Physics and Reality

What came before atoms?

There is a point in the history of the universe beyond which our reconstruction becomes increasingly uncertain and model dependent. But from that point onward, its history is sufficiently well understood that we can follow it step by step.

That stage came before atoms existed, and long before stars and galaxies appeared. It was a time when the universe was a hot plasma of a small number of particle species.

At that stage the universe was made mostly of the following:

photons — the particles of light.
electrons and positrons. A positron is the antiparticle of the electron: it has the opposite electric charge, but is otherwise identical.
protons and neutrons, which are the building blocks of atomic nuclei. Different nuclei differ simply by how many protons and neutrons they contain. A neutron is unstable in isolation and decays in about fifteen minutes. But when bound together with a proton inside a nucleus, it can become stable.
neutrinos — extremely light and very weakly interacting particles. They appear, for example, in the decay of a neutron into a proton and an electron: n → p + e + neutrino

This was the era of a rapidly interacting hot soup of particles, at a temperature of about ten billion degrees Celsius. Our universe, as we observe it across its full visible expanse, seems to require such a plasma to have been spread almost perfectly homogeneously over distances a billion times shorter than the size of our present horizon.

The particles were constantly colliding, annihilating, being produced, and transforming into one another. Electrons and positrons could annihilate into photons, while photons with enough energy could produce electron-positron pairs again. Protons and neutrons could convert into one another through weak interactions involving electrons, positrons, and neutrinos.

Nothing seemed to remain fixed for long. And yet, despite all this activity, the system was held in balance precisely by these interactions.

This did not mean that nothing happened. On the contrary, it meant that everything happened in both directions, at matching rates. A process and its reverse continuously compensated one another, so that the overall composition remained steady.

What made this state possible was the sheer rapidity of interactions.

As long as collisions and transformations occurred quickly enough, equilibrium could be maintained. But the universe was expanding, and expansion steadily changed the conditions. The density dropped. The temperature fell. The time between interactions grew.

This mattered because not all particles interact with the same strength.

Photons interact readily with charged particles. Electrons and positrons also interact efficiently. Neutrinos do not. Their link to the rest of the plasma is much weaker. So although all these particles once lived together in the same thermal world, they were not equally tied to it.

This difference would eventually matter.

The later universe — with its atoms, light elements, stars, and galaxies — emerged from this simpler stage. But before any of that could happen, the universe first had to live through this hot and tightly coupled particle era.

That is the world out of which nuclei would eventually emerge.