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WHERE DOES MATTER COME FROM? - cont.

       He suggested that three conditions must be satisfied in order to produce the matter excess. First, there must be a way of creating both more matter and antimatter particles of the kinds which are important to us—that is, the kinds that make up the atoms we are made of. Then, there must be a mechanism to bias the creation of more matter than antimatter. And finally, once we have an excess of matter particles over their antimatter partners, we must make sure that this excess is not erased as the universe continues to expand.

       The first of these conditions is the creation of both baryons and anti-baryons from collisions involving the other particles present in the primordial soup. Baryons are particles which interact via the strong nuclear force, the force responsible for holding the nucleus together. Protons and neutrons (a.k.a. nucleons), and their constituent parts called quarks, are all baryons. At low energies, the number of baryons participating in collisions between different particles is conserved: that is, just like electric charge, the total number of baryons before an interaction equals the total after. If we are interested in making baryons, as we must in order to create matter in the universe, this conservation law is not very useful. According to Sakharov’s requirement, however, at very high energies the interactions between elementary particles should not conserve the number of baryons. That is, at high energies both baryons and anti-baryons can be created from “other” particles. These high energies are naturally realized in the hot furnace of the early universe.

         But this first condition does not differentiate between baryons and anti-baryons. At high temperatures we could still create the same number of each, and that wouldn’t cause a bias toward matter over antimatter. We need a second condition. Once the high energies of the early universe allow for the creation of baryons and anti-baryons, we need a condition that selects, or biases, the creation of baryons over anti-baryons, an arrow pointing in the correct direction (i.e., toward matter).

        In 1964, J.H. Christenson and his collaborators found experimental evidence that interactions between certain baryons do indeed exhibit this bias.

        It is as if Nature has its own biases, in this case toward more baryons. If this is true in laboratory experiments, no doubt this will also be true in the early universe. Making excess matter over antimatter is not as hard as it initially seemed to be. But this is still not the whole story. One more challenge remains, which has to do with the physics of hot systems, also known as thermodynamics.  

       One of the properties of very hot systems is that they have no memory of their past. Imagine a coffee spoon which is initially cold. Now immerse one of its ends into a very hot cup of coffee. What happens? Although initially only the end in the coffee will be hot, very quickly the whole spoon will be equally hot. You won’t be able to tell which of the two ends was immersed into the coffee cup; the system (coffee spoon and hot coffee) lost its “memory.” Another term for this loss of memory is thermal equilibrium. If the early universe was in thermal equilibrium, any excess baryons would have been deleted; in equilibrium, the net baryon number is zero. In order to maintain the baryon bias as the universe cools, we need to make sure the universe doesn’t “lose its memory” and delete the new baryons. Therefore, we need a third condition.    
 

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