A century after their discovery, we still have a relatively poor understanding of where cosmic rays come from and how they reach Earth. Despite this, cosmic rays have revealed examples of matter previously unknown, such as the muon, positron and the group known as strange particles. The hunt is now on for dark matter and other possible exotic forms, known to nature but not yet to us.
Where do they come from?
Cosmic ray is a catch-all name for radiation of extraterrestrial origin. We now know that over 95 per cent of the 鈥減rimary鈥� cosmic rays that crash into Earth鈥檚 upper atmosphere are high-energy protons, with helium and other nuclei making up the remainder. Primary cosmic rays originate in deep space, but where exactly remains a mystery. Theories favour huge explosions, such as supernovae and gamma ray bursts, or supermassive black holes.
One reason why it is hard to identify the source of primary cosmic rays is that the vast majority of them are charged particles and so they are deflected by the magnetic fields that thread through space. By the time they arrive at Earth they come randomly from all over the sky.
Advertisement
Clues come from cosmic rays鈥� collisions with gas in interstellar space, which create electrically neutral pions, gamma rays and neutrinos. All three are unaffected by magnetic fields but gamma rays preserve their direction of travel best. High-energy gamma rays have been spotted coming from remnants of supernovae by NASA鈥檚 Fermi space telescope. Young supernovae seem to possess both the strongest magnetic fields and emit the highest-energy cosmic rays. These observations support Enrico Fermi鈥檚 , made in 1949, that cosmic rays get their high energies as a result of interactions between particles and wandering magnetic fields.
The latest theory for the source of the biggest hitting cosmic rays 鈥� with energies of around 100 million teraelectronvolts (TeV) 鈥� is supermassive black holes at the centres of galaxies. The previous candidate, gamma ray bursts, which are believed to arise from the collapse of massive stars to black holes, looks less likely thanks to observations from the IceCube Neutrino Observatory at the South Pole. It was thought that these catastrophic collapses would spew out protons and accelerate them to vast energies. These protons would interact with the gamma rays, producing pions and high-energy neutrinos. IceCube has been looking for these energetic neutrinos but has so far found none, suggesting there must be a different source for 100 million TeV cosmic rays.
Extreme energies
How nature accelerates particles to extreme energies far beyond anything possible with purpose-built accelerators has long been one of the main questions in astrophysics. The Large Hadron Collider at CERN is our most powerful accelerator, smashing proton beams each carrying 4 teraelectronvolts of energy. Cosmic rays can reach 100 million TeV (see chart). Although the flux of particles arriving at Earth with energy exceeding that is small, such particles pervade the cosmos and so in total represent a huge amount of energy.
Cosmic ray protons with these extreme energies may be the result of acceleration over long distances, perhaps involving several stages, rather than being the result of a single dramatic event. The strong magnetic fields in young supernovae, for example, can keep the highest-energy particles in the remnant鈥檚 shock wave long enough to speed them to the energies observed.
However, theory says that there should be a rapid fall off in the number of protons with energies above 50 million TeV coming from distant sources. That鈥檚 because the cosmos is filled with photons left over from the big bang, known as the cosmic microwave background. Protons of very high energies are unable to travel more than about 150 million light years before interacting with these photons. Such interactions produce pions and lead to a loss of energy. In effect, the universe is not transparent to ultra-high-energy protons, so they cannot penetrate space and reach Earth as cosmic rays. This is known as the GZK cut-off, after Kenneth Greisen, Georgy Zatsepin and Vadim Kuz鈥檓in, who first pointed out that this phenomenon should occur.
Even so, experiments occasionally claim to spot cosmic rays with energies that exceed this limit. One possibility is that these particles have been produced within 150 million light years of us and hence their numbers have been dimmed somewhat but not entirely cut off. While this cannot be ruled out, no obvious sources have been identified within that radius.
The evidence for such ultra-high-energy cosmic rays remains controversial. Determining the energy of cosmic rays up to and beyond the GZK limit is one of the outstanding challenges in the field.
Cosmic neutrinos
Neutrinos emitted by the sun pass through the Earth continuously. These solar neutrinos are traditionally not regarded as cosmic rays. Collisions of cosmic rays in the upper atmosphere spawn neutrinos similar to solar ones but also produce neutrinos of other flavours. By comparing these cosmic neutrinos with their solar counterparts, we have gained fundamental insights into the nature of these enigmatic particles. The findings have revealed that neutrinos have mass. The challenge for physicists is to explain why.
Antimatter and dark matter
Antimatter and matter were created equally from the energy of the big bang, theory has it. When British physicist Paul Dirac predicted antimatter, he remarked that there could be as many stars made of antimatter as of matter, if the laws of nature are the same for both. Individual particles of antimatter have indeed been seen in cosmic rays and created in experiments at particle accelerators. But there is no evidence for antimatter in bulk in our galaxy, nor throughout the observable universe.
If anti-stars had exploded in the cosmos, we would expect some examples of anti-nuclei to appear in cosmic rays. So far nothing even as complicated as a nucleus of anti-helium has been found.
Perhaps Earth鈥檚 atmosphere is confounding us. To find out, physicists have built an experiment called the Alpha Magnetic Spectrometer and mounted it on the International Space Station (see picture). AMS records thousands of cosmic rays each second. Being above the atmosphere, the particles are from primary cosmic rays and give information about the make-up of the wider universe. AMS is searching for antimatter and other exotic objects, including the constituents of mysterious dark matter.
Dark matter is an enigma. Cosmologists have inferred the existence of this invisible stuff, but we know nothing about its constituents other than that they must be massive and electrically neutral. If such particles exist, then they are new to science. Dark matter outweighs the visible matter in the universe by a ratio of about 4 to 1, and so one may expect that examples may be present in cosmic rays.
The challenge is to detect 鈥渄ark particles鈥�. They do not feel the forces that other particles do and will give themselves away, like H. G. Wells鈥檚 invisible man 鈥� by jostling the crowd. Deep underground, shielded from the majority of cosmic rays, supersensitive detectors seek evidence of a dark particle hitting an atom and making its nucleus recoil. This is exceedingly difficult to detect, and it is essential to reduce natural radioactive backgrounds to a minimum to give any genuine signal a chance of showing up.
