Astronomers see time moving in slow motion in the early universe

Geraint Lewis, University of Sydney

According to our best understanding of physics, the fact that space is expanding should affect the apparent flow of time, with the distant universe appearing to run in slow motion. But observations of highly luminous and variable galaxies — also known as 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 observations to untangle the complex flicker of nearly 200 quasars. Buried within this flickering is the imprint of expanding space. It shows that the universe seems to be ticking five times slower when it was just a billion years old.

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

Time moves in slow motion

In 1905, Albert Einstein, through his special theory of relativity, told us that the speed of ticking of clocks is relative. As in, it depends on how the clocks move. In his 1915 general theory, he told us that gravity can also affect the relative speeds of clock ticks.

In the 1930s, physicists realized that the expanding space of the cosmos, described in Einstein’s language of general relativity, also affects the tick-tock universe.

Because of 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 the ticking of the clock grows.

The prediction of Einstein’s math is clear: We should see the distant universe unfold in slow motion.

Supernova clock tick tock

Measuring this universe in slow motion is difficult. Nature provides no standard clocks throughout the cosmos whose relative ticks astronomers can easily compare.

It took until the 1990s for astronomers to discover and understand the ticking of suitable clocks: a particular type of exploding star, a supernova. Each supernova explosion was strikingly similar. They brighten quickly and then fade within a few weeks.

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

To achieve this, astronomers had to smooth out the peculiarities of each supernova. This involved putting them on an equal footing and pairing them with a standard intrinsic brightness and a standard clock.

They found that the flashes of more distant supernovae extended exactly in line with Einstein’s predictions. The most distantly observed supernovae, which exploded when the universe was half its present age, brightened and faded twice as slowly as the most recent supernovae.

The problem with quasars

Supernovas aren’t the only variable objects in the cosmos.

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

Quasars are extremely bright, some burning furiously when the universe was a newborn. Quasars are also variable, varying in brightness as matter tumbles turbulently towards destruction.

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

However, searches for the expected signal came up empty. The samples of hundreds of quasars observed over decades varied markedly, but the variations of those near and far seemed to be identical.

Some have suggested that this variability is not inherent but due to black holes scattered throughout the universe, with gravity making some quasars bigger. More extravagantly, others have argued that the lack of expected cosmological signal is a sign that our cosmology is all wrong and needs to go back to the drawing board.

Time moves in slow motion into new data

In 2023, astronomers have released a new set of data on quasars. This dataset featured 190 quasars originally identified in the highly successful Sloan Digital Sky Survey, but observed over two decades in multiple colors: green, red, and infrared light.

Data sampling was mixed, with many observations in some periods and fewer in others. But the wealth of this data has allowed astronomers, led by graduate student Zachary Stone of the University of Illinois, to be able to statistically characterize the variability of each quasar as what is known as a damped random walk. This characterization assigned a time scale – a tick – to each quasar.

Like every supernova, every quasar is different. The observed variability may depend on their intrinsic properties. But with this new data, we could match similar quasars together, eliminating the impact of these differences. As astronomers had already done for supernovae, we had standardized the tick-tock of quasars.

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

Due to their brightness, however, we could see the influence of this cosmic time dilation much further out. The most distant quasars, from when the universe was only 1/10 its current age, passed time five times slower than today.

Proving again that Einstein was right

At its core, this is a story about how Einstein is right again and how his mathematical description of the cosmos is the best we have. It silences 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 “slowly ticking” quasars in the early universe. Seeing time move in slow motion is another confirmation of Einstein’s special theory of relativity.