Dark matter is proving less shadowy than its name suggests. Its signature may have been detected by a balloon-borne experiment that measured a surprisingly high number of energetic electrons streaming in from space.
High-energy electrons are found throughout space and are accelerated when stars explode in supernovae. But a balloon-borne detector flying over Antarctica called the (ATIC) has detected 70 more high-energy electrons than the normal background level attributed to supernova blasts.
of Louisiana State University in Baton Rouge, who led the collaboration, says there are two possible explanations.
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The electrons could come from a nearby astrophysical object, such as a pulsar, that lies within 3000 light years from Earth. But the team has spent four years trying to fit the signal to such an object and has yet to find a good match.
The alternative is that the electrons were produced when two dark matter particles met and destroyed each other. That hypothesis is strengthened by the electrons鈥� observed energies, which range from 300 to 800 gigaelectronvolts.
鈥淭here is nothing that we know of in high-energy physics or astrophysics that happens in this energy range,鈥� says Wefel.
Extra dimensions
What鈥檚 more, the signal peaked at 650 GeV and then rapidly declined to the background level at 800 GeV. According to Wefel, this is the kind of signature you would expect if a type of exotic particle known as a particle was the dark matter culprit, with the peak at 650 GeV corresponding to its mass.
This type of particle is a WIMP (weakly interacting massive particle), one of the most promising candidates for dark matter, and comes from theories in which the universe has extra spatial dimensions. These extra dimensions can only be detected by observing WIMPS that have leaked into the four dimensions (three of space and one of time) that are familiar to us.
The past few years have been good for dark matter hunters. In 2007, NASA鈥檚 WMAP satellite, which measures the big bang鈥檚 afterglow, picked up an excess of microwaves from around the centre of our galaxy. This 鈥榃MAP haze鈥� could be radiation produced when dark matter particles collide.
Other signals
A few months ago, another group found tantalising hints of dark matter in antimatter measurements taken by a detector known as PAMELA.
So how do the results from ATIC fit in with these?
Even though the data from PAMELA cover a different energy range from the ATIC signal, Wefel believes that 鈥渢here is no contradiction between ATIC and PAMELA, at least to within the uncertainties on the presently available data鈥�, he told 麻豆传媒. 鈥淚t is possible that we may be observing the same source.鈥�
But ATIC has detected 200 times more potential dark matter than WMAP did at the galactic centre. 鈥淲e need a boost factor of 200 for the results to be compatible,鈥� says Wefel. 鈥淪o either the WMAP haze is wrong, the theory is wrong, or dark matter is not uniformly distributed all over the place.鈥�
The search continues
With so many unanswered questions, will we ever be able to say conclusively that dark matter has been spotted? Wefel thinks that experiments such as the recently launched Fermi Gamma Ray Space Telescope should continue to discover new possible sources of dark matter. These sources will need to be studied in other wavelengths and with other instruments in order to determine their properties.
鈥淭hen we shall see if any of them have the capability to produce the electron signal that ATIC observed,鈥� says Wefel. 鈥淗ow long do you search before you give up? I can鈥檛 say but I suspect we鈥檒l keep going until Fermi and other instruments run out of new source discoveries. Meanwhile other experiments will try to study the electrons in more detail to see if they can 鈥榩in down鈥� the signature of dark matter annihilation.鈥�
However the pieces fit together, other experts say the ATIC discovery is intriguing. That鈥檚 because there are still some questions about what accelerates electrons and other charged particles in space, called cosmic rays.
鈥淓ven if it proves not to be dark matter, the puzzle of how very high-energy cosmic rays are produced is still a mystery, and this work will help shed some light on it,鈥� thinks , an astrophysicist from the University of Edinburgh.
Journal reference: (vol 456, p 362)
