Nonparametric extensions of nuclear equations of state: probing the breakdown scale of relativistic mean-field theory

TL;DR

This paper introduces a hybrid nonparametric model for dense matter equations of state, combining relativistic mean-field theory at low densities with flexible, agnostic modeling at higher densities, to better quantify uncertainties and identify potential breakdowns.

Contribution

It develops a novel nonparametric approach that transitions from theory-informed to agnostic modeling of neutron star matter, enabling uncertainty quantification and detection of phase transitions.

Findings

No evidence of breakdown of relativistic mean-field theory within neutron stars.

Method can identify the pressure at which a strong phase transition occurs.

Enhanced future observational constraints improve detection of breakdowns.

Abstract

Phenomenological calculations of the properties of dense matter, such as relativistic mean-field theories, represent a pathway to predicting the microscopic and macroscopic properties of neutron stars. However, such theories do not generically have well-controlled uncertainties and may break down within neutron stars. To faithfully represent the uncertainty in this breakdown scale, we develop a hybrid representation of the dense-matter equation of state, which assumes the form of a relativistic mean-field theory at low densities, while remaining agnostic to any nuclear theory at high densities. To achieve this, we use a nonparametric equation of state model to incorporate the correlations of the underlying relativistic mean-field theory equation of state at low pressures and transition to more flexible correlations above some chosen pressure scale. We perform astrophysical inference under various choices of the transition pressure between the theory-informed and theory-agnostic models. We further study whether the chosen relativistic mean-field theory breaks down above some particular pressure and find no such evidence. Using simulated data for future astrophysical observations at about two-to-three times the precision of current constraints, we show that our method can identify the breakdown pressure associated with a potential strong phase transition.