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Title: Does the sound speed exceed the conformal bound inside neutron stars?
Plain: In matter made of free quarks at very high density, sound travels at the speed of light divided by the square root of 3. Neutron-star data suggest sound travels faster than this somewhere inside, but the evidence depends on modeling assumptions.
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.
Settled by: Equation-of-state inference using 2 percent radius measurements across masses from 1.2 to 2.1 solar masses.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: partially-resolved
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.
Posed since: (empty)