Image of a supernova remnant taken by the Hubble Space Telescope

Supernova remnants such as this are the source of many cosmic rays – however, this doesn’t explain the origin of many of the positrons that are found in cosmic rays.

Image: NASA, ESA, CXC, SAO, the Hubble Heritage Team (STScI/AURA), J. Hughes (Rutgers University)

You might not feel it, but it’s always raining – not raindrops, but cosmic rays. These high-energy particles from outer space could contain clues to some of the mysteries of the Universe.

Cosmic rays are charged particles moving at extremely high speeds that come from outside the Earth’s atmosphere. Some come from the Sun. However, most come from outside the Solar System. The main sources of cosmic rays are the remnants of exploding stars called supernovae.

Most cosmic rays are protons; positively charged particles that are also found in the nucleus of an atom. However, there are many different charged particles that make up cosmic rays, including fragments of atomic nuclei, electrons and positrons.

Positrons are relatively rare on Earth. A positron has the same mass as an electron, but an equal and opposite electric charge. Positrons are a type of antimatter, and a positron is said to be an electron’s antiparticle. When electrons and positrons meet, they annihilate each other and release energy. This is why positrons are rare – on Earth they will quickly collide with an electron and annihilate.

Positrons are created when cosmic rays interact with gas molecules between the stars. Physicists made predictions of the amount and energies of positrons that would be formed this way. Using particle detectors on balloons, these predictions appeared to be confirmed.

Recently, physicists have been able to use particle detectors on space craft in orbit around the Earth. The results from these experiments show more positrons than were expected above a certain energy level. This suggests that there is another, unknown source of antimatter.

Physicists don’t yet know what this source is. One theory is that cosmic bodies might create antimatter by acting as particle accelerators. Another theory is that positrons might be created by interactions between particles of dark matter, which makes up about a quarter of the mass of the Universe but whose true nature is unknown.

These extra positrons demonstrate that sometimes scientists’ predictions aren’t completely accurate, but by continuing to experiment and observe they are able to develop a clearer picture of what is going on in the Universe.

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