The project has three working groups (WGs), which are respectively devoted to testing model using three of the most promising cosmological probes: weak gravity lensing (WG1), galaxy clustering (WG2) and clusters of galaxies (WG3).
In WG1, one of the tasks is to run a large number of simulations of various models, to measure one of the main observables of weak lensing: the convergence power spectrum. The original plan was slightly changed as during the preparation we found some other interesting observables that can be extracted from lensing observations. A lot of effort was spent on the study of weak lensing by cosmic voids, and a novel probe developed in this process is cosmic voids detected from 2D weak lensing maps, which were found to have a greater potential for testing models using future lensing data. We also looked at the statistics of weak lensing peaks, which had not been studied in great details before, and discovered some surprisingly simple relations they follow. Consequently, the details of the planned simulations were modified from the original proposal so that they can be suitable for studying these new probes.
In WG2, our main goal is to use measurements of galaxy clustering to test models, an approach that relies on three building blocks: (1) accurately predicting the clustering of dark matter in real space, usually quantified by the matter power spectrum or correlation function, (2) understanding how the clustering of galaxies differs from that of dark matter, given that the former are biased tracers of the latter, and (3) understanding how best to model redshift space distortions (RSD), which can mislead people into obtaining wrong galaxy positions from observations. We have tackled (1) using three approaches: (1a) a halo model approach to predict the matter power spectrum, by improving our understanding of the different ingredients of the halo model, (1b) a reaction method to predict the matter power spectrum for dark energy and modified gravity models at precent accuracy, and (1c) an emulation approach by running simulations for a selection of models and doing interpolation to get the matter power spectra for models which are not simulated. We have tackled (3) using three approaches: (3a) ay measuring RSD from simulations of modified gravity models, (3b) an improvement of the traditional Gaussian streaming model for RSD based on a more accurate modelling of the probability distribution of galaxy pairwise velocities, and (3c) a novel backward modelling approach, with an iterative reconstruction method to remove the RSD effect from observed galaxy clustering for galaxy pairs separated by 20Mpc or more. The reconstruction approach has also been applied to study galaxies bias for building block (2). Along this process, we have developed the first self-consistent galaxy formation simulations in modified gravity theories. Furthermore, a few papers were published about the use of the so-called marked correlation function to test gravity.
In WG3, we have followed the work plan to develop a novel framework to test gravity using the abundance of galaxy clusters. We have analysed a large number of simulations for a popular modified gravity model (chameleon f(R) gravity), to study the most important properties of dark matter haloes, including their masses, density profiles and abundances. These analyses have led to some surprisingly simple and accurate fitting functions for these properties. We have worked on the first ever large realistic hydrodynamical cluster simulations: these not only helped to calibrate cluster scaling relations which are critical for measuring cluster abundances, but also showed interesting effect of modified gravity on the population and clustering of galaxies.
