ASTRONOMERS have been searching everywhere for it. But the elusive 鈥渄ark
matter鈥� could be hiding right under their noses in the brightest spot
around鈥攖he Sun. According to researchers from the University of Oxford,
our dazzling star is harbouring a vast reservoir of the stuff. If they鈥檙e right,
measurements of neutrinos emitted by the Sun could provide our first insight
into the mysterious nature of dark matter particles.
Although we can鈥檛 detect dark matter directly, physicists think that it makes
up to 90 per cent of the mass in the Universe. Without its extra gravitational
pull, rapidly spinning galaxies would fly apart. Researchers have coined the
term WIMPs, or Weakly Interacting Massive Particles, to describe whatever it is
that dark matter is made of.
Astrophysicists Ilidio Lopes and Joe Silk reasoned that passing WIMPs should
be captured by the gravity of heavy bodies like the Sun, so they calculated how
many WIMPs would have collected in the centre of the Sun since it formed 5
billion years ago. Assuming each WIMP has a mass of around 100 billion
electronvolts, right in the middle of the possible range, Lopes reckons about
1044 of them are skulking around inside. 鈥淥ur question was then, how does this
change the Sun?鈥� he says.
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Although WIMPs by definition rarely interact with normal matter, some of them
would hit atoms involved in the fusion reactions that power the Sun, speeding
the transport of energy from its centre to its outer layers. This would help the
star to shine, but also decrease its core temperature by up to 0.1 per cent, in
turn reducing the rate of fusion by as much as 1 per cent.
As the rate of fusion affects the number of neutrinos produced, experiments
that count neutrinos should be able to detect whether WIMPS are in the Sun and
if so, how strong their effect is. David Wark of the Sussex University and the
Rutherford Appleton Laboratory in Oxfordshire says the idea provides an
interesting use for data from the Sudbury Neutrino Observatory in Ontario. In
2001, SNO announced the first accurate count of solar neutrinos
(快猫短视频, 23 June 2001, p 7).
鈥淭hat鈥檚 the sort of thing that makes science
fun鈥攁ll the connections you didn鈥檛 think of,鈥� he says.
Because the SNO results did not reveal any evidence of solar cooling so far,
any WIMPs in the Sun must be heavier than 40 billion electronvolts, as heavier
WIMPs would interact less with normal matter, say Silk and Lopes in a
forthcoming issue of Physical Review Letters.
But this conclusion could change when Silk and Lopes update their
calculations using the very latest data on the rate of the reaction that makes
neutrinos in the Sun. Kurt Snover and colleagues at the University of Washington
in Seattle say that the fusion of beryllium-7 with hydrogen runs about 17 per
cent faster than people had thought (Physical Review Letters, vol 88, p
041101). This means that the number of neutrinos SNO measured is lower than
expected after all. Although within experimental error, this could be the first
tantalising hint of WIMPs in the Sun.