Astronomers solve supernova mystery
Scientists thought they had lost one of their most reliable indicators of interstellar distance. Phil Dooley explains how order was restored.
Astronomers have solved a cosmic mystery that had threatened to undermine a long-trusted yardstick for measuring the immense distances in the universe.
Robert Quimby and his team at the Kavli Institute for the Physics and Mathematics of the Universe in Tokyo, have found that the uncharacteristic brightness of a supernova that exploded in 2010 was caused by a galaxy in front of it acting like a giant lens.
The solution to the puzzle means scientists can continue to rely on certain supernovae as standards to measure distance in the universe, as they have since a Chilean survey in the early 1990s.
A white dwarf star explodes to become a type 1a supernova when it reaches a critical mass. As this mass is the same for all white dwarfs, they always explode with the same amount of light intensity. They also have two unique signatures – a characteristic spectrum and a regular pattern in the way they fade in the month following the explosion.
So reliable are these features that astronomers can use supernovae as remarkably precise distance beacons - the fainter they appear, the further away they are.
But in 2010, something unsettling happened. An explosion was detected. It lasted for as long as a type 1a supernova and had a spectrum that exactly matched that of a type 1a supernova, but it was far too bright.
“This one, named PS1-10afx, seemed very wrong,” says Quimby. “If this was a type 1a supernova and it was 30 times brighter than we think it should be, that would be a huge problem.”
The explosion was too bright, the team thought, because they had another independent means of estimating its distance.
Its spectrum was redshifted (which happens when wavelengths of light are lengthened by the ubiquitous expansion of the universe) indicating it was extremely distant – about nine billion light years away. Judging from the light intensity alone the explosion was much closer.
Spacetime curved by the galaxy would act like a giant lens,
intensifying the supernova's rays.
But Quimby had a hypothesis that there might be an invisible galaxy in front of the supernova that was acting as a gravitational lens – a phenomenon first observed almost a century ago.
A gravitational lens is formed by something so enormously heavy that it warps spacetime – a prediction of Einstein’s general theory of relativity. Like someone standing on a mattress, a galaxy made of tens of billions of stars creates a dent in spacetime.
Quimby suspected there must be just such a galaxy directly between us and the supernova. Spacetime curved by the galaxy would act like a giant lens, intensifying the supernova's rays, much the same way a magnifying lens can intensify sunlight to burn paper.
But because of the vast distances, telescopes could only detect as a faint blob what might be a galaxy lying in front of the one containing the supernova. To differentiate the two, the group looked at the light spectrum of the blob and measured its redshift. If it was a galaxy that was closer it would have a smaller redshift than the more distant supernova.
“It was not an easy observation, we spent basically a whole night on Keck, one of the biggest telescopes in the world, looking at one object. But at the end of that, it was pretty clear,” says Quimby.
As suspected, the team found two sets of redshifts which indicated there were two galaxies, one 8.3 billion light years away that had lensed the light from the supernova in a galaxy 0.7 billion light years behind it.
Cosmologist Geraint Lewis from the University of Sydney was not surprised to hear the mystery had been solved in this way. Because the universe is peppered with galaxies, “roughly one in a few thousand sight lines through the universe should be strongly gravitationally lensed”, he says.
PS1-10afx may have been returned to the rank and file of standard type 1a supernovae, but it may still offer some extraordinary opportunities, says Quimby. “The strong gravitational lens actually forms multiple images of the supernova. The light of each of these images is travelling through a different pathway in the universe, and arrives at slightly different times. By timing the delay between them you can directly measure the Hubble constant – how fast the universe is expanding.”