1) My team has led the development of a software package called gwcosmo, which is one of the main analysis tools used for cosmology by the LIGO-Virgo-KAGRA (LVK) collaboration (who carry out science with the current ground-based gravitational wave detectors). As such, our work is directly linked to some of the most high-impact results from the current gravitational wave (GW) data. In the current reporting period we re-designed and implemented a new statistical formulation of the dark sirens calculation performed by gwcosmo; this sped up the run time of the code by a factor of ~1000, allowing for more sophisticated analysis. We also extended the code to perform tests of the laws of gravity beyond the standard cosmological model. We tested our software on current public data and other mock data, and wrote up our work in two publications (both currently undergoing review for publication in the journal JCAP).
2) My team has produced a new simulation code to study the collapse of dark matter in the universe, called Hi-COLA. In particular, Hi-COLA can study the formation of the cosmic web within a very broad class of gravity theories beyond Einstein's General Relativity. Version one of Hi-COLA has been publicly released, and we presented the code and its initial results in a paper (published in JCAP).
3) We have developed a new algorithm for completing galaxy catalogues, i.e. modelling the missing galaxies from a survey. This is a key prescription that is used in the dark sirens analysis of 1).
4) We have performed a new set of calculations for relativistic effects in large-scale structure in luminosity distance space. These are a necessary step for future cross-correlations of gravitational wave data with galaxy surveys. This work also enabled us to model for the first time key properties of gravitational wave sources called evolution bias and magnification bias.
5) We studied how a change in the propagation speed of gravitational waves could be detected by both the future LISA space-based detector, and a `multi band' combination of future detectors. We find that a departure from GR can be strongly constrained via one or both of these methods, at lower energy scales than is possible currently. This will push our tests of General Relativity into a new regime, informing future ideas regarding dark energy (the mysterious substance which is thought to drive the expansion rate of the universe). This work is written up in two published papers.
