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Dark Matter Proof Found Over Antarctica?

Anne Minard
for National Geographic News
November 19, 2008
 
High-energy electrons captured over Antarctica could reveal the presence of a nearby but mysterious astrophysical object that's bombarding Earth with cosmic rays, researchers say.

Or the electrons may be the long-awaited physical evidence of elusive dark matter.

Either way, the unusual particles are exciting for astrophysicists, who say they could someday confirm or deny decades of unproven theories.

"In the first case, we have now seen for the first time a nearby source of cosmic rays. Nobody's seen that before," said study co-author John Wefel, a physicist at Louisiana State University in Baton Rouge.

"In the second case, we may be seeing something even more stupendous."

Annihilation Signal

Cosmic rays are not beams per se but are any protons, electrons, and other subatomic particles that careen toward Earth from a variety of sources, including the supernova explosions that mark the deaths of stars.

Most of the cosmic electrons that reach Earth are low-energy, because the highest-energy ones fizzle the fastest and don't last long enough to get here.

Capturing any electrons at all from the high end of the energy spectrum requires a sustained sampling effort.

The authors of the new study flew a balloon-borne particle collector called the Advanced Thin Ionization Calorimeter (ATIC) over Antarctica.

Circular winds at that latitude allow the balloon to stay aloft for up to 30 days at a time, capturing electrons and measuring their charges, energies, and trajectories.

The team got a surprise: ATIC found inflated numbers of high-energy electrons that match the signal expected from the destruction of dark matter.

Dark matter is one of astrophysics' greatest enigmas. It is thought to be five times more common than visible matter, but there is no proof of what it is made of.

The existence of dark matter has largely been inferred from its gravitational effects, such as the fact that most galaxies have enough mass to remain as well-defined objects despite having too little visible matter to account for the necessary gravity.

(Related photo: "Cluster Smashup Is Dark Matter Proof" [August 27, 2008].)

A few exotic particles have been suggested as dark matter ingredients. One of these, named the Kaluza-Klein particle, is predicted to have the same mass as 550 to 650 protons.

When these theoretical Kaluza-Klein particles collide and annihilate, they're expected to produce electrons with energies between 550 and 650 gigaelectron volts, or GeV. One GeV is roughly the energy locked up in the mass of a single proton, according to Einstein's famous formula E=mc2.

At 620 GeV, the odd energy spike in the Antarctic electrons falls within that range, the authors report in this week's issue of the journal Nature.

Mystery Object

As an alternative theory, the authors say, a nearby astrophysical object could be churning out high-energy electrons that are reaching Earth.

Possibilities include a pulsar, which is the highly magnetic, rotating remnant of a collapsed star, or a microquasar, the luminous, energetic collection of material orbiting a small black hole.

Astrophysicist Okkie de Jager, of North-West University in Potchefstroom, South Africa, and colleagues announced the discovery in April that two pulsars—Geminga and B0656+14—are local sources of high-energy cosmic rays.

These pulsars could be producing the newly discovered electrons, de Jager said.

"I would put my money on a local source, simply because we do have the smoking gun to this effect," said de Jager, who was not involved in the new study.

Yousaf Butt, an astrophysicist at the National Academy of Sciences, wrote a commentary on the work also appearing in Nature.

"Let's not forget that a completely new type of astrophysical object could also produce the detected electron excess," Butt said.

"After all, pulsars were discovered only in 1967, and until 1992 we were blissfully unaware of microquasars."

For its high-energy electrons to reach Earth, such an object would need to be close, astrophysically speaking—within about 3,000 light-years of Earth.

Undecided

Study co-author Wefel said his research team doesn't favor either theory just yet.

"We're sort of stuck in between the two. We can't decide."

No known object precisely matches the data on hand, and the results aren't conclusive for the detection of dark matter, he noted.

"We just do not have enough events to prove they're responsible," he said, referring to the Kaluza-Klein particles. "It's suggestive but it's not proven."

Further sampling is key, he said, but funding has not been renewed for his team to continue using ATIC over Antarctica.

Giant neutrino telescopes like IceCube, A University of Wisconsin-led project built at the South Pole, could find more dark matter clues.

And an instrument called CALorimetric Electron Telescope, or CALET, is now being designed in Japan with the hope that it will join the International Space Station in 2013.

CALET would collect electrons over Earth for at least 1,000 days, as opposed to ATIC's 30.

Fermi, NASA's gamma-ray space telescope formerly known as GLAST, is also capable of measuring an electron spectrum. And the European Union's Cherenkov Telescope Array, now under development, may be able to locate dark matter hot spots in the universe.

Finally, when it's running smoothly, the Large Hadron Collider in Europe will function in part as an experimental dark matter factory, producing collisions at 14,000 GeV that could help shed light on dark matter's exotic particles.
 

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