EVEN those whose idea this is hope they are wrong 鈥� pulsars, rather than dark matter, might be responsible for an usually large number of high-energy particles discovered in space.
In August, the team behind the PAMELA satellite reported seeing an excess of positrons 鈥� the antimatter equivalent of electrons 鈥� zipping around space at high speed. This caused excitement among physicists, who wondered if it could be a signature of dark matter. Although this mysterious substance is thought to make up a large proportion of the universe, so far it has only been detected indirectly, through gravitational effects. It is possible that the positrons might be the by-product of the annihilation of dark-matter particles (快猫短视频, 23 August, p 8).
The snag is that high-energy positrons can also be produced in the vicinity of pulsars 鈥� neutron stars that rotate rapidly and emit regular light pulses. Pulsars鈥� powerful magnetic fields accelerate electrons to high speeds, and these may collide with photons and generate pairs of electrons and positrons as a result.
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Now a team led by Dan Hooper of Fermilab in Batavia, Illinois, has calculated that pulsars in our galaxy could indeed produce enough high-energy positrons to account for the PAMELA observations ().
The recently launched Fermi Gamma-ray Space Telescope may be able to settle the issue. The hope is that it will have sufficient resolution to determine if the positrons are coming from nearby neutron stars.
鈥淧ulsars should be considered and studied as possible sources of positrons,鈥� says PAMELA team member Mirko Beozio of the National Institute of Nuclear Physics in Trieste, Italy, though he declines to say if he thinks they could explain the observations. Ever since his team presented their initial data at the International Conference on High Energy Physics in Philadelphia, Pennsylvania, they have chosen to say little else until their paper on the finding is published in a journal.
Jonathan Ormes of the University of Denver in Colorado, a member of the Fermi team, is putting his money on the neutron-star explanation. 鈥淒ark matter is exciting and everybody loves it, but you have to eliminate all of the other possibilities,鈥� he says.
However, Hooper is still rooting for dark matter, which he says remains 鈥渢he most exciting of the hypotheses that have been put out so far鈥�.
鈥淒ark matter is the most exciting of the hypotheses that have been put out so far to explain the excess of positrons鈥�
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