Matters of Antimatter
As promised in my last post, I thought I’d spend a bit of time discussing the source of the fuss that cosmologists make over matter and antimatter.
Antimatter is just like ordinary matter in every way, except that every quantity you can think of (apart from mass and spin), is reversed. As an example, the electron is a particle with a specific mass and carrying a specific amount of negative electric charge. The antiparticle of the electron is a positron, which has the identical mass to an electron, but precisely the opposite charge. The thing about particles and their antiparticles is that, if one puts them together, the net value of any quantity (called a quantum number by physicists) carried by the pair of them is zero. Therefore, a particle and an antiparticle together are merely mass which, thanks to Einstein’s E=mc2, can be converted entirely into energy. As a result of this, when matter and antimatter come together, they annihilate, producing energy in the form of light (photons).
We know so much about antimatter for two reasons. The first is that it is a natural part of quantum field theories, which we use to describe matter, and which are among the best-tested theories in all of science. The second is that we can make and investigate antimatter in large amounts. For example, the purpose of the Fermi National Accelerator Laboratory near Chicago is to make vast numbers of antiprotons to study how they annihilate with protons.
Antimatter is important in cosmology because of the extreme temperatures and densities of the early universe, as described in a previous post. One consequence of such an extreme environment is that there is so much energy around that any kind of matter (including antimatter) can be created. Therefore, in the early universe, one expects there to have been equal amounts of both matter and antimatter and then, as the universe cooled, for these particles to find each other, annihilate, and leave our present universe with very little matter around (and an equally small amount of antimatter).
This is clearly at odds with what we observe in the universe, where we have relatively large amounts of matter and essentially no evidence of primordial antimatter. In fact, this asymmetry between matter and antimatter can be made quantitative (for baryons such as protons and neutrons) through observations of the abundances of light elements in the universe (Big Bang Nucleosynthesis) and also from the pattern of anisotropies in the cosmic microwave background radiation (CMB). At some point in the future I will post about both these topics, which are fascinating in their own rights. However, for now, suffice it to say that there is clear evidence that the universe is composed of matter, with negligible antimatter.
This all constitutes a puzzle for cosmologists. How did the universe evolve from early times, in which there were equal numbers of baryons and antibaryons, to the present universe, in which there is a precisely measured baryon asymmetry of the universe (BAU)?
Potential solutions to this puzzle provide a wonderful example of the interplay between particle physics and cosmology. A beautiful feature of many theories beyond the standard model of particle physics is that, when considered in the context of the expanding universe, they automatically contain such a dynamical mechanism that can, in principle, explain the origin of the BAU. The generation of the BAU through one of these mechanisms is what is known as baryogenesis. This isn’t enough of course; we don’t yet know which, if any, of these theories might be the right one. However, upcoming experiments, such as those at the Large Hadron Collider (LHC), provide the exciting possibility of either ruling out some of them or providing significant evidence for one of them. Since I’ve worked a lot on such ideas during my career, this is a topic close to my heart.
Antimatter is just like ordinary matter in every way, except that every quantity you can think of (apart from mass and spin), is reversed. As an example, the electron is a particle with a specific mass and carrying a specific amount of negative electric charge. The antiparticle of the electron is a positron, which has the identical mass to an electron, but precisely the opposite charge. The thing about particles and their antiparticles is that, if one puts them together, the net value of any quantity (called a quantum number by physicists) carried by the pair of them is zero. Therefore, a particle and an antiparticle together are merely mass which, thanks to Einstein’s E=mc2, can be converted entirely into energy. As a result of this, when matter and antimatter come together, they annihilate, producing energy in the form of light (photons).
We know so much about antimatter for two reasons. The first is that it is a natural part of quantum field theories, which we use to describe matter, and which are among the best-tested theories in all of science. The second is that we can make and investigate antimatter in large amounts. For example, the purpose of the Fermi National Accelerator Laboratory near Chicago is to make vast numbers of antiprotons to study how they annihilate with protons.
Antimatter is important in cosmology because of the extreme temperatures and densities of the early universe, as described in a previous post. One consequence of such an extreme environment is that there is so much energy around that any kind of matter (including antimatter) can be created. Therefore, in the early universe, one expects there to have been equal amounts of both matter and antimatter and then, as the universe cooled, for these particles to find each other, annihilate, and leave our present universe with very little matter around (and an equally small amount of antimatter).
This is clearly at odds with what we observe in the universe, where we have relatively large amounts of matter and essentially no evidence of primordial antimatter. In fact, this asymmetry between matter and antimatter can be made quantitative (for baryons such as protons and neutrons) through observations of the abundances of light elements in the universe (Big Bang Nucleosynthesis) and also from the pattern of anisotropies in the cosmic microwave background radiation (CMB). At some point in the future I will post about both these topics, which are fascinating in their own rights. However, for now, suffice it to say that there is clear evidence that the universe is composed of matter, with negligible antimatter.
This all constitutes a puzzle for cosmologists. How did the universe evolve from early times, in which there were equal numbers of baryons and antibaryons, to the present universe, in which there is a precisely measured baryon asymmetry of the universe (BAU)?
Potential solutions to this puzzle provide a wonderful example of the interplay between particle physics and cosmology. A beautiful feature of many theories beyond the standard model of particle physics is that, when considered in the context of the expanding universe, they automatically contain such a dynamical mechanism that can, in principle, explain the origin of the BAU. The generation of the BAU through one of these mechanisms is what is known as baryogenesis. This isn’t enough of course; we don’t yet know which, if any, of these theories might be the right one. However, upcoming experiments, such as those at the Large Hadron Collider (LHC), provide the exciting possibility of either ruling out some of them or providing significant evidence for one of them. Since I’ve worked a lot on such ideas during my career, this is a topic close to my heart.
posted by Mark Trodden at 12:04 AM
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