title: THE EFFECTS OF DARK ENERGY AND BARYONS AND THEIR INTERPLAY ON THE GROWTH OF STRUCTURE
Creators: name: Pfeifer, S
Publisher: name: Liverpool John Moores University
Year: 2020

description: The discovery of the accelerated expansion of our Universe brought with it a new theoretical entity called dark energy. Within our standard model of cosmology, \lcdm{}, this dark energy component is described by a cosmological constant with a small energy density that does not evolve with space or time. Attempts to attribute physical meaning to the cosmological constant have been unsuccessful, culminating in a collection of problems known as the ``cosmological constant problem" and the ``coincidence problem". These have motivated alternative theories of dark energy that aim at relieving some of the theoretically unsatisfactory characteristics of the cosmological constant. Over recent years, observational cosmology has made great leaps in constraining the parameters of the standard model of cosmology. However, with the increasing quantity and quality of data available, a few tensions between different observational probes have started to appear. These have only grown over time and are now of statistical significance. These tensions could have any of the following origins; they arise from unknown and unaccounted systematic errors in the data, from unknown errors in the theoretical modeling and/or from an incomplete model of cosmology. The latter possibility has added motivation for extending the standard model of cosmology, where alternate forms of dark energy are one of many available avenues. The main aim of this work is to explore forms of dark energy with a greater degree of freedom than the cosmological constant. Specifically, dynamical dark energy (DDE) models which allow dark energy to evolve with time and are parametrised by two additional free parameters: $w_0$ and $w_a$. I investigate the current cosmological parameter constraints from a combination of observation data sets and devise a strategy to select 6 cosmologies of interest. I independently modified and ran a total of 12 simulations, evenly split between collissionless and hydrodynamic simulations. Since dark energy affects the expansion history, geometric probes, such as Type Ia supernovae and baryon acoustic oscillations, can constrain the dark energy parameters in a conceptually straightforward manner. However, changes to the expansion history also affect the growth of structure which could make large-scale structure (LSS) statistics potentially powerful and complementary probes. The first part of this work investigates the effect that these cosmologies have on a variety of LSS statistics using large cosmological hydrodynamical simulations. I find that DDE can affect the clustering of matter and haloes at the $\sim10\%$ level, which should be distinguishable with upcoming large-scale structure surveys. DDE cosmologies can also enhance or suppress the halo mass function (with respect to $\Lambda$CDM) over a wide range of halo masses. The internal properties of haloes are minimally affected by changes in DDE, however. The second part of this work investigates the separability of the cosmology and baryonic physics. I quantify to what extent these two processes affect each other, or in other words, how correlated they are. I show that the impact of baryons and associated feedback processes is largely independent of the change in cosmology and that these processes can be modelled separately to typically better than a few percent accuracy.