MASSIVE optical telescopes on mountain tops have been the main tools for exploring dark energy – the mysterious stuff that is accelerating the expansion of the universe. Soon the quest could move underground. Neutrinos born in stellar cataclysms and detected in gigantic water tanks buried in mines may become the new probes for dark energy.
Dark energy was discovered in the late 1990s by astronomers studying the light from stellar explosions known as type 1a supernovae. Since then telescopes around the world, such as the Very Large Telescope on Cerro Paranal in Chile, have been used to study the light from more and more supernovae. Now Lawrence Hall of the University of California at Berkeley and colleagues think that neutrinos spewed out in another type of stellar explosion, a core-collapse supernova, could be just the tool for studying dark energy.
When the core of a massive star grows too large, it collapses under its own gravity, releasing a flood of neutrinos – a theory confirmed in 1987 when a supernova went off in a nearby dwarf galaxy, the Large Magellanic Cloud, and a sudden wave of the particles hit neutrino detectors on Earth. Two of them, Kamiokande-II in Japan and the IMB detector in the US, were underground water tanks. Photomultiplier tubes lining these tanks detected the distinctive and rare blue flashes of light emitted when a neutrino hits an electron.
The millions of core-collapse supernovae that have gone off throughout the history of the universe must have created a background of supernova relic neutrinos. But the diffuse nature of these neutrinos makes them very difficult to detect. However, the next generation of neutrino detectors, such as the planned Underground Nucleon decay and Neutrino Observatory, which will be about 20 times larger than Super-Kamiokande in Japan (see right), will have tanks that can hold a million tonnes of water and so should be up to the job. "If I have to bet on it, the next neutrinos of astrophysical origin we see will be supernova relic neutrinos," says physicist Chang Kee Jung of the State University of New York at Stony Brook, who is involved with the UNO proposal.
Hall's team has worked out that the spectrum of these relic neutrinos could hold cosmological treasure. That's because the flux of neutrinos measured today is affected by how the universe expanded in the past. So measuring the flux of supernova relic neutrinos of different energies could reveal how the universe is expanding.
This is similar to the way dark energy was first discovered, when astronomers found that type 1a supernovae were dimmer than expected. The accelerating expansion of space had diluted their light, and this was put down to some kind of invisible repulsive force whose nature is still unknown.
Relic neutrinos might merely confirm the acceleration, leaving the exact nature of dark energy a mystery – or they could reveal new physics. Neutrinos might bounce off dark energy, in which case their spectrum will be distorted in such a way as to tell us something more about this mysterious force. Or it may be that light from distant supernovae is being distorted in some strange way – perhaps by being gradually converted into particles called axions. Finally, the spectrum of supernova relic neutrinos could reveal whether anything is awry with optical supernova studies.
Author: Stephen Battersby
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