Does the sound speed exceed the conformal bound inside neutron stars?
In plain words
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 statement
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.
What would settle it
Equation-of-state inference using 2 percent radius measurements across masses from 1.2 to 2.1 solar masses.
Status in the literature
Unverified 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.
Related problems
- More general than Location and origin of the sound-speed maximum in dense matter