  The properties of high-density nuclear and neutron matter are studied using a
relativistic mean-field approximation to the nuclear matter energy functional.
Based on ideas of effective field theory, nonlinear interactions between the
fields are introduced to parametrize the density dependence of the energy
functional. Various types of nonlinearities involving scalar-isoscalar
($\sigma$), vector-isoscalar ($\omega$), and vector-isovector ($\rho$) fields
are studied. After calibrating the model parameters at equilibrium nuclear
matter density, the model and parameter dependence of the resulting equation of
state is examined in the neutron-rich and high-density regime. It is possible
to build different models that reproduce the same observed properties at normal
nuclear densities, but which yield maximum neutron star masses that differ by
more than one solar mass. Implications for the existence of kaon condensates or
quark cores in neutron stars are discussed.
