Speeding upN-body simulations of modified gravity: chameleon screening models

Abstract

We describe and demonstrate the potential of a new and very efficient method for simulating certain classes of modified gravity theories, such as the widely studied f (R) gravity models. High resolution simulations for such models are currently very slow due to the highly nonlinear partial differential equation that needs to be solved exactly to predict the modified gravitational force. This nonlinearity is partly inherent, but is also exacerbated by the specific numerical algorithm used, which employs a variable redefinition to prevent numerical instabilities. The standard Newton-Gauss-Seidel iterative method used to tackle this problem has a poor convergence rate. Our new method not only avoids this, but also allows the discretised equation to be written in a form that is analytically solvable. We show that this new method greatly improves the performance and efficiency of f (R) simulations. For example, a test simulation with 512 3 particles in a box of size 512 Mpc/h is now 5 times faster than before, while a Millennium-resolution simulation for f (R) gravity is estimated to be more than 20 times faster than with the old method. Our new implementation will be particularly useful for running very high resolution, large-sized simulations which, to date, are only possible for the standard model, and also makes it feasible to run large numbers of lower resolution simulations for covariance analyses. We hope that the method will bring us to a new era for precision cosmological tests of gravity.

Simulating galaxy formation in f(R) modified gravity: matter, halo, and galaxy statistics

“…This way, it can compute the fifth force and capture the effects of the chameleon screening mechanism. The modified gravity solver is based on the f (R)-solver in mg-gadget (Puchwein et al 2013) but employs the optimised u 3 method of Bose et al (2017) and a local time-stepping scheme (Arnold et al 2016) for higher efficiency. The modified gravity module is combined with the Il-lustrisTNG galaxy formation model (Pillepich et al 2018a;Springel et al 2018;Genel et al 2018;Marinacci et al 2018;Nelson et al 2018) implemented in arepo.…”

An accurate perturbative approach to redshift space clustering of biased tracers in modified gravity

“…In this section, we briefly introduce the N-body simulations we will use to assess the performance of our analytical model, which is a crucial step for our analysis. The first set of simulations, that we will refer to from now on as Group I simulations, are the Extended LEnsing PHysics using ANalaytic ray Tracing (ELEPHANT) simulations [94], that were performed with two modified versions of the GR code (RAMSES): the ECOSMOG module [95,96] produced snapshots for the F6, F5 and F4 cases at a cosmological redshift of z = 0.5, while ECOSMOG-V [97][98][99] was used to produce the nDGP N1 and N5 realizations, also at z = 0.5. 1024 3 dark matter particles were evolved, in a simulation box with a side L box = 1024M pc/h and a cosmology specified by the following parameters:…”

Marked clustering statistics in f(R) gravity cosmologies

“…In order to robustly test gravity on cosmological scales, reliable N-body simulations of modified gravity models are essential. The non-linear nature of the scalar field equation requires the implementation of novel numerical techniques, which is what makes N-body simulations of modified gravity challenging (Winther et al 2015;Barreira et al 2015;Bose et al 2017). Once such simulations are ready, we can use measurements of clustering statistics from surveys to test and constrain cosmological models (see e.g., Reid et al 2010).…”