the early universe as a natural accelerator

When physicists want to study particles at extreme energies, they build a collider to smash them together hard. But the early universe did this for free, everywhere, all at once. In the first tiny fractions of a second after the Big Bang, everything was unimaginably hot and dense, and in physics, heat is just energetic motion. The particles in that primordial fireball were slamming into each other at energies that dwarf anything our biggest accelerators can reach. In a real sense, the young universe was the most powerful particle accelerator that has ever existed.

The key idea is that temperature and particle energy are two ways of saying the same thing. The hotter a soup of particles, the harder they collide, so the early universe was a kind of natural laboratory running every particle-physics experiment at once, simply by being hot. As it expanded it cooled, sweeping down through energy scale after energy scale: at the very highest energies, exotic processes we can only theorise about; a little later, at energies our colliders can almost reach, the Higgs field switching on and giving particles their masses; later still, quarks binding into protons and neutrons; and eventually nuclei and atoms forming. Each cooling stage is a chapter of particle physics played out across the whole cosmos.

This is why particle physics and cosmology are so deeply intertwined. The early universe took the laws of particle physics and ran them at energies we cannot achieve in a machine, then left fossils we can still read today — the abundances of the light elements, the patterns in the cosmic microwave background, the amount of dark matter, the leftover excess of matter over antimatter. By studying these relics, physicists test theories of particle physics against the one experiment no laboratory can rival, and conversely, every advance in particle physics sharpens our story of the universe's first moments.