Collisions of heavy nuclei at RHIC (and soon LHC) energies create a state of matter similar to that of the early universe a few microseconds after the big bang, characterized by a temperature of T ~200-400 MeV. Temperature is one useful intensive thermodynamical variable for depicting the phase diagram of nuclear matter. Another is the net baryon density ρB (or alternatively, the baryon chemical potential μB), reflecting the fact that baryon number is a conserved quantity. The central region of a collision at RHIC is nearly (net!) baryon free, with a value of μB/T ~10% << 1, and therefore crudely approximates the early universe, where this ratio was of order 10-9.
As exciting as it is to explore the early universe in the laboratory, very relativistic nuclear collisions access only a ‘small’ region of the phase diagram near μB =0. There is also enormous astrophysical relevance along the μB axis very near T=0. This is the domain that describes nuclear matter as found in neutron stars. (more…)