Big Bang.”
Previously, astrophysicists have confirmed this slow-motion Universe back to about half the age of the Universe using supernovae as ‘standard clocks.’
But while supernovae are exceedingly bright, they are difficult to observe at the immense distances needed to peer into the early Universe.
By observing quasars, this time horizon has been rolled back to just a tenth the age of the Universe, confirming that the Universe appears to speed up as it ages.
“Where supernovae act like a single flash of light, making them easier to study, quasars are more complex, like an ongoing firework display,” Professor Lewis said.
“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.”
The results further confirm Einstein’s picture of an expanding Universe but contrast earlier studies that had failed to identify the time dilation of distant quasars.
“These earlier studies led people to question whether quasars are truly cosmological objects, or even if the idea of expanding space is correct,” Professor Lewis said.
“With these new data and analysis, however, we’ve been able to find the elusive tick of the quasars and they behave just as Einstein’s relativity predicts.”
The findings appear today in the journal Nature Astronomy.
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Geraint F. Lewis & Brendon J. Brewer. 2023. Detection of the cosmological time dilation of high-redshift quasars. Nature Astronomy, in press; doi: 10.1038/s41550-023-02029-2
