The assumption that General Relativity is the fundamental law of gravity requires an extrapolation of many orders of magnitude from solar system length scales where the theory is well tested. This uncertainty undergirds explorations of modified gravity theories, which are further motivated by the possibility of explaining dark energy and dark matter that have hitherto eluded detection.   However, model-independent constraints on modified gravity models to date exist mainly on linear scales.  

In work using DiRAC resources, we present the first N-body simulations of modified gravity models based on a consistent parameterisation that is valid on all length scales. We investigate the impact of a time-dependent modification of the gravitational force on the matter power spectrum and consequently on weak-lensing observables, with a particular focus on the constraining power gained by including non-linear scales. We also examined how well existing fitting functions reproduce the non-linear matter power spectrum of the simulations.  

In the figure below, we show the performance of our ReACT formalism on the weak lensing observables which are used to interpret galaxy surveys. We have employed our DiRAC simulations to forecast modified gravity effects at the level required for future surveys such as Euclid and the Vera Rubin Telescope (LSST). This paves the way for full model-independent tests of modified gravity using the huge influx of data that will become available from these upcoming surveys. 

Reference: 

Sankarshana Srinivasan, Daniel B. Thomas, Francesco Pace and Richard Battye, “Cosmological gravity on all scales. Part II. Model independent modified gravity N-body simulations”, Journal of Cosmology and Astroparticle Physics, Volume 2021, June 2021, https://doi.org/10.1088/1475-7516/2021/06/016.