As the Universe expands, tiny perturbations that are imprinted upon the universe during inflation eventually collapse under their own weight. These then provide the formation sites for the rich structure of the cosmos that is observed today. The details of this collapse process are governed by the constituent parts of the universe: If there is more dark matter then perturbations weigh more and collapse faster; if there is more dark energy the universe expands faster and this slows down structure formation. It follows that by measuring the progress of this collapse process over the history of the cosmos one can learn about the nature of dark matter and dark energy. The collapse can be measured indirectly via weak gravitational lensing, whereby light from distant sources is bent in a spatially coherent way by the spatial coherence of the intervening structure.
This project aims to address one of the main challenges in inference from contemporary cosmological data using weak lensing: On very large scales, the collapse of cosmological structure is driven by gravity and this is the only force that is important. However, on smaller scales, other processes influence the collapse, particularly processes that originate on the scale of galaxies. These are electromagnetic in origin, and the most important of these processes are the heating of galactic gas by both accreting super-massive black holes in the centres of the galaxies and by the supernovae explosions of massive stars. The influence of these complicated electromagnetic (baryonic) processes make ab-initio theoretical calculations of structure formation very difficult. It becomes necessary to run computationally-intensive hydrodynamic simulations to understand how these baryonic processes redistribute gas, and therefore matter, in the cosmos. This project attempts to expand the knowledge of humanity with regards to how gas dynamics affects the distribution of structure in the Universe.
This project has furthered knowledge regarding the fundamental connection between the unobservable dark-matter Universe and the observable visible Universe. Detailed knowledge of this connection is essential to be able to exploit data from current and future surveys to the full extent possible. The project has three main conclusions: First, a completely novel model has been generated that relates the observable gas and star distributions to the dark matter. In the future it will be possible to use a combination of observables, including the observed galaxy distribution and the electron pressure in conjunction with weak gravitational lensing, to learn about the expansion of the Universe. Second, the underlying predictions for the matter distribution in the absence of baryonic feedback processes have been significantly improved via HMcode-2020. It is essential to have this ingredient well calibrated as other state-of-the-art approaches, such as that mentioned in the first conclusion, rely on this keystone. Third, a new technique has been generated to properly incorporate the physics of dark-matter halo clustering within the existing modelling framework. This is important because previously models have been forced to make the assumption of a linear relationship between haloes and dark matter. Including the non-linearity in this relationship removes a bias that would otherwise affect future cosmological constraints and potentially provides a new window into interesting non-linear physics.
