Conventionally, we assume that there was some asymmetric process that happened in the early Universe to give rise to this asymmetry. Instead, however, what we see is a Universe with 'only' about 1-or-2 billion photons for every proton. There would be approximately the same number of electrons and positrons as there are protons and antiprotons, and that would be it. If we didn't have any matter/antimatter asymmetry, we would have wound up with a Universe that has an incredible 10 20 photons for every proton, and one antiproton for every proton as well. matter-antimatter pairs annihilate and photons can no longer collide at high enough energies to create new particles. As the Universe cools, you run out of energy to make new pairs, and the annihilation dominates.Īs the Universe expands and cools, unstable particles and antiparticles decay, while. In equal amounts, these pairs annihilate, producing pure energy (photons) again. This is because, at high temperatures and densities, you can spontaneously produce new particle-antiparticle pairs from pure energy, via Einstein's E = mc 2. In the standard picture of the hot Big Bang, when the Universe was in its very early stages, particle-antiparticle pairs of all the known (and even any yet-to-be-discovered) particles were created in tremendous abundance. The standard interpretation of these facts is that, even though we aren't entirely sure how, we must have created more matter than antimatter in the Universe's past. This creation-and-annihilation process, which obeys E = mc^2, is the only known way to create and destroy matter or antimatter. reaction (right), with matter/antimatter annihilating back to pure energy. The production of matter/antimatter pairs (left) from pure energy is a completely reversible.

Matter antimatter