The Alpha Magnetic Spectrometer (AMS) is one of the largest and most expensive particle physics detectors of all time. Unlike its cousins at CERN, it operates thousands of miles above the surface of the Earth, orbiting on board the International Space Station. It is designed to detect particles of antimatter flying through space as cosmic rays, ultra high energy particles from stars and other sources.
Antimatter is much like the normal matter we see around us– it has mass, and it feels gravity and magnetic fields. The first difference is that the ‘anti’ version of a particle has the opposite electric charge. For example, an electron is negatively charged whereas its antimatter counterpart, the positron, is positively charged. The real difference comes when an antiparticle comes into contact with the normal matter version of itself. When this happens an event called ‘annihilation’ occurs: converting all the mass of the two particles into energy in the form of photons, the particle which light is made from.
Most theories about the beginning of the Universe state that matter and antimatter should have been created in equal quantities in the Big Bang. However, the present day Universe appears to be made almost entirely of matter. We don’t see huge annihilation events when we look out into space – there are no planet-antiplanet collisions, or antimeteors annihilating as they hit our atmosphere. So the antimatter must have gone somewhere. There are many theories that seek to explain this missing antimatter, but it could be that at least some of it is out there in galaxies and galaxy clusters made entirely of antistars and antimatter. Of course these would have to be far, far away from those made of matter – annihilation between a galaxy and an antigalaxy would be unmissably visible. As far as we know, there is nothing in our local universe that looks like this, but that doesn’t rule out the existence of antigalaxies further away. The difficult part is finding them.
An antistar should act in the same way as a star made of matter. It would burn its fuel of antihydrogen and antihelium just like other stars, emitting photons as it went. It should also produce light elements just like our Sun – in this case antihydrogen or antihelium. Antihydrogen can be produced through other means as well, but antihelium is so hard to produce that it could almost only originate in an antistar. Thus finding antihelium nuclei passing through space would be a strong suggestion that antistars are out there somewhere. If this is the case it is possible that some, possibly many, of the galaxies we observe far, far away are made of antimatter.
The AMS hopes to do this using a huge magnet, approximately a meter across, which will deflect the course of antiparticles flying through space as they pass through the instruments in the detector. By looking at these tracks it will be possible to determine what interstellar debris the AMS is picking up, whether or not it is antimatter, and crucially whether any antihelium is out there. There is still a huge amount of data to analyse from the detector, so the question is still very much open. An antigalaxy could still be out there somewhere!