Scalarization of isolated black holes in scalar Gauss-Bonnet theory in the fixing-the-equations approach

One of the most promising avenues to perform numerical evolutions in theories beyond general relativity is the fixing-the-equations approach, a proposal in which new "driver" equations are added to the evolution equations in a way that allows for stable numerical evolutions. In this direction, we extend the numerical relativity code spectre to evolve a "fixed" version of scalar Gauss-Bonnet theory in the decoupling limit, a phenomenologically interesting theory that allows for hairy black hole solutions in vacuum. We focus on isolated black hole systems both with and without linear and angular momentum, and propose a new driver equation to improve the recovery of such stationary solutions. We demonstrate the effectiveness of the latter by numerically evolving black holes that undergo spontaneous scalarization using different driver equations. Finally, we evaluate the accuracy of the obtained solutions by comparing with the original unaltered theory.