The tidal polarizability of neutron stars, the neutron skin thickness of heavy nuclei, and the nature of dense matter

The historic first detection of a binary neutron star merger is providing fundamental insights into the nature of dense matter.  A set of realistic models of the equation of state (EOS) that yield an accurate description of the properties of finite nuclei, support neutron stars of two solar masses, and provide a Lorentz covariant extrapolation to dense matter are used to confront its predictions against tidal polarizabilities extracted from the gravitational-wave data. Limits on the tidal polarizability translate into constraints on the neutron-star radius. Based on these constraints, models that predict a stiff symmetry energy, and thus large stellar radii, can be ruled out. Indeed, we deduce an upper limit on the radius of a 1.4M_sun neutron star of R_1.4 < 13.76 km. Given the sensitivity of the neutron-skin thickness of 208Pb to the symmetry energy, albeit at a lower density, we infer a corresponding upper limit of about R208 ≲ 0.25 fm. However, if the upcoming PREX-II experiment measures a significantly thicker skin, this may be evidence of a softening of the symmetry energy at high densities—likely indicative of a phase transition in the interior of neutron stars.