Neutron stars in extended scalar-Gauss-Bonnet gravity: the richness of the solution spectrum

TL;DR

This paper explores the diverse spectrum of neutron star solutions within extended scalar-Gauss-Bonnet gravity, revealing new behaviors, phase transitions, and solution branches unique to high-mass neutron stars, enhancing understanding of strong-field gravity.

Contribution

It combines scalar-tensor theories with Gauss-Bonnet modifications to extensively analyze neutron star solutions, uncovering qualitatively new phenomena and solution branches.

Findings

Discovery of new solution branches for high-mass neutron stars.

Identification of novel phase transition behaviors.

Solutions cannot be mimicked by simple equation of state changes.

Abstract

Neutron stars are natural laboratories for testing gravity in the strong field regime. That is why the full spectrum of neutron star solutions in different modified theories should be thoroughly studied. Among the most natural modifications of general relativity are the theories in which additional scalar degrees of freedom are present. That is why scalar-tensor theories like Brans--Dike and Damour--Esposito--Farese theories, as well as their extensions such as scalar-Gauss-Bonnet gravity, attracted attention throughout the years. In the present work, we combine those theory families and explore extensively the neutron star solution space in their realm. We identify qualitative new behavior of the solutions, including the existence of new types of phase transitions and new branches of solutions present only for high neutron star masses. Due to the peculiarities of the solutions, they can not be easily mimicked by a simple change of the equation of state.