Matter at high densities

Abstract

The equation of state of matter in its most stable form at different densities ϱ and at zero temperature is discussed. Using effective range theory and the low density limit of the nuclear many-body formalism, the energy and pressure of a neutron gas is calculated at densities where the interparticle distance is large compared with the effective range. Energy and pressure of a neutron gas at densities comparable to nuclear densities are estimated next, using a semiempirical method. Most probably neutron matter is not bound at any density. Coulomb effects and the effect of beta-decays are considered and it is found that free neutrons are at equilibrium with ionized nuclei like Tc 145 at a density of about 3 × 10 11 g/cc with an electron Fermi energy of about 23 Mev. For the unlikely case of bound neutron matter an “electron skin” surrounding large neutron drops contributes a negative surface tension and neutron drops of radius about 100 fermis would probably be most stable. At densities above 10 15 g/cc hyperons coexist with protons and neutrons and their effect on the pressure is considered. Finally, at very high densities, a simple approximation is given for the pressure due to the proximity of the hard repulsive cores of nuclear forces.