It’s the oldest question in the world and it occurs to every child, sooner or later: where do I come from? Mum and Dad of course, but where did they come from? Genetics only takes us so far; our line of ancestors actually stretches back beyond our first single celled forebears. Chemistry proceeds biology, and before that the world was made only of protons, neutrons and electrons. Now this takes us pretty far back, to the first second of the universe. In many ways, our fate was decided in this instant. The protons and neutrons we are made of formed a millionth of a second after the proverbial lights went on, condensing out of quarks. But where did the quarks come from?
Photo courtesy of NASA
Baryogenesis as a concept is not too difficult to follow. Every molecule you see around you is a survivor of a vast catastrophe that struck the early universe, when 30,000,000 of every 30,000,001 quarks in the universe were destroyed. The culprit of this disaster is antimatter – the bizarro version of matter. The crux of the matter is that matter and antimatter have a love-hate relationship; they annihilate each other, but also prefer to be created together. In the present day our universe is just too cold to create matter out of thin air (actually, through interactions with particles like photons), but this was not always so. When we go far enough back, at temperatures of about 10^13 degrees Celsius pair creation kicks off and the universe is filled with massive amounts of matter and antimatter. While this is lukewarm for a particle physicist there are more orders of magnitude between this temperature and the sun’s core than the sun’s core and you. From what I have said, the origin of matter doesn’t seem like much of a mystery; pair creation made matter. The problem is that it also made antimatter, and (according to the Standard Model) in equal amounts. When the universe cooled, matter could no longer be created, only destroyed, and so both matter and antimatter dwindled into nothing.
Creation (or destruction) of an electron-positron pair. Canny readers will notice that I have used this little diagram before.
Clearly this is not the case – as any child can see, our universe is a populated and interesting one, filled with stars and planets and puppies. Above all, our universe is made of matter – no antimatter allowed. So there must have been a kind of discrimination against antimatter for some matter to survive this rampant destruction. Either this asymmetry between matter and antimatter existed from the start, as some sort of initial condition, or it somehow has dynamically evolved since then. Inflation dilutes any primordial asymmetry even more than a homeopathic remedy, so there must have been some matter creating process – baryogenesis. In any case, simply citing “initial conditions” is almost like saying “just because”, which never really seems to work with children.
When you need to explain something, it is usually best to start by what broad features your theory must have. For baryogenesis, Sahkarov did this back in 1967. For any theory (that doesn’t violate CPT) to create an asymmetry between matter and antimatter, three conditions must be satisfied:
- Baryon number must be violated. If you wish to count the number of protons and neutrons, it turns out that assigning them a quantity called “baryon number” is useful, a proton and neutron each have a baryon number of 1, and a quark has a baryon number of 1/3. Antimatter versions have a negative baryon number. The process that leads to the predominance of matter over antimatter, and so baryons over anti-baryons, is referred to as “baryogenesis”. It turns out that the total baryon number of the universe is conserved under perturbative effects in the Standard Model, what is known as an “accidental symmetry”. If we want more protons than antiprotons this number cannot be inviolate. There is a similar counting of electrons and neutrinos called lepton number, which is also believed to be broken. Unfortunately as neutrinos are extremely difficult to observe there is no direct evidence of a total lepton asymmetry.
- Matter and antimatter must be treated differently. This means that charge conjugation (where you swap particles with antiparticles) and charge-parity conjugation (swap particles with antiparticles and also reflect them like a mirror image) cannot leave the physics unchanged. More succinctly, C and CP must be broken. While C violation is trivial (the weak force violates C maximally), CP is almost entirely preserved in the Standard Model. This is both a major failing of the Standard Model and a fantastic prediction – we know that CP violation is absolutely fundamental to our universe, and that there must be more of it than we have seen so far. You have probably seen CP violation mentioned many times, both on this site and through news reports. The necessity for CP violation to explain our own existence is the real reason why CP violation deserves our attention.
- The universe must go out of thermal equilibrium. In thermal equilibrium any process creating a baryon asymmetry would be balanced by its reverse. Fortunately for us, the fact that the universe expands creates periods of thermal non-equilibrium, such as phase changes (like when the Higgs mechanism breaks the electroweak symmetry of the Standard Model).
While the Standard Model does technically satisfy all three of these, it does so in a trivial way. The amount of CP violation is far too low, and a universe in which the Standard Model is entirely correct never gets far enough out of equilibrium to allow a large difference in matter and antimatter to form even if it did violate CP more. The only really useful element that the Standard Model has is baryon number violation; a non-perturbative process called sphalerons occurs above the electroweak phase transitions which violates baryon and lepton number. More importantly, it preserves a linear combination of the two, so if you manage to make a baron asymmetry or a lepton asymmetry, you automatically get both. Theories like leptogenesis use this to turn a lepton asymmetry into a baryon asymmetry. While there are many possible scenarios that could have lead to the present day world (my own work is in one of these, asymmetric dark matter), the truth is that we simply don’t know which of these, if any, is correct.
Despite this being a question of the most fundamental kind, baryogenesis does not get nearly the same kind of media attention as dark matter or dark energy. Partly this is because we have little chance of experimentally finding an answer – baryogenesis could have occurred at almost any energy scale, which includes a good many far out of the reach of our colliders. But it is still important to push for an answer. Nothing is a better mark of our progress in understanding our origins than seeing how the question we ask about our origin evolves.
