Constraining the equation of state in modified gravity via universal relations

TL;DR

This paper investigates universal relations between neutron star parameters at maximum mass across various equations of state and modified gravity theories, highlighting the impact of gravity modifications on EOS constraints.

Contribution

It extends the universal relations to scalarized neutron stars in scalar-tensor theories and shows their dependence on gravity parameters, affecting EOS constraints.

Findings

Universal relations hold for scalarized neutron stars in modified gravity.

Relation parameters depend strongly on gravity theory parameters.

Modified gravity can significantly alter EOS constraints.

Abstract

Modern multi-messenger astronomical observations and heavy ion experiments provide new insights into the structure of compact objects. Nevertheless, much ambiguity remains when it comes to super dense matter above the nuclear saturation density such as that found within neutron stars. This work explores equation of state (EOS)-independent universal relations between the physical parameters of static neutron stars at the maximum-mass point, previously proven for General Relativity (GR) and used to constraint EOS candidates within the GR framework. We explore 53 different EOS candidates and prove that these relations hold for scalarized neutron stars in massive scalar-tensor theories, exploring also the effect of non-baryonic EOS. We further show that the relation's fit parameters are highly dependent on the theory's parameters. On the basis of these relations multiple constraints on the EOS can be derived and it turns out that they can be significantly different than the GR ones. This demonstrates the importance of taking into account the modified gravity effects even when imposing constraints on the EOS.