Astronomers see time moving in slow motion in early universe

Geraint Lewis, University of Sydney

According to our best understanding of physics, the fact that space is expanding should influence the apparent flow of time, with the distant universe appearing to run in slow motion. But observations of highly luminous and variable galaxies – aka quasars – have failed to reveal this cosmic time dilation … until now.

Nature Astronomy published a new study on July 3, 2023. In it, we use two decades of observation to untangle the complex flickering of almost 200 quasars. Buried within this flickering is the imprint of expanding space. It shows the universe appearing to be ticking five times slower when it was only a billion years old.

Thus, we found that quasars obey the rules of the cosmos, putting to bed the idea they represented a challenge to modern cosmology.

Time moving in slow motion

In 1905, Albert Einstein, through his special theory of relativity, told us the speed of clocks’ ticking is relative. As in, it’s dependent on how the clocks are moving. In his 1915 general theory, he told us gravity too can influence the relative rates of clock ticks.

By the 1930s, physicists realized the expanding space of the cosmos, which is described in the language of Einstein’s general relativity, also influences the universe of ticks and tocks.

Due to the finite speed of light, as we look through our telescopes, we are peering into the past. The further we look, the further back into the life of the universe we see. But in our expanding universe, the further back we look, the more time space has had to stretch. Therefore, the more the relative nature of clock ticks grows.

The prediction of Einstein’s mathematics is clear: we should see the distant universe playing out in slow motion.

Tick-tock supernova clock

Measuring this slow-motion universe is difficult. Nature does not provide standard clocks across the cosmos whose relative ticks astronomers can easily compare.

It took until the 1990s for astronomers to discover and understand the tick of suitable clocks: a particular kind of exploding star, a supernova. Each supernova explosion was surprisingly similar. They brighten rapidly and then fade away over a matter of weeks.

Supernovae are similar, but not identical, meaning their rate of brightening and fading was not a standard clock. But by the close of the 20th century, astronomers were taking another look at these exploding stars, using them to chart the expansion of the universe. (This expansion turned out to be accelerating, leading to the unexpected discovery of dark energy.)

To achieve this goal, astronomers had to iron out peculiarities of each supernova. That involved putting them on an equal footing and matching them to a standard intrinsic brightness and a standard clock.

They found the flash of more distant supernovae stretched precisely in line with Einstein’s predictions. The most distant observed supernovae, exploding when the universe was half its present age, brightened and faded twice as slowly as more recent supernovae.

The trouble with quasars

Supernovae are not the only variable objects in the cosmos.

Astronomers discovered quasars in the 1960s. We believe they’re supermassive black holes, billions of times more massive than the sun, lurking at the hearts of galaxies. Matter swirls around these black holes, heating up and glowing brightly on its journey to oblivion.

Quasars are extremely bright, some burning furiously when the universe was an infant. Quasars are also variable, varying in luminosity as matter turbulently tumbles on its way to destruction.

Because quasars are so bright, we can see them at much greater distances than supernovae. So the impact of expanding space and time dilation should be more pronounced.

However, searches for the expected signal have turned up blank. Samples of hundreds of quasars observed over decades definitely varied, but it seemed that the variations of those nearby and those far away were identical.

Some suggested this variability is not intrinsic but due to black holes scattered through the universe, with gravity magnifying some quasars. More outlandishly, others claimed the lack of the expected cosmological signal is a sign that our cosmology is all wrong and need to go back to the drawing board.

Time moving in slow motion in new data

In 2023, astronomers published a new set of quasar data. This data set presented 190 quasars originally identified in the highly successful Sloan Digital Sky Survey, but with observations over two decades in multiple colors: green, red and infrared light.

The data sampling was mixed, with lots of observations over some times, and less over others. But the wealth of this data meant the astronomers, led by graduate student Zachary Stone at the University of Illinois, could statistically characterize each quasar’s variability as what is known as a damped random walk. This characterization assigned a time scale – a tick – to each quasar.

Like each supernova, each quasar is different. The observed variability can depend upon their intrinsic properties. But with this new data, we could match similar quasars with each other, removing the impact of these differences. As astronomers had done for supernovae before, we had standardized the tick-tock of quasars.

The only remaining influence on the observed variability of quasars was the expansion of space, and we unambiguously revealed this signature. Quasars obeyed the rules of the universe exactly as Einstein’s theory predicted.

Due to their brightness, however, we could see the influence of this cosmic time dilation much further. The most distant quasars, from when the universe was only 1/10 of its present age, were ticking away time five times more slowly than today.

Proving Einstein right again

At its heart, this is a story about how Einstein is right again, and how his mathematical description of the cosmos is the best we have. It puts to rest ideas of a sea of cosmic black holes, or that we truly inhabit a static, unchanging universe. And this is precisely how science advances.

Geraint Lewis, Professor of Astrophysics, University of Sydney

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Bottom line: Astronomers have observed quasars “ticking slowly” in the early universe. Seeing time moving in slow motion is another confirmation of Einstein’s special theory of relativity.