Theoretical studies suggest that quarks at very low temperatures and high densities enter a so-called color-superconducting state, which fundamentally alters the nature of matter. This state is analogous to the transition of an electron gas into an electrical superconductor – except that, instead of electrons, quarks pair up and create an energy gap in their excitation spectrum. Unlike conventional superconductors, however, a color superconductor generally does not conduct electric current without resistance, but rather the color charge of the quarks.

The formation of quark pairs and the resulting energy gap fundamentally change the behavior of matter. Even relatively weak pairing effects have a significant impact on the matter and its thermodynamic properties.

Significant increase in the speed of sound

Andreas Geißel, Tyler Gorda, and Jens Braun calculate correction terms arising from quark pairing and interactions, while accounting for the specific conditions inside neutron stars. Their results show that the color-superconducting ground state is thermodynamically favored at high densities. Moreover, this state leads to a significant increase in the speed of sound – a direct measure of the mechanical stability of matter. According to the calculations by Geißel, Gorda, and Braun, the speed of sound in the interior of neutron stars may even exceed 60 percent of the speed of light.

Numerical simulations also suggest that such high sound speeds may be necessary to explain the stability of the most massive known neutron stars. The work by Geißel, Gorda, and Braun now indicates that color-superconducting matter may be a crucial ingredient in explaining massive neutron stars – and that observations of these stars could, in turn, help to better constrain the energy gap in the quark spectrum.