According to our best understanding of physics, the fact 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, known as quasars, have failed to reveal this cosmic time dilation – until now.
In a new study published in Nature Astronomy, 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, with the Universe appearing to be ticking five times slower when it was only a billion years old.
This shows quasars obey the rules of the cosmos, putting to bed the idea they represented a challenge to modern cosmology.
Time is a funny thing
In 1905, Albert Einstein, through his special theory of relativity, told us the speed of clocks’ ticking is relative, 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 realised 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.
Read more: Timeline: the history of gravity
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, and 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, as nature does not provide standard clocks across the cosmos whose relative ticks could be compared.
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, brightening rapidly and then fading 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.)
Read more: From dark gravity to phantom energy: what's driving the expansion of the universe?
To achieve this goal, astronomers had to iron out peculiarities of each supernova, putting them on an equal footing, matching them to a standard intrinsic brightness and a standard clock.
They found the flash of more distant supernovae was 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.
Quasars were discovered in the 1960s, and are thought to be supermassive black holes, some many billions of times more massive than the Sun, lurking at the hearts of galaxies. Matter swirls around these black holes on its journey to oblivion inside, heating up and glowing brightly as it does so.
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 that this demonstrated that the variability of quasars is not intrinsic but is instead due to black holes scattered through the Universe, magnifying some quasars by the action of gravity. More outlandishly, others have claimed that the lack of the expected cosmological signal is a clear sign that we have cosmology all wrong and need to go back to the drawing board.
New data, new approaches
In 2023, a new set of quasar data was published. This presented 190 quasars originally identified in the highly successful Sloan Digital Sky Survey but observed over two decades in multiple colours – 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 characterise each quasar’s variability as what is known as a “damped random walk”. This characterisation assigned a time scale, a tick, to each quasar.
Like each supernova, each quasar is different, and 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 had been done for supernovae before, we had standardised 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, the influence of this cosmic time dilation could be seen much further. The most distant quasars, seen when the Universe was only a tenth of its present age, were ticking away time five times more slowly than today.
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.