Striving to uncover clues about the early universe, scientists worldwide are smashing together heavy ions to produce exotic states of matter that may not have existed naturally since shortly after the Big Bang. The ultimate goal is a quark-gluon plasma (QGP), a soup of particles that existed before the universe cooled off to form protons and neutrons. The exact temperature and density required to form a quark-gluon plasma are not well determined: The green band shows an earlier estimate (circa 1998) of the temperatures and densities needed (a baryon density of 1 refers to the density of a typical nucleus; a baryon density of 5-10, typical in neutron stars, means 5-10 times denser than ordinary nuclear matter.) In state-of-the-art heavy-ion experiments, scientists have estimated that temperatures of about 2 billion Celsius may be required for quark-gluon plasmas to appear; here the temperatures are expressed in terms of the energy of the particles, in the millions of electron volts (MeVs). Interestingly, QGPs may also exist in the inaccessible cores of neutron stars.
Even without the appearance of a quark-gluon plasma, these heavy-ion experiments can form new states of nuclear matter--and their properties, which differ markedly from the matter we observe today, can offer tantalizing clues about the early universe.
(Figure courtesy Lawrence Berkeley Lab.)