The visible mass in the Universe emerged at the Big Bang when hadrons — the building blocks of atomic nuclei — formed from a hot fireball made of quarks and gluons reports Nature. When temperatures raise to trillions of degrees, particles deep inside the atoms start to shift into new, non-atomic states. Protons and neutrons inside the fireball of a neutron-star merger reach a density analogous to the effect of cramming New York City into a sugar cube.
Quarks, the smallest building-blocks of matter, never appear alone in nature. They are always tightly bound inside the protons and neutrons. However, neutron stars, weighing as much as the Sun, but being just the size of a city like Frankfurt, possess a core so dense that a transition from neutron matter to quark matter may occur. Physicists refer to this process as a phase transition, a fancy term to describe a holistic change in the overall arrangement of a system’s structure, and in turn, its function.