Ten microseconds after the Big Bang, the Universe resembled a pot of boiling broth. Floating in it were quarks and gluons — the particles that would later assemble into protons and neutrons. As the broth cooled, at some point the quarks clumped together into lumps — the very protons and neutrons that make up all visible matter.
This transition is like molten wax hardening into a candle. Except instead of wax, we have quark-gluon plasma, and instead of a candle, the entire observable Universe. Scientists have learned to recreate this moment in accelerators by slamming heavy nuclei together at enormous speeds. For an instant, a tiny droplet of that primordial plasma is born, and you can watch it cool and solidify.
Over forty years of experiments, a wealth of data has accumulated. New measurements show how particles are born and how their numbers fluctuate from collision to collision. These fluctuations are like the trembling of solidifying wax, revealing its composition. By comparing measurements with calculations, physicists are refining the map of the strong interaction — the very glue that holds quarks together.
Ahead are new accelerators and new collisions. They will help us understand exactly how the quark broth turned into the matter that makes up stars, planets, and ourselves. That pot of broth is still simmering — now in laboratories around the world.
In the first microseconds after the Big Bang, the entire observable Universe was the size of a small city, yet it weighed as much as all the galaxies combined.
In Greg Egan's novel Schild's Ladder, exotic states of matter reminiscent of quark-gluon plasma are explored.