In a rare moment of cosmic alignment, the Hubble space telescope has captured an event astronomers have waited 50 years to observe: a distant supernova whose light has been magnified by passing through a “gravitational lens” on its way to Earth. But the magnified supernova didn’t only appear at one spot in the night sky – in an intergalactic trick of the light, it appeared at four places at once, creating an “Einstein cross” which could help uncover the secrets of dark matter.
The work was published in a special March issue of Science marking the centennial of Einstein’s theory of general relativity.
“Besides being really cool,” says Alexei Fillipenko, an astrophysicist at University of California Berkeley and co-author on the paper, “it’s a big deal because there are painfully few ways to learn about the distribution of dark matter”.
Einstein crosses were not predicted by the German-born physicist but result from general relativity, his theory explaining how massive objects warp space and time to create gravity.
That gravity could bend light was a cornerstone of the theory. British astronomer Arthur Eddington verified this in 1919. During a solar eclipse – a rare moment when starlight isn’t swamped by sunlight – he saw the position of stars close to the Sun appear to shift as their light was bent by the Sun’s gravity, like a straw seen through a glass of water. This was hailed as proof of Einstein’s theory.
Czech emigre and amateur engineer Rudi Mandl took the idea a step further. In 1936 he knocked on the door of Einstein's New Jersey home to ask whether the Sun could bend a distant star's light so that it focussed like a magnifying glass. Einstein calculated that yes, it could, although the magnification would be weak, and you'd have to go to a point way beyond Pluto to see the effect. After much prodding, Einstein eventually published this lensing work, although he doubted it would ever be observed in reality.
The idea remained largely dormant until the early 1960s when Norwegian astrophysicist Sjur Refsdal refined Einstein’s calculations and applied them to galaxies – with their much larger masses, they would make stronger gravitational lenses.
“The story goes, when he started working on it, Refsdal was told not to bother about this frivolous stuff and to get on to something more serious,” says Geraint Lewis, an astrophysicist at the University of Sydney, who met Refsdal a couple of times.
Refsdal’s prescient work lay dormant for 15 years until 1979, when the first gravitational lensing was detected – a galaxy magnifying the light of a distant quasar. Now gravitational lensing is an important tool in the arsenal of cosmology, boosting the power of our telescopes often by more than 100 times, and so letting us see much fainter, more distant objects than would otherwise be possible. The most distant objects are also the oldest, because of the amount of time it takes for their light to reach us – “like a magnifying glass looking back into the early Universe,” says Lewis.
In 1964 Refsdal predicted that light from a supernova – the ferocious death-throes of a star which can outshine its entire galaxy – could be an incredibly powerful tool for cosmology. Depending on its angle to the Earth, a magnifying galaxy could act as a warped lens, and multiply a supernova’s image as well as magnify it. We now call this the Einstein cross.
Like trains all departing from the same station but taking different tracks, split light from the supernova would take different paths through the cosmos and arrive at Earth at different times. In effect, the multiple supernova images would appear to brighten and then dim, one by one.
The problem is supernovae are rare, short-lived events visible in the sky for only a few weeks as the explosion fades. “For a galaxy like our own Milky Way, we get one big supernova every few hundred years,” says Lewis. The chance alignment between an exploding star and a gravitational lens had never been seen – until late last year.
On 11 November 2014, astrophysicist Patrick Kelly was at home in Berkeley after a long day at work when an email from NASA landed – images from time he’d been allocated on Hubble. Over the past year, Kelly, a supernova expert, trawled through more than 100 images of galaxy clusters searching for supernovae, uncovering about 10. His process was to play “spot the difference” between a new image and an older one of the same region.
Opening the most recent images, he spotted not one difference, but four. He spun around to his pregnant wife and said, “I think I’ve just made a once-in-a-lifetime discovery.”
Back when Refsdal did his calculations, he was thinking of using the Einstein cross to weigh galaxies – a tough problem at the time. But in the decades since the 1960s a much grander mystery has emerged in cosmology – dark matter. Though we can’t see it, we know, from the tug of its gravity on stars and galaxies, that it must be out there, somewhere. Now Kelly and his team are using Refsdal’s idea – of measuring the timing between the brightening of each supernova image on the Einstein cross – to uncover where it is. This timing depends on the amount of matter, including dark matter – along the light’s path to Earth. Pinpointing where dark matter lurks should offer more clues as to what it is.
Thanks to general relativity, we haven’t seen the last of this supernova. The galaxy that appears in the centre of the Einstein cross is one in a cluster of galaxies – each of which has also bent light from the same supernova. The team predicts we will see the supernova again in the next 10 years, thanks to another galaxy that sent light on a slightly longer path. If astronomers manage to catch the replay, they can refine their models and get an even stronger grasp of dark matter distribution.
Computer modelling of the cluster shows we’ve missed opportunities to see the exploding star twice before. Coincidently, the first opportunity was likely around 1964 – the same year Refsdal predicted the effect.
Refsdal sadly died of a progressive nerve disease in 2009 but the supernova has been named in his honour. Somewhere out there, another glimpse of SN Refsdal is racing towards us at the speed of light.
“We'll be keeping an eye on it for sure,” says Fillipenko.
