When the universe was born, space was too hot and dense for atoms to exist. Even the constituents of atoms—the protons and neutrons that make up atomic nuclei—couldn’t immediately coalesce. Instead the newborn cosmos was filled with even smaller and more fundamental building blocks, quarks and gluons, which formed a smoothly flowing soup called quark-gluon plasma.
Soon, though, the universe expanded, cooling down the soup. Gluons glued quarks together to make protons and neutrons, which floated around as a gas rather than a plasma. The change marked a phase transition, just as water can switch from vapor to liquid to solid ice. But key details of this profound primordial transition, such as its threshold energy and density, have remained deeply mysterious.
That began to change around the turn of the millennium, when physicists started smashing gold ions together at nearly the speed of light inside the giant particle accelerator at Brookhaven National Laboratory on Long Island in New York State. Those smashups were so powerful that they briefly re-created the quark-gluon plasma from the big bang. Now researchers analyzing data from those collisions have seen an unexpected signal that offers new clues about how the cosmic transition happened eons ago.
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