The project has been able to construct a consistent framework for constraining general relativity and cosmological physics with large scale structure and astrophysical data. The project was constructed along three main strands. The A strand focussed on developments in theory, the B strand on developments in phenomenology and methods and the C strand on data analysis.
Strand A of the project was constructed to develop the theoretical framework of the programme and build connections with other methods for constraining gravity. The first main subtheme was to construct a formalism that could completely describe all types of gravitational physics on (linear) cosmological scales. The team was able to achieve this goal successfully with an approach which truly comprehensive. A second subtheme was to connect constraints along a vast range of scales. Unexpectedly, this was one of the most successful achievements of this project. In late 2017, a binary neutron star was detected, GW170817, allowing the team to place the tightest constraints on cosmological gravity to date. This led us to explore the connection between cosmological gravity and black hole physics and gravitational wave propagation. It also led us to the accretion of cosmological scalar fields around black holes. Finally, we proposed a completely new approach for incorporating priors assumptions in the analysis of cosmological theories of gravity
Strand B of the project was oriented to developing the algorithms and methods which would be used in the data analysis. We released Hi_Class, a code for solving the linear regime for general scalar tensor theories as well as generalized parametrized theories. Along with EFT Camb, this means that there are now general purpose codes for the type of analysis we advocate. We also lead a world-wide code comparison of over a dozen such solvers which focusd on specific theories. An important strand was the non-linear regime. It has become apparent to the wider community that there are serious impediments to achieving the level of accuracy and generality desired due to a number of effects. We opted to get a full understanding of the impact that non-linearities will have on cosmological and other observables and to construct a halo model code, calibrated against fast N-body codes which could approximate the non-linear regime with the desired accuracy.
Strand C was oriented towards data analysis and observations. We devoted a substantial amount of time to defining the road-map for understanding the effects of baryons on observables. From our work in strand A on black hole accretion we were able to model gravitational dynamical friction on black hole orbits. In parallel we constructed the first fully consistent model for the noise for the anisotropic gravitational wave background arising from large scale structure. A key achievement was the complete analysis of the base scalar-tensor theory, Jordan-Brans-Dicke theory in a way which allowed us to explore the interplay between the main cosmological parameters, systematic effects and gravity. In parallel, we focussed on tomographic data leading to the tightest constraints on the evolution of the growth rate in terms of the largest compilation of tomographic data to date. Members of my team were able to obtain constraints from ACT, KIDS and BOSS on general relativity. Finally we have produced some of the most comprehensive forecasts for the next generation of surveys for scalar-tensor theories and are finishing a forecast for future Euclid data.
