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Cosmological time dilation in the early Universe

Scientists have confirmed that just 1.5 billion years after the Big Bang, time ran five times slower than it does today, 13.8 billion years later. Though scientists have long been aware that conditions just after Big Bang were radically different than those in the cosmos we see around us today, the discovery shows that time is relative in regards to the age of the Universe, too, just like Einstein predicted.

The breakthrough is detailed in a new paper published in Nature Astronomy authored by Geraint Lewis, an astrophysicist at the Sydney Institute for Astronomy at the University of Sydney, and Brendon Brewer from the University of Auckland’s Department of Statistics, who made the long-awaited observation of cosmological time dilation in the early Universe.

“What we observed essentially is that when we look at the distant universe, we see it run in slow motion. It’s like watching a movie that’s been slowed down; whatever is going on out there is running slower,” Lewis explained. “The further back we look, the slower things seem to occur. At its heart, this is ‘Einstein is right… again’ type research.”

The ticks of quasar clocks prove time is relative

Albert Einstein’s 1915 theory of general relativity makes the staggering prediction that not only would space be radically different in the early Universe, but time itself would progress differently in the infant cosmos, which was substantially hotter and denser than the Universe we see today. As the Universe evolved, space expanded, and matter cooled and became less dense, time should have begun to run more quickly. 

This is because general relativity, also known as the geometric theory of gravity, hinges on the fact that the three dimensions of space and the one dimension of time exist as a unified 4D entity called “spacetime”.

General relativity predicts that when an object on mass is placed in spacetime, it warps its very fabric, creating a “dent” in this 4D surface — for space, that can be visualized in two dimensions as a stretched rubber sheet with balls of increasing mass placed upon it. A marble sitting on the sheet creates more of a “dent” than a pea, a bowling ball creates more warping than a marble, and a cannonball creates the most extreme warping of all. 

The same is true of cosmic objects and their effect on space. Thus, stars create more warping than planets, and black holes create a greater “dent” in space than stars (or probably anything else). While that is a mind-blowing concept in itself, because space and time are unified in general relativity, that warping effect also extends to time. That means that time runs more slowly close to an object of great mass. 

This effect, called time dilation, has been confirmed many times around Earth. For instance, the global positioning system (GPS) technology we rely on for navigation wouldn’t work if the satellites they use didn’t have clocks that account for the fact that time runs more slowly at the surface of Earth than it does at their position in orbit. The time difference is minor but would quickly accumulate, eventually rendering GPS useless.

Time dilation should be present in the early Universe ,when the cosmos existed in a much denser state. Scientists have hunted for this cosmological time dilation in the light from quasars, the active hearts of galaxies powered by feeding supermassive black holes, but it has been elusive in the early Universe, leading researchers to wonder if general relativity’s gravitational recipe could be missing an ingredient or if astrophysicists don’t quite know what quasars actually are — that is, until now. 

Lewis emphasizes that what is important about this observation is that the time dilation predicted by Einstein is a fundamental aspect of the Universe, and it works just like the great physicist predicted.

“It’s the expansion of space itself, which is the thing that changes spacetime between then and now. And that’s the key thing that gives you this time dilation,” Lewis explained. “This slowing down of time doesn’t care about things like dark energy, dark matter, or any of the stuff in the universe. It is a property of spacetime itself.”

Quasars finally play ball…

To finally uncover cosmological time dilation, Lewis and Brewer searched through data collected over two decades by the Sloan Digital Sky Survey, which allowed them to study almost 200 quasars in detail.

Previously, researchers have used exploding stars in the form of supernovas to investigate time dilation as far back as half the age of the Universe, but quasars are more useful for going further back in time. This is because, unlike the single flashes of light from supernovas, the light from quasars evolves over time. The problem is, until now, Lewis explained that quasars have simply refused to cooperate with scientists. 

“So there was a mystery there; why didn’t quasars play ball? Why didn’t they play by the rules of the universe?” he said. While many scientists believed that this was the result of a lack of data, there was the possibility that quasars may actually be closer and less bright objects that had fooled astronomers into believing they were more distant. 

“That was my motivation to go back to this dataset. I thought the Sloan Digital Sky Survey data is probably the best data set available at the moment because they had 20 years’ worth of data, and they had regions with really, really good quasar sampling,” Lewis said. “And in the intervening period, during the last ten years, people have really gotten a handle on the statistical nature of quasar light curves.”

Lewis explained that the brightening and fading of quasar light, known as their variability is like the stock mark, unpredictable on the surface but with statistical properties that allow it to be modelled over time. This variability can be used like the ticking of a cosmic clock. It was this understanding and the complex quasar firework display from 190 of these feeding supermassive black holes that allowed the team to spot time dilation when the universe was just 1.5 billion years old, less than 10% of its current age. 

How would a time traveler experience time dilation?

Cosmological time dilation can clearly be seen by using powerful telescopes to look back in time, using light that has been travelling to us for around 12.3 billion years from ancient quasars. But how would a human, able to travel back to cosmic epoch, experience the progression of time?

“We get this entire picture from Einstein that all clocks are actually relative, but if I could actually take you and plunk you back 1.5 billion years after the Big Bang, time would seem completely normal,” Lewis said. “One second would feel like one second everything around you would be proceeding at the rate which physics predicts.”

In this time travel experiment, it would only be possible to spot the effect of time dilation when two synchronised clocks, one which journeys with the time traveller to the Universe’s early epoch and one which stayed in our time, were brought back together. 

Fortunately, the ability to look back in time using distant quasars does away with the need for such a likely impossible time-travel experiment. It may be a while, however, before scientists can look for cosmological time dilation even earlier in cosmic history. This is because investigations like that conducted by Lewis and Brewer need many quasars to be observed for decades to develop a light curve.

“We are actually in a bit of a transition here in astronomy,” he said. “We have all these new facilities coming online in the next few years, and the James Webb Space Telescope is starting to find quasars and galaxies at ridiculously great distances, but we also need the light curves, and we need to know how things vary over time, and that takes decades.”

While Lewis expressed that as a cosmologist, he has a love of finding new and surprising elements of the Universe, in this case, he said that he is happy not to be surprised.

“We want to get to our next generation of theories, so it’s relieving that we don’t have to go back and rewrite a century’s worth of cosmology and relativity and everything else associated with gravity,” he concluded. 

Reference: G. F. Lewis., B. J. Brewer., Detection of the cosmological time dilation of high-redshift quasars, Nature Astronomy, 2023, DOI: 10.1038/s41550–023–02029–2

Feature image credit: Daniele Levis Pelusi on Unsplash

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