Ab-initio QCD calculations impact the inference of the neutron-star-matter equation of state

We demonstrate that ab-initio calculations in QCD at high densities offer significant and nontrivial information about the equation of state of matter in the cores of neutron stars, going beyond that which is obtainable from current astrophysical observations. We do so by extrapolating the equation of state to neutron-star densities using a Gaussian process and conditioning it sequentially with astrophysical observations and QCD input. Using our recent work, imposing the latter does not require an extrapolation to asymptotically high density. We find the QCD input to be complementary to the astrophysical observations, offering strong additional constraints at the highest densities reached in the cores of neutron stars; with the QCD input, the equation of state is no longer prior dominated at any density. The QCD input reduces the pressure and speed of sound at high densities, and it predicts that binary collisions of equal-mass neutron stars will produce a black hole with greater than $95\mathrm{\%}$ ($68\mathrm{\%}$) credence for masses $M\ge 1.38M_{\odot }$ ($M \geq 1.25 M_\odot$). We provide a Python implementation of the QCD likelihood function so that it can be conveniently used within other inference setups.