  Recent progress in quantum Monte Carlo with modern nucleon-nucleon
interactions have enabled the successful description of properties of light
nuclei and neutron-rich matter. As a demonstration, we show that the agreement
between theoretical calculations of the charge form factor of 12C and the
experimental data is excellent. Applying similar methods to isospin-asymmetric
systems allows one to describe neutrons confined in an external potential and
homogeneous neutron-rich matter. Of particular interest is the nuclear symmetry
energy, the energy cost of creating an isospin asymmetry. Combining these
advances with recent observations of neutron star masses and radii gives
insight into the equation of state of neutron-rich matter near and above the
saturation density. In particular, neutron star radius measurements constrain
the derivative of the symmetry energy.
