Due to a peculiar effect that speed has on the appearance of the passage of time, it seems that time slowed down when the universe was just a baby.
At least, it seems to us, at a light travel time of almost 13 billion years away. This is called time dilationand astrophysicist Geraint Lewis of the University of Sydney in Australia and statistician Brendon Brewer of the University of Auckland first spotted it in the early universe by studying the fluctuations of bright galaxies called quasar galaxies during the Cosmic dawn.
Because of accelerated expansion of the universethey found, we see those fluctuations unfold five times slower than if they happened nearby.
It’s the most distant we’ve ever seen time dilation in action, and it solves several problems. It shows that quasars are consistent with the effect over vast waves of space-time, meaning they are not alone in accordance with the standard model of cosmology, we can account for time dilation in studies of their behavior.
“Looking back to a time when the universe was just over a billion years old, we see that time seems to flow five times slower,” explains Lewis.
“If you were there, in this young universe, a second would seem like a second — but from our position, more than 12 billion years in the future, that early time seems to be dragging.”
Although it is not really noticeable in our daily lives, space and time are inextricably linked in the universe. This way we can see the accelerated expansion of the universe. Light from much greater distances stretches out as space expands, shifting to longer, redder wavelengths as the distance from the source increases.
This effect is called the Doppler effects, and it can also be experienced here on Earth. Think of the way the sound of an ambulance siren seems to stretch as the ambulance pulls away from you.
In this analogy, the ambulance becomes a distant galaxy and the light is the siren. At the source, the emission is normal, but from our perspective it all stretches out.
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Something similar should — and does — happen over time, as we’ve seen with supernova explosions about halfway through the observable universe.
Time passes normally for us. For someone loitering near the supernova explosion, time also seems to pass normally. But because of the relative speed between the two points, the supernova appears to us to be happening in slow motion.
It has been predicted that quasars in the early universe should show a similar effect, but they are different kinds of objects than supernovae. Quasar galaxies are galaxies with an actively feeding supermassive black hole at their center. The feeding process produces a lot of light as the material around the black hole heats up and flickers with turbulence.
“Where supernovae behave like a single flash of light, making them easier to study, quasars are more complex, like a continuous firework display,” says Lewis. “What we’ve done is unravel this firecracker, which shows that quasars, too, can be used as standard markers of time for the early universe.”
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Lewis and Brewer studied a sample of 190 quasars between 2.45 and 12.17 billion years ago (the Big Bang happened 13.8 billion years ago), with data on a range of wavelengths collected over a two-decade period. They had about 200 observations for each quasar, allowing for detailed reconstructions of their fluctuations.
Previously, scientists thought that quasar variability did not show the effects of time dilation, but the samples were small and observed over a much shorter period of time.
By dramatically expanding both the number of quasars and the duration of observations, the two researchers found that they appear to flicker in slow motion, compared to more recent quasars.
“Previous studies led people to question whether quasars are really cosmological objects, or even whether the idea of expanding space is correct,” says Lewis. “However, with this new data and analysis, we have been able to find the elusive tick of the quasars, and they behave exactly as Einstein’s theory of relativity predicts.”
The research has been published in Nature Astronomy.