Neutron skins and neutron stars in the multi-messenger era.

The historical first detection of a binary neutron star merger by the LIGO-Virgo collaboration [B. P. Abbott {\sl et al.} Phys. Rev. Lett. 119, 161101 (2017)] is providing fundamental new insights into the astrophysical site for the $r$-process and on 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 nuclear matter are used to confront its predictions against tidal polarizabilities extracted from the gravitational-wave data. Given the sensitivity of the gravitational-wave signal to the underlying EOS, limits on the tidal polarizabilities inferred from the observation translate into stringent 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, under a particular binary-mass scenario, we deduce an upper limit on the radius of a $1.6\,M_{\odot}$ neutron star of $R_{\star}^{1.6}\!<\!13.25\,{\rm km}$. Given the sensitivity of the neutron-skin thickness of ${}^{208}$Pb to the symmetry energy, albeit at a lower density, we infer a corresponding upper limit of $R_{\rm skin}^{208}\!\lesssim\!0.25\,{\rm fm}$. 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.