Supernova Superstar: The Most Distant and Important One Yet

One ancient, exploding star could help explain two great mysteries

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NASA

This image of Supernova SN 1987A, one of the brightest stellar explosions since the invention of the telescope, is based on observations done with the High Resolution Channel of Hubble’s Advanced Camera for Surveys and was released on Dec. 10, 2012.

Astronomers can make a surprising amount of hay over relatively small observations. That faint hiss in your AM radio that just seems like spotty reception? It’s actually the cosmic background radiation pouring in from all over the universe. That subtle redshift in the light from distant stars? Turns out the universe is expanding.

Now investigators may be ready to turn a small discovery into big science again. Researchers at the Kavli Institute for the Physics and Mathematics of the Universe in Japan have discovered an exploding star known as a type 1a supernova at the edge of the cosmos, and while 1a’s aren’t that common — stars like the sun don’t blow up that way, for example — they’re not all that uncommon either. Still, this particular 1a could help solve the mysteries of both dark matter and dark energy — huge stuff by any measure — and all thanks to a single optical illusion that has Albert Einstein’s fingerprints all over it.

One weird prediction of the great physicist’s general-relativity theory is that massive objects like stars and galaxies should warp the space around them, bending the paths of light rays that stream by. They act as a “gravitational lens,” distorting or magnifying the image of another object farther in the background. In this case, the background object was the 1a supernova, known awkwardly as PS1-10afx; the massive foreground object is probably a huge cluster of galaxies.

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Supernovas like PS1-10afx are valuable tools not just because they’re bright but also because they’re uniformly bright; all 1a’s put out pretty much the same amount of visible energy. This constant, known as a standard candle, makes it easy for astronomers to determine how distant the supernovas are because they can calculate how bright any one of them should look from earth at any particular distance. Significantly, this also allows them to calculate how fast the universe is expanding, simply by watching the rate at which the brightness falls off. Back in 1998, two teams of astronomers looking at Type 1a’s discovered, to their astonishment, that the most distant supernovas weren’t moving as fast as they expected — or to put it another way, nearby supernovas were moving too fast. Somehow, the universe was expanding faster and faster as it aged.

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The Nobel Prize–winning explanation: some mysterious force, a sort of antigravity, was speeding the expansion. The force, now known as dark energy, has Einstein’s fingerprints on it too: he predicted it in 1916, then retracted it in the 1920s as the crude observations of the day seemed to rule it out.

In order to understand what dark energy actually is — and astronomers aren’t nearly there yet — it’s crucial to find more, and more distant, Type 1a’s. That’s why the new discovery is so important: PS1-10afx is easily the most distant supernova seen. The explosion happened more than 9 billion years ago, when the universe was only about 5 billion years old, and its light has been racing toward us ever since. If scientists can find several more from around that time, they’ll be able to nail down the cosmic expansion rate when the universe was young. That will flesh out their understanding of how dark energy behaves — whether it changes in strength over time, for example — and ultimately help them figure out what the heck it is.

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Not only that: the foreground cluster of galaxies that magnified the supernova’s image is undoubtedly filled with dark matter, the invisible, still unidentified stuff that makes up 80% of the mass of the cosmos. Dark matter has the opposite effect of dark energy — pulling the universe together rather than apart. The more of it there is in the foreground mass, the more powerful the gravitational-lensing effect — so by figuring out how much the supernova was magnified, astronomers can get a handle on exactly how much of the mysterious stuff the cluster holds.

The Kavli Institute is not alone in its interest in scouring the remote cosmos for 1a’s that could be lensed. The ground-based Large Synoptic Survey Telescope, slated to go online in 2018, and the space-based Euclid mission, planned for a 2020 launch, will almost certainly add significantly to astronomers’ stash of magnified supernovas. Those could help answer some questions that even Einstein himself didn’t know to ask.

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1 comments
robert.kirshner
robert.kirshner

This story has some errors and omissions. It would be good to fix them.

First of all, the object in question, PS1-10afx was not "discovered" by Quimby et al.  As the (awkward) name suggests, it was found in data from PanSTARRS, the telescope system in Hawaii, and reported by Ryan Chornock and a bunch of us in the PanSTARRS 1 science consortium. Chornock wrote quite a good paper on this object that you can easily find at arXiv:1302.0009.

Second, "PS1-10afx" is not "easily the most distant supernova seen." There have been higher-redshift supernovae reported by Adam Riess & co-conspirtitors (I am one), including the recent paper by David Jones conveniently entitled "The Discovery of the Most Distant Type Ia Supernova at Redshift 1.914"  (arXiv:1304.0768)

Third, there are many good things about Quimby's analysis of PS1-10afx suggesting it is a lensed object, rather than an exceptionally bright one, but the thing that is missing is the lens!  As yet, they have no clear evidence for its existence.  

And finally, I think you are mistaken that the very distant objects will be the key to learning the nature of dark energy.  They are important because they can show whether the evolution of supernovae is misleading us, but everyone expects the universe to be dominated by dark matter and decelerating at high redshift.  This effect has been seen in the supernova data (Riess et al.) and recently in the BOSS evidence from the Lyman-alpha forest.

Robert Kirshner

Clowes Professor of Science

Harvard University