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Accepted · first revision · record

Title: Location and origin of the sound-speed maximum in dense matter
Plain: If sound in neutron-star matter first speeds up beyond the free-quark value and then slows down again, there must be a peak. Where the peak lies and what causes it, for example a gradual change from nucleons to quarks, is unknown.
Precise: Given $c_s^{2}(n) > c^{2}/3$ somewhere below the central density of a maximum-mass star, determine the density $n_{\mathrm{peak}}$ of the maximum of $c_s^{2}$ and its height, and the microscopic mechanism: quarkyonic matter (a phase with a quark Fermi sea surrounded by a shell of nucleons near the Fermi surface), a hadron-quark crossover, a first-order transition, or strongly repulsive hadronic interactions. Answer: $n_{\mathrm{peak}}/n_0$ and the maximum $c_s^{2}$ with uncertainties, plus a mechanism consistent with chiral EFT at low density and perturbative QCD at high density.
Settled by: Equation-of-state inference using 2 percent radius measurements across 1.2 to 2.1 solar masses and post-merger gravitational-wave spectra, compared with model calculations of each mechanism.
Kind: phenomenon
Answer type: mechanism
References: (empty)
Literature status: open
Status note: (empty)
Posed since: (empty)