One of the more annoying questions in physics is why are we here? Bear with me for a minute, as this is a real question. The Universe is supposed to have started with what Fred Hoyle called “The Big Bang”. Fred was being derisory, but the name stuck. Anyway what happened is that a very highly intense burst of energy began expanding, and as it did, perforce the energy became less dense. As that happened, out condensed elementary particles. On an extremely small scale, that happens in high-energy collisions, such as in the Large Hadron Collider. So we are reasonably convinced we know what happened up to this point, but there is a very big fly in the ointment. When such particles condense out we get an equal amount of matter and what we call antimatter. (In principle, we should get dark matter too, but since we do not know what that is, I shall leave that.)
Antimatter is, as you might guess, the opposite of matter. The most obvious example is the positron, which is exactly the same as the electron except it has positive electric charge, so when a positron is around an electron they attract. In principle, if they were to hit each other they would release an infinite amount of energy, but nature hates the infinities that come out of our equations so when they get so close they annihilate each other and you get two gamma ray photons that leave in opposite directions to conserve momentum. That is more or less what happens when antimatter generally meets matter – they annihilate each other, which is why, when we make antimatter in colliders, if we want to collect it we have to do it very carefully with magnetic traps and in a vacuum.
So now we get to the problem of why we are here: with all that antimatter made in equal proportions to matter, why do we have so much matter? As it happens, the symmetry is violated very slightly in kaon decay, but this is probably not particularly helpful because the effect is too slight. In the previous post on muon decay I mentioned that that could be a clue that there might be physics beyond the Standard Model to be unraveled. Right now, the fact that there is so much matter in the Universe should be a far stronger clue that something is wrong with the Standard Model.
Or is it? One observation that throws that into doubt was published in the Physical Review, D, 103, 083016 in April this year. But before coming to that, some background. A little over ten years ago, colliding heavy ions made a small amount of anti helium-3, and a little later, antihelium-4. The antihelium has two antiprotons, and one or two antineutrons. To make this, the problem is to get enough antiprotons and antineutrons close enough. To give some idea of the trouble, a billion collisions of gold ions with energies of two hundred billion and sixty-two billion electron volts produced 18 atoms of antihelium 4, with masses of 3.73 billion electron volts. In such a collision, the energy requires a temperature of over 250,000 times that of the sun’s core.
Such antihelium can be detected through gamma ray frequencies when the atoms decay on striking matter, and apparently also through the Alpha Magnetic Spectrometer on the International Space Station, which tracks cosmic rays. The important point is that antihelium-4 behaves exactly the same as an alpha particle, except that, because the antiprotons have negative charge, their trajectories bend in the opposite direction to ordinary nuclei. These antinuclei can be made through the energies of cosmic rays hitting something, however it has been calculated that the amount of antihelium-3 detected so far is 50 times too great to be explained by cosmic rays, and the amount of antihelium-4 detected is 100,000 times too much.
How can this be? The simple answer is that the antihelium is being made by antistars. If you accept them, gamma ray detection indicates 5787 sources, and it has been proposed that at least fourteen of these are antistars, and if we look at the oldest stars near the centre of the galaxy, then estimates suggest up to a fifth of the stars there could be antistars, possibly with antiplanets. If there were people on these, giving them a hug would be outright disastrous for each of you.Of course, caution here is required. It is always possible that this antihelium was made in a more mundane way that as yet we do not understand. On the other hand, if there are antistars, it solves automatically a huge problem, even if it creates a bigger one: how did the matter and antimatter separate? As is often the case in science, solving one problem creates even bigger problems. However, real antistars would alter our view of the universe and as long as the antimatter is at a good distance, we can accept them.