The Primordial Soup Protons and neutrons were originally thought to be indivisible, but we now know that they have constituent parts—quarks. Quarks are bound together tightly through gluons and can only come apart under extreme conditions—conditions believed microseconds after the Big Bang to have formed a hot, dense “soup” of quarks and gluons, called the quark-gluon plasma (QGP). QGP can be made experimentally in heavy ion colliders, where extremely high temperatures are reached. Jacak and Müller (p. 310 ) review the progress in this field and its unexpected ties to cold atomic gases and string theory.