Prof. says 'WIMPZILLAS' source of high-energy cosmic rays
By Kristina Bergen
Ultra-high energy (UHE) cosmic rays travelling at nearly the speed of light shoot through the galaxy from all directions, striking Earth's atmosphere roughly every six seconds.
Most cosmic rays originate from stars, but the energy source for these UHE cosmic rays has been a mystery - until now.
University of Saskatchewan astrophysicist Rainer Dick and international colleagues have proposed a new theory that may explain this momentous puzzle in contemporary particle astrophysics.
"Scientists have been trying to determine the origin of ultra-high energy cosmic rays for almost 40 years," said Dick.
"These cosmic rays strike Earth with such high energies (comparable to the energy of a tennis ball moving at 100 km per hour) that we know they can't have travelled from another galaxy. So they must have originated within our own."
Dick hypothesizes that super-heavy particles of dark matter, called 'WIMPZILLAS', are responsible for the existence of UHE cosmic rays.
Dark matter is made of invisible particles generated at extremely high temperatures mere seconds after the universe was created. These particles usually don't give off or absorb light except when two of them collide and annihilate in a burst of radiation.
WIMPZILLAS are the Godzilla-sized version of dark matter particles called Weakly Interacting Massive Particles (WIMPS). WIMPZILLAS weigh only a trillionth of a gram, but that's more than 100 billion times the mass of an atom - heavier than any other known elementary particle.
Dick thinks that WIMPZILLAS clump together in the dark matter halo surrounding our galaxy. When two of them collide and are annihilated, they release extremely high amounts of energy - UHE cosmic rays.
His theory challenges a 1997 theory put forward by a Russian team and a U.S.-Italian-German collaboration that a single dark matter particle deteriorates, resulting in UHE cosmic rays.
Cosmic rays were first discovered back in 1912, but UHE cosmic rays weren't observed until the 1960s with the advent of ground-based cosmic ray detectors.
In its June issue, New Scientist published an article on WIMPZILLAS by Dick's team . A paper explaining their idea will appear in the September issue of Astroparticle Physics.
The existence of WIMPZILLAS may also solve a second puzzle - the missing mass of the universe.
Most of the matter in the universe is clustered into galaxies, but only a tiny fraction of it, such as stars or planets, is visible. Scientists measure the mass of a galaxy by determining how fast it rotates, or by observing how light rays are bent in its gravitational field. What they find is that galaxies weigh more than the sum of their stars.
"We know that the mass of the galaxy is made up of much more than stars," said Dick. "Massive dark matter particles, WIMPZILLAS, account for the missing mass."
The theory will be tested at the Pierre Auger Observatory now under construction in Argentina. Over the first three or four years of operation, hundreds of UHE cosmic rays will be tracked and studied using 1,600 cosmic ray detectors over 3,000 sq. km.
Co-developers of the WIMPZILLA theory are Edward Kolb of the Enrico Fermi Institute in Chicago and Pasquale Blasi of the Arcetri Astrophysical Observatory in Florence, Italy.
Funding was provided by NSERC (Natural Sciences and Engineering Research Council) and NASA/Fermilab Astrophysics Center.