GRFolres: A code for modified gravity simulations in strong gravity

Gravitational waves (GWs) are generated by the mergers of dense, compact objects like black holes (BHs) and neutron stars (NSs).This provides an opportunity to study the strong field, highly dynamical regime of Einstein's theory of general relativity (GR) at higher curvature scales than previous observations (Arun T. Baker et al., 2015;Barack Barausse Gnocchi et al., 2019; Perkins, Yunes, et al., 2021).It is possible that at such scales modifications to GR may start to manifest.However, in order to detect such modifications, we need to understand what deviations could look like in theories beyond GR, in particular in the merger section of the signal in near equal mass binaries, which are key targets of the LIGO-Virgo-KAGRA network of detectors (and their future 3G successors).Such predictions necessitate the use of numerical relativity (NR), in which the (modified) equations of GR are evolved from an initial configuration several orbits before merger, through the merger period and the subsequent "ringdown", during which the gravitational wave signal can be extracted near the computational boundary.1 Folres (pronounced fol-res) is a word meaning covers or linings in the Catalan language.It has a specific application in the tradition of Castells (Human Towers), denoting the second layers of reinforcement above the base pinya.We use it here in analogy to our understanding of effective field theories (EFTs) of gravity as an infinite sum of terms organised as a derivative expansion, in which the first one corresponds to GR (with up to 2 derivatives), and the second one to modified theories up to 4 derivatives, which are those that we are able to simulate with GRFolres.