Shortly after the Big Bang, time slowed down five times


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

By tracking the flickering glow of luminescent matter swirling through galaxies when the universe was only a billion years old (less than a tenth of its current age), two researchers have found that events appear to have unfolded five times slower than they did back then. 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 are fixed, and every clock in the universe ticks at exactly the same rate. Then Einstein shattered this view by suggesting that time is actually rubbery and relative,” said Geraint Lewis, an astrophysicist at the University of Sydney and the study’s lead author. “Now we have shown that Einstein was right again.”


The Einsteinian concept of slower time in the early universe emerged in the late 1920s when astronomers discovered cosmic expansion. Galaxies in the sky were found to be flying away from the Milky Way at high speed, swept along by the ceaselessly expanding void – and the farther away they were, the faster they flew. Not only did this mean that the universe was once much smaller and denser – created in a “big bang” from a compact, primordial point – but also that the farthest galaxies visible to us would recede at nearly the speed of light .

According to Einstein’s special and general theories of relativity, both conditions change over time. As light from one of those distant galaxies travels through the ever-expanding universe from the heavier gravitational grasp of the deep, dense early cosmos, it must traverse ever-expanding expanses of space to reach Earth. Consequently, time is stretched in a phenomenon known as time dilation: A clock running 10 billion years ago would tick at a normal rate to an observer of that era, but from the perspective of someone today, it appears to be ticking much more slowly.

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


So Lewis and astrostatistician Brendon Brewer instead examined much larger, more luminous objects known as quasars — bright astrophysical beacons formed from supermassive black holes that fill up with gas at the centers of distant galaxies. Gas piles up and spins as it flows at near-light speed into a feeding black hole, where it heats up to temperatures of several trillion degrees Fahrenheit and emits a glowing glow visible throughout the cosmos.

But this glow is not stable. Black holes are messy, fickle eaters — and trillion-degree gas can go down less like a smooth milkshake and more like thick peanut butter. While this variability makes quasars easier to identify, it complicates their use as standard markers of cosmic time. If supernovae look like fireworks, burn bright and fade quickly, then quasars change brightness like the stock market, with an unpredictable pattern of turbulent flickering. Previous studies have not even found a time dilation effect between quasars that are far from us and quasars that are relatively close.

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


Lewis and Brewer, on the other hand, used a new, much more comprehensive data set: They looked at a total of 190 quasars, covering a cosmic period from about 2.5 billion to 12 billion years ago. The flickering of each quasar was observed hundreds of times at multiple wavelengths over a two-decade period.

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

Eventually, the researchers found that the ticking of the quasar clocks behaved exactly as Einstein’s theory of relativity predicts. Quasars found in distant galaxies ticked slower than those born in the later, near universe, with time dilation that made the most distant quasars appear to be moving at a glacial one-fifth of standard speed.


Katie Mack, an astrophysicist who holds the Hawking Chair in Cosmology and Science Communication at Ontario’s Perimeter Institute for Theoretical Physics, says these findings shed light on several uncertainties surrounding quasar behavior. In particular, the study confirms that quasars match consensus expectations — and reinforces the need for astronomers to account for 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 reassuring to know that nothing bizarre is happening there,” says Mack, who was not involved in the study.

Although astronomers had anticipated the presence of the effect in the ancient universe, this prediction had yet to be tested. Michael Hawkins, a researcher emeritus 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 general theory of relativity has turned centuries of science upside down. set when it was introduced. Hawkins himself has done previous research that failed to detect time dilation in quasars, which he says underscores the importance of ongoing research and refinement in the field.


“To uphold scientific practice, you have to remain skeptical to the end, so it’s critical to keep testing even the most established theories of the universe,” says Hawkins. As a next step, he would like to see future studies replicate the analysis with a larger sample of quasars that come from galaxies even deeper in the cosmic past.

For Lewis, the work is more than a vindication of Einstein and modern cosmology. Accurate timestamps of ancient quasars could also be useful for further investigating the nature of dark energy, the mysterious force believed to be responsible for a startling acceleration in the universe’s expansion.

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

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