Heidegger

Relic neutrinos join the hunt for dark energy
* 21 July 2006
* NewScientist.com news service
* Stephen Battersby

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.
““Neutrinos might bounce off dark energy so that their
spectrum is distorted in a way that tells us”
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.

Posted on Saturday, July 22nd, 2006 at 12:10 pm
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