Periodic Reporting for period 4 - GravityLS (Large Scale Structure Constraints of General Relativity)
Período documentado: 2021-03-01 hasta 2022-08-31
Thus far, we have made steady progress on a number of projects. For a start we have developed a mathematical framework that allows us to describe all possible deviations from General Relativity on large scales. This also allows us to figure out if there are interesting observational effects to look for in the data. We have also developed a publicly available piece of software that implements a great part of this framework so that we will be able to constrain it with observations. A key aspect has been to cross-check our codes with over a dozen other codes - this has been useful to correct errors in all the codes so that they are completely trustworthy.
An important development has been our links with observations which are not cosmological. In particular, the recent detection of a binary neutron star merger allowed us to place constraints on a vast swathe of different theories, allowing us to focus on the modifications to general relativity that really matter. We have also understood how to use measurements of gravitational wave signals to place constraints in a completely different regime (where the gravitational field is much stronger) but which can then be combined with cosmological measurements to place even tighter constraints.
We expect the next decade to bring us a wealth of cosmological data which will allow us to place even more stringent constraints on gravity. As preparation for this new wave of information, we have been exploring the various techniques that may be applied to, in some sense a forecast of what we might expect. But we have also taken stock and analyzed on of the current most comprehensive data sets, KIDS-450, and combined it with other data sets to place state of the art constraints on cosmological gravity.
Understanding the fundamental laws that operate on largest scales and placing stringent constraints on them will allow us to be confident about our model of the origin and evolution of the Universe.
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.