In the inner crust of a neutron star, nuclear matter forms exotic geometric phases: spheres, cylinders, slabs, cylindrical holes, spherical holes — the “nuclear pasta” sequence, named for its resemblance to gnocchi, spaghetti, lasagna, bucatini, and Swiss cheese. At zero temperature, the pasta phases are well studied: they arise from competition between nuclear surface tension (which favors compact shapes) and Coulomb repulsion between protons (which favors dispersed shapes).
Zhou et al. compute nuclear pasta at finite temperature, in the conditions relevant to proto-neutron stars — the hot, dense remnants of core-collapse supernovae. The result is not merely “the same pasta, but blurrier.” Temperature changes which pasta phases exist, where they appear, and how thick the pasta layer is.
The mechanism is through surface tension. At zero temperature, the surface tension between nuclear matter and the surrounding neutron gas is determined by the nuclear interaction. At finite temperature, thermal fluctuations soften the surface, reducing the tension. But the reduction is not uniform across all densities and compositions — it depends on the isospin asymmetry (the ratio of neutrons to protons) through the symmetry energy.
Different nuclear interaction models predict different symmetry energy slopes. The TM1e model (softer symmetry energy) and the TM1 model (stiffer) produce qualitatively different pasta configurations at the same temperature and density. Some phases that exist in one model are absent in the other. The pasta layer spans approximately 1.2 kilometers in proto-neutron star inner crusts, but its internal structure — which shapes appear, in what order, at what density — depends on the symmetry energy slope, a quantity that is still poorly constrained by terrestrial experiments.
The structural point: nuclear pasta is not a fixed sequence of shapes. It is a phase diagram, and the axes of the phase diagram include temperature and the symmetry energy slope — a parameter of the nuclear interaction that we do not yet know precisely. The pasta is a thermometer and a constraint: observing it (through neutron star cooling curves, gravitational wave signals, or neutrino transport) would simultaneously measure the temperature and constrain the nuclear interaction.