Supernova Slowdowns Confirm Einstein's Predictions of Time Dilation

Jonathan O'Callaghan reports via Scientific American: 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 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. [...] 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. [...] 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," says [Tamara Davis of the University of Queensland, a co-author of the paper]. "You don't really need to do any better." Jonathan O'Callaghan is an award-winning freelance journalist covering astronomy, astrophysics, commercial spaceflight and space exploration.

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