Category: SCIENCE

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:

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:

In the very early Universe, practically all particles were massless. Then the Higgs symmetry broke, and suddenly everything was different.

In the earliest stages of the hot Big Bang, the Universe was filled with all the particles, antiparticles, and quanta of radiation it had the energy to create. As the Universe expanded, it cooled: the stretching fabric of space also stretched the wavelengths of all the radiation within it to longer wavelengths, which equates to lower energies.

If there are any particles (and antiparticles) that exist at higher energies that are yet to be discovered, they were likely created in the hot Big Bang, so long two things are true:

• that all necessary quantum conservation laws (spin, energy, charge, angular momentum, etc.) are still obeyed, and
• there was enough energy (E) available to create a particle of that particular mass (m) via Einstein’s E = mc².

It’s possible that a slew of puzzles about our Universe, including the origin of the matter-antimatter asymmetry and the creation of dark matter, will wind up being solved by new, yet undiscovered physics at these early times. But the massive particles we know today would all appear foreign to us under the earliest conditions of the hot Big Bang. At these early stages, none of them have any mass at all.

The particles and antiparticles of the Standard Model are easy to create in the early stages of the hot Big Bang, even as the Universe cools and the fractions-of-a-second tick by. The Universe might start of at energies as large as 10¹⁵ or 10¹⁶ GeV; even by time it’s dropped to 1000 (10³) GeV, there’s still enough energy to easily create each and every Standard Model particle and antiparticle. At the energies achievable by the LHC, we can create the full suite of particle-antiparticle pairs that are known to physics, which includes every species of particle described by the Standard Model.