Despite more than a century of efforts to show otherwise, it seems Albert Einstein can still do no wrong. Or at least that’s the case for his special theory of relativity, which predicts that time ticks slower for objects moving at extremely high speeds. Called time dilation, this effect grows in intensity the closer to the speed of light that something travels, but it is strangely subjective: a passenger on an accelerating starship would experience time passing normally, but external observers would see the starship moving ever slower as its speed approached that of light. As counterintuitive as this effect may be, it has been checked and confirmed in the motions of everything from Earth-orbiting satellites to far-distant galaxies. Now a group of scientists have taken such tests one step further by observing more than 1,500 supernovae across the universe to reveal time dilation’s effects on a staggering cosmic scale. The researchers’ findings, once again, reach an all-too-familiar conclusion. “Einstein is right one more time,” says Geraint Lewis of the University of Sydney, a co-author of the study.
In the paper, posted earlier this month on the preprint server arXiv.org, Ryan White of the University of Queensland in Australia and his colleagues used data from the Dark Energy Survey (DES) to investigate time dilation. For the past decade, researchers involved with DES had used the Victor M. Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile to study particular exploding stars called Type 1a supernovae across billions of years of cosmic history. Using this vast dataset of supernovae, DES seeks to fine-tune our understanding of the accelerating expansion of the universe, which appears to be driven by mysterious dark energy; in January researchers used this dataset to hint that this acceleration may be changing over time.
As a bonus, the DES supernovae data offered scientists a new chance to study cosmological time dilation—that is, time dilation caused by the universe’s expansion. One outcome of this expansion is that more distant objects are moving away from us much faster than closer ones—meaning that the farther into the universe DES looked, the stronger time dilation’s effect should have been upon the supernovae it observed there. “If we saw something else, that would show something is really fundamentally wrong with the foundation of cosmology,” says Tamara Davis of the University of Queensland, a co-author of the paper. “I love the fact that we can actually see time dilation happening. It’s blindingly obvious from the time you look at the data that it’s there.”
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The relationship itself is beautifully simple: the amount that a supernova’s characteristic flash-and-fade will be elongated is a factor of 1 + z, where z is the supernova’s redshift, a measure of how much cosmic expansion has stretched out the supernova’s emitted light as it traveled to Earth. Higher redshifts correspond to greater cosmic distances. “We live in an expanding universe, and one of the consequences of that should be that we observe the more distant universe running in slow motion compared to the universe today,” Lewis says.
For objects in the nearby universe, where redshifts are near zero, the effect of cosmological time dilation is vanishingly small. But the universe is huge—the James Webb Space Telescope (JWST), for instance, recently detected a distant galaxy at a record-setting redshift of 14.32, just 290 million years after the big bang. Typically from its first outburst to its final afterglow, a supernova might last for three months or so, but when time dilation comes into play, a supernova at a redshift of 1 will appear to double in length.
Cosmological time dilation has long been known, but measuring it is difficult. Some of our best efforts have timed gamma-ray bursts, extraordinarily bright flashes of energy seen across the universe, or quasars, bright and hot regions of swirling material around supermassive black holes. Last year Lewis used about 200 quasars to investigate cosmological time dilation, and he was nearly able to see this exact 1 + z relationship in action but with somewhat large uncertainties. White’s work, using a much larger sample of supernovae that are more predictable than quasars, allowed for a much more accurate measurement.
Type 1a supernovae are keystone cosmic explosions caused when a white dwarf—the slowly cooling corpse of a midsized star—siphons so much material from a companion that it ignites a thermonuclear reaction and explodes. This explosion occurs once the growing white dwarf reaches about 1.44 times the mass of our sun, a threshold known as the Chandrasekhar limit. This physical baseline imbues all Type 1a supernovae with a fairly consistent brightness, making them useful cosmic beacons for gauging intergalactic distances. “They should all be essentially the same kind of event no matter where you look in the universe,” White says. “They all come from exploding white dwarf stars, which happens at almost exactly the same mass no matter where they are.”
The steadfastness of these supernovae across the entire observable universe is what makes them potent probes of time dilation—nothing else, in principle, should so radically and precisely slow their apparent progression in lockstep with ever-greater distances. Using the dataset of 1,504 supernovae from DES, White’s paper shows with astonishing accuracy that this correlation holds true out to a redshift of 1.2, a time when the universe was about five billion years old. “This is the most precise measurement” of cosmological time dilation yet, White says, up to seven times more precise than previous measurements of cosmological time dilation that used fewer supernovae.
The result is “really impressive,” says Amitesh Singh of the University of Mississippi, noting that measuring time dilation is “one of the most direct pieces of evidence of the expansion of the universe.” Making this measurement is not in itself a revolutionary result, however, given that few, if any, reputable cosmologists would argue that the universe is not expanding or that special relativity is wrong. “I’m not trying to be cynical when I say it’s not surprising,” says Nicole Lloyd-Ronning of the University of New Mexico–Los Alamos. But, she adds, “it is a confirmation of the physics that we feel we know. This is a manifestation of special relativity and cosmic expansion in general.”
Time dilation does pose some interesting dilemmas, though, particularly with studies of the far universe. Recently, JWST revealed supernovae stretching back into the distant cosmos, including a Type 1a supernova at a redshift of 2.9, or about two billion years after the big bang, the most distant one yet seen. Because of time dilation, “at a redshift of 2, you multiply by 3,” says Ori Fox, an astronomer at the Space Telescope Science Institute. This means events at a redshift of 2 would last “maybe nine months to a year” as seen from Earth, he says. But at much higher redshifts, “you’re talking about timescales of years,” Fox says, which makes supernovae in the even earlier universe hard to spot as astronomers seek them out when comparing before and after images of potentially supernova-hosting galaxies. “If you’re at a redshift of 10, now you’re talking a minimum of four years,” to see a supernova switch on and off, he says.
This particular supernova-focused facet of the Dark Energy Survey has concluded, so until a new dataset is taken, White’s measurement of cosmological time dilation is unlikely to be beaten. “It’s a pretty definitive measurement,” Davis says. “You don’t really need to do any better.” With that measurement in hand, anyone wringing their hands over our supposed cosmic ignorance can rest easy: our best theory describing the cosmos at large appears to be holding true—which doesn’t mean, of course, that we shouldn’t have checked. “One of the assumptions is that we live in a universe that’s described by Einstein’s equations,” Lewis says. “We can’t just say that and not do anything. We need to test our assumptions.”