In the earliest stages of the hot Big Bang, there equal amounts of matter and antimatter should have existed. Why aren’t they equal today?
13.8 billion years ago, at the moment of the Big Bang, the Universe was the hottest it’s ever been in history. Every single known particle exists in great abundance, along with equal amounts of their antiparticle counterparts, all smashing rapidly and repeatedly into everything around them. The spontaneously create themselves from pure energy, and annihilate away into pure energy whenever particle-antiparticle pairs meet up.
Additionally, anything else that can exist at these energies — new fields, new particles, or even dark matter — will spontaneously create itself under these conditions, too. But the Universe cannot sustain these hot, symmetric conditions. Immediately, it not only expands, but cools. In a fraction of a second, these unstable particles and antiparticles vanish, leaving a Universe favoring matter over antimatter. Here’s how it happens.
At the moment of the Big Bang, the Universe is filled with everything that can be created up to its maximum total energy. There are only two barriers that exist:
- You have to have enough energy in the collision to create the particle (or antiparticle) in question, as given by E = mc².
- You have to conserve all the quantum numbers that need to be conserved in every interaction that takes place.
That’s it. In the early Universe, energies and temperatures are so high that you not only make all of the Standard Model particles and antiparticles, you can create anything else that energy allows. This could include: