In a first, astronomers have observed time ticking in slow motion in the early cosmos, confirming Albert Einstein’s century-old ideas about the reality-warping effects of our universe’s expansion.

By tracking the flickering glows of luminous matter swirling through galaxies when the universe was a mere one billion years old (less than a tenth of its present age), two researchers have found that events back then appear to have unfolded at a pace five times slower than normal. Their findings were published earlier this month in Nature Astronomy.

“For decades Isaac Newton gave us this vision of a universe where space and time is fixed, and every clock across the universe ticks at exactly the same rate. Then Einstein shattered this vision by proposing that time is actually rubbery and relative,” says Geraint Lewis, an astrophysicist at the University of Sydney and lead author of the study. “Now we’ve shown that Einstein was, once again, correct.”

The Einsteinian concept of time running slower in the early universe arose in the late 1920s as astronomers were discovering cosmic expansion. Galaxies in the sky were found to be flying away from the Milky Way at high speed, swept along by the ceaselessly growing void—and the farther off they were, the faster they flew. This not only meant that the universe was once much smaller and denser—arising in a “big bang” from some compact, primordial point—but also that the most distant galaxies visible to us should be receding at close to the speed of light.

According to Einstein’s special and general theories of relativity, both circumstances alter the flow of time. As light from one of those far-distant galaxies travels from the heavier gravitational grip of the deep, dense early cosmos and across the continuously expanding universe, it must traverse increasingly greater expanses of space to reach Earth. Consequently, time becomes stretched in a phenomenon known as time dilation: a clock running 10 billion years ago would tick at a normal rate to an observer from that time, but from the perspective of someone today, it would appear to be ticking much slower.

Astronomers had previously validated this slow-motion cosmos about halfway back through the universe’s 13.8-billion-year history by examining the light from massive exploding stars called supernovae that detonated six to seven billion years ago. But such supernovae are too faint to observe at the immense distances needed to probe earlier cosmic epochs.

So Lewis and astrostatistician Brendon Brewer instead investigated much larger, more luminous objects known as quasars—bright astrophysical beacons formed from supermassive black holes gorging on gas at the centers of distant galaxies. Gas piles up and spirals around as it funnels at nearly light speed into a feeding black hole, where it heats up to temperatures of several trillion degrees Fahrenheit and emits an incandescent glow visible across the cosmos.

But this glow isn’t steady. Black holes are messy, fitful eaters—and trillion-degree gas can go down less like a smooth milkshake and more like chunky peanut butter. Although this variability makes quasars easier to identify, it complicates their use as standard markers of cosmic time. If supernovae are akin to a firework, burning bright and quickly fading away, then quasars change brightness more like the stock market, with an unpredictable pattern of turbulent flickers. In fact, prior studies have failed to find a time dilation effect between quasars very distant from us and ones relatively close by.

“Those early findings inspired some fringe cosmologists to question whether quasars’ variability adheres to our existing models of the universe. There were even suggestions that our long-held, fundamental idea that the universe is expanding was wrong,” Lewis says. He adds that these studies used small samples or observed quasars over a short time period.

In contrast, Lewis and Brewer used a new, much more expansive data set: they looked at 190 quasars in all, covering a range of cosmic time from about 2.5 billion to 12 billion years ago. Each quasar’s flickering was observed hundreds of times at multiple wavelengths across a span of two decades.

The duo also grouped the quasars by intrinsic luminosity. “We boxed bright quasars with bright quasars and faint quasars with faint quasars,” Lewis says. This approach minimized the chance of making “apples-to-oranges” comparisons between distinctly different quasar types and allowed the researchers to calibrate each quasar’s “ticks,” yielding more certainty that some of the observed discrepancies in light fluctuations were caused by time dilation.

Ultimately, the researchers found that the tick-tock of the quasar clocks behaved just as Einstein’s relativity predicts. Quasars found in faraway galaxies ticked slower than ones born in the later, nearby universe, with time dilation making those most distant appear to run at a glacial one fifth of the standard speed.

Katie Mack, an astrophysicist who holds the Hawking Chair in Cosmology and Science Communication at the Perimeter Institute for Theoretical Physics in Ontario, says that these findings provide clarity on various uncertainties surrounding quasar behavior. Specifically, the study confirms that quasars align with consensus expectations—and it reinforces the need for astronomers to consider time dilation when studying them.

“This is the first time that the effect of time dilation has been clearly observed with quasars, and it’s comforting to know that there’s nothing bizarre happening there,” says Mack, who was not affiliated with the study.

While astronomers had anticipated the presence of the effect in the ancient universe, this prediction still needed to be tested. Michael Hawkins, an emeritus researcher at the University of Edinburgh’s Institute for Astronomy, says the study serves as a valuable reminder for scientists to avoid complacency with established cosmological models, adding that Einstein’s theory of general relativity upended centuries of science when it was introduced. Hawkins himself has previously conducted research that failed to detect time dilation in quasars, which he says underscores the significance of ongoing investigation and refinement in the field.

“To uphold scientific practice, you have to maintain skepticism until the very end, so it’s critical to continue testing even the most well-established theories of the universe,” Hawkins says. As a next step, he would like to see future studies replicate the analysis with a larger sample of quasars originating from galaxies even deeper in the cosmic past.

For Lewis, the work is more than a vindication of Einstein and modern cosmology. Accurate timestamping of ancient quasars might also prove useful for further exploring the nature of dark energy, the mysterious force thought responsible for a surprising acceleration in the universe’s expansion.

 “Standardizing and confirming our models is ultimately a step into the next generation,” Lewis says. “The goal now is to chart the expansion of the universe in as much detail as possible.”