What is the equation of state of matter inside neutron stars?
Neutron-star cores compress matter to several times nuclear density — a regime no laboratory can reach. The equation of state determines the maximum mass, radius, tidal deformability and whether cores contain hyperons, quark matter or new phases.
Why it is hard
QCD at high density and low temperature is numerically intractable (the sign problem). Mass–radius measurements carry multi-km uncertainties, and different EOS families can mimic each other in any single observable.
Current evidence
Two-solar-mass pulsars set a hard floor on maximum mass; NICER X-ray pulse-profile modeling gives R ≈ 12–13 km for PSR J0030+0451 and J0740+6620; GW170817's tidal signal disfavors overly stiff EOS. The result is a narrowed but still ~10 km-wide band of viable EOS.
Possible approaches
More precise NICER/STROBE-X radius measurements, gravitational-wave tidal parameters from future merger detections (and post-merger ringdown), heavy-ion collision data anchored to multi-messenger constraints, and advances in lattice QCD and chiral effective field theory.
Related topics
Sources
- Neutron Stars — NASA Science
- Neutron Stars — NASA Science updates — NASA Science
- Gravitational-Wave Science — LIGO Scientific Collaboration
- Pulsars — NASA Science