How Perturbative QCD Constrains the Equation of State at Neutron-Star Densities

We demonstrate in a general and analytic way how high-density information about the equation of state (EOS) of strongly interacting matter obtained using perturbative quantum chromodynamics constrains the same EOS at densities reachable in physical neutron stars. Our approach is based on utilizing the full information of the thermodynamic potentials at the high-density limit together with thermodynamic stability and causality. This requires considering the pressure as a function of chemical potential p(mu) instead of the commonly used pressure as a function of energy density p(epsilon). The results can be used to propagate the perturbative quantum chromodynamics calculations reliable around 40n(s) to lower densities in the most conservative way possible. We constrain the EOS starting from only a few times the nuclear saturation density n greater than or similar to 2.2n(s), and at n = 5n(s) we exclude at least 65% of otherwise allowed area in the epsilon-p plane. This provides information complementary to astrophysical observations that should be taken into account in any complete statistical inference study of the EOS. These purely theoretical results are independent of astrophysical neutron-star input, and hence, they can also be used to test theories of modified gravity and beyond the standard model physics in neutron stars.

Automated summary

This study demonstrates how perturbative quantum chromodynamics can provide high-density information about the equation of state of strongly interacting matter, and how this information can be used to constrain the equation of state in physical neutron stars. By utilizing thermodynamic potentials and stability considerations, the study shows that the EOS can be reliably propagated from high to lower densities in a conservative manner.