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Neutron-star data suggest sound travels faster than this somewhere inside, but the evidence depends on modeling assumptions.","posed_since":"","precise":"Determine whether $c_s^{2} = \\mathrm{d}P/\\mathrm{d}\\epsilon$ exceeds $c^2/3$ (the value for free massless quarks) at some density below the central density of a maximum-mass star. Answer: yes or no, with posterior probability given pulsar masses, NICER radii, tidal deformabilities, chiral EFT below about $2 n_0$ and perturbative QCD limits.","problem_ref":null,"references":"","settled_by":"Equation-of-state inference using 2 percent radius measurements across masses from 1.2 to 2.1 solar masses.","status_note":"Bayesian analyses including two-solar-mass pulsars and NICER radii have found $c_{s}^{2} > c^{2}/3$ in most posteriors since about 2018, with the probability depending on the priors as of 2026.","title":"Does the sound speed exceed the conformal bound inside neutron 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