The mysterious stuff known as dark matter may have left a calling card at the edge of the Earth’s atmosphere where a space-faring satellite named PAMELA could pick it up. Researchers are reporting that PAMELA detected a high number of the subatomic particles called positrons, the positively-charged counterpoints to electrons, which could have been created by collisions between dark matter particles. “PAMELA found a number of positrons much higher than expected,” the mission’s principal investigator Piergiorgio Picozza [said]. “Many think this could be a signal from dark matter” [SPACE.com]. But of course, others think there’s a more mundane explanation.
Dark matter is one of the greatest enigmas in astrophysics: It cannot be observed directly, so researchers have to study its effects on normal matter to try to deduce what it’s made of. The top candidates for dark matter, the heavy but invisible stuff that makes up 23 percent of the universe, are weakly-interacting massive particles. Contrary to their WIMPy name, when two of these particles collide, they annihilate each other in a burst of energy and propel a cloud of matter and antimatter particles into space. The theory has been a favorite of physicists for years, but until now, no one had detected evidence of these collisions [Wired].
The new study, published in Nature, describes the PAMELA satellite’s investigations of the cosmic rays that constantly bombard our planet. Cosmic rays are actually particles, accelerated by supernovae remnants, then knocked around in a game of stellar pinball. They ultimately slam into the Earth’s atmosphere…. The rays are made up of various atomic and subatomic particles, and we detect them by observing the cascade of particles that are created when one hits our atmosphere [Ars Technica]. PAMELA found an unexpected amount of high-energy positrons, and say that there are only two likely sources for these particles: dark matter collisions, or the dense, spinning stars known as pulsars that emit beams of radiation.
While most physicists agree that the new findings are exciting, many are not convinced that dark matter’s signature has finally been detected, and are hoping further studies will clarify the positrons’ source. Nasa’s Fermi Gamma-ray Space Telescope, which was launched in June 2008, is already taking measurements from pulsars and should produce data that could clarify the mysterious signal. [Astrophysicist Nigel] Smith thinks pulsars provide the most likely explanation. “It’s the simplest solution,” he said. “I think everyone will be waiting for the Fermi data to come in” [BBC News].
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Image: PAMELA Collaboration