Bounds on the minimum sound speed above neutron star densities

TL;DR

This paper derives conservative upper bounds on the minimum sound speed in ultra-dense matter within neutron stars, based on astrophysical observations and QCD principles, with implications for phase transitions at high densities.

Contribution

It establishes the most conservative upper limits on the sound speed in neutron star cores, linking astrophysical data with fundamental physics constraints.

Findings

Upper bounds on the minimum sound speed are robust and conservative.

Discovery of neutron stars over 2.6 solar masses would support phase transitions.

Current constraints suggest possible first-order phase transitions at high densities.

Abstract

We show that the existence of massive neutron stars and asymptotic freedom of QCD place robust upper bounds on the lowest sound speed of the ultra-dense matter unattainable in neutron stars. Centered on worst-case scenarios, our limits are the most conservative among physical equations of state in the density range $\sim 2-40 n_0$. Discovery of $\gtrsim 2.6 M_\odot$ neutron stars, in combination with current multimessenger astrophysical constraints on the equation of state, would strongly support first-order phase transitions at high baryon densities.