# Reduced-order surrogate models for scalar-tensor gravity in the strong field and applications to binary pulsars and GW170817

## TL;DR

This paper develops fast, accurate surrogate models for scalar-tensor gravity in neutron stars, enabling efficient analysis of pulsar and gravitational wave data to test strong-field gravity theories.

## Contribution

The authors create reduced-order surrogate models for scalar-tensor gravity predictions, significantly speeding up calculations while maintaining high accuracy, aiding in gravitational tests.

## Key findings

- Models achieve ~1% accuracy
- Speed up calculations by 100x
- Constrain gravity parameters using pulsar and GW data

## Abstract

We investigate the scalar-tensor gravity of Damour and Esposito-Far\`ese (DEF), which predicts non-trivial phenomena in the nonperturbative strong-field regime for neutron stars (NSs). Instead of solving the modified Tolman-Oppenheimer-Volkoff equations, we construct reduced-order surrogate models, coded in the pySTGROM package, to predict the relations of a NS radius, mass, and effective scalar coupling to its central density. Our models are accurate at $\sim1\%$ level and speed up large-scale calculations by two orders of magnitude. As an application, we use pySTGROM and Markov-chain Monte Carlo techniques to constrain parameters in the DEF theory, with five well-timed binary pulsars, the binary NS (BNS) inspiral GW170817, and a hypothetical BNS inspiral in the Advanced LIGO and next-generation GW detectors. In the future, as more binary pulsars and BNS mergers are detected, our surrogate models will be helpful in constraining strong-field gravity with essential speed and accuracy.