Question

Many descriptions about matter-antimatter asymmetry starts with the statement that observed baryon to photon density (about 6.1x10^(-10)) is too small. What is the reason? What ratio is expected?

Answer 1

What ratio is expected?

If we postulate matter and antimatter to be completely symmetric, then they should annihilate completely and there shouldn't be any matter or antimatter at all. And the ratio should be zero.

too small

I've never heard that people say that it is "too" small. The point is that it is just "very" small. From experiments we observe that matter and antimatter are almost symmetric with respect to the laws of physics -- the difference between matter and antimatter is really tiny. Why then there is so much matter and no antimatter at all? Doesn't look symmetric to me -- even if we add "almost" symmetric.

The answer is that we have to compare amount of matter to amount of photons. Photons was dominating in Early Universe. They had all the energy and was constantly producing matter-antimatter pairs. And these pairs was constantly annihilating back to photons.

If one had some little asymmetry between matter and antimatter in the laws of physics (one actually needs some extra conditions). Then one can produce some extra tiny amount of matter which will not annihilate with antimatter and remain after Universe cools.