SCIENCE

What was it like when matter defeated antimatter? | by Ethan Siegel | Starts With A Bang! | Nov, 2023

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At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. Although the laws of physics are largely symmetric between matter and antimatter, it’s very clear that today’s Universe is filled with matter and nearly completely devoid of antimatter. Any asymmetry must have been generated in the very early Universe, shortly following the hot Big Bang. (Credit: zombiu26 / Adobe Stock)

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 very hot temperatures and high densities in the early Universe, there are no bound protons or neutrons, but only a plasma of free quarks and gluons, along with other subatomic particles and quanta. As the Universe expands and cools, particle-antiparticle pairs annihilate away and become more difficult to recreate with subsequent, lower-energy collisions. (Credit: Models and Data Analysis Initiative/Duke University)

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:

  1. You have to have enough energy in the collision to create the particle (or antiparticle) in question, as given by E = mc².
  2. 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:

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