  We present a model-independent description of low-density neutron matter
based on the virial expansion. The virial equation of state provides a
benchmark for all nuclear equations of state at densities and temperatures
where the interparticle separation is large compared to the thermal wavelength.
We calculate the second virial coefficient directly from the nucleon-nucleon
scattering phase shifts. Our results for the pressure, energy, entropy and the
free energy correctly include the physics of the large neutron-neutron
scattering length. We find that, as in the universal regime, thermodynamic
properties of neutron matter scale over a wide range of temperatures, but with
a significantly reduced interaction coefficient compared to the unitary limit.
