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

We investigate the scalar-tensor gravity of Damour and Esposito-Far\`ese (DEF), which predicts nontrivial 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 $\ensuremath{\sim}1%$ level and speed up large-scale calculations by 2 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.