  We investigate asymmetric nuclear matter with two- and three-nucleon
interactions based on chiral effective field theory, where three-body forces
are fit only to light nuclei. Focusing on neutron-rich matter, we calculate the
energy for different proton fractions and include estimates of the theoretical
uncertainty. We use our ab-initio results to test the quadratic expansion
around symmetric matter with the symmetry energy term, and confirm its validity
for highly asymmetric systems. Our calculations are in remarkable agreement
with an empirical parametrization for the energy density. These findings are
very useful for astrophysical applications and for developing new equations of
state.
