Constraining the speed of sound inside neutron stars with chiral EFT interactions and observations

The dense matter equation of state (EOS) determines neutron star (NS) structure but can be calculated reliably only up to 1-2 times nuclear saturation density, using accurate many-body methods that employ nuclear interactions constrained by scattering data and chiral effective eld theory. In this work, we use physically motivated ansatzes for the speed of sound cS at high density to extend microscopic calculations of neutron-rich matter to the highest densities encountered in stable neutron star cores. We confirm earlier expectations that cS is likely to violate the conformal limit of cS^2<1/3, possibly reaching values closer to the speed of light c at a few times nuclear saturation density, independent of the nuclear Hamiltonian. If QCD at finite baryon density obeys the conformal limit we conclude that the rapid increase of cS required to accommodate a 2M NS suggests a form of strongly interacting matter where a description in terms of nucleons will be unwieldy even between 1-2 times nuclear saturation density. We also discuss how future observations could constrain the EOS and guide theoretical developments of nuclear interactions.