Neutron rich matter on heaven and earth

Neutron stars are unique cosmic laboratories for the exploration of matter
under extreme conditions of density and neutron-proton asymmetry. The
historical first detection of the binary neutron star merger GW170817 by
the LIGO-Virgo collaboration is providing fundamental new insights into the
astrophysical site for the r-process and on the nature of dense matter.
Limits inferred from the gravitational wave signal seem to suggest that
neutron stars are fairly compact—implying that the symmetry energy
(which encodes the cost of converting protons into neutrons) is relatively
soft. In turn, these limits translate into an upper limit on the neutron-skin
thickness of 208Pb that is significantly lower than the central value
measured by the PREX collaboration using parity violating electron
scattering. This suggests an intriguing possibility. If the upcoming PREX-II
experiment confirms that the neutron- skin thickness of 208Pb is large, this
may be evidence in favor of a softening of the symmetry energy at the
higher densities probed by GW170817—likely indicative of a phase
transition in the stellar core.