Influence of cosmology on long term star formation
Oh, Boon Kiat
This dissertation explores cosmological simulations of galaxy formation, focusing on the asymptotic efficiency of conversion of baryons into stars and asking how this quantity varies in different cosmological models. By exploring the behaviour into the future and in counter-factual universes with altered cosmological parameters, I hope to gain insight into the robustness of galaxy formation codes for their behaviour in different parameter regimes, addressing the concern that the models may be fine-tuned to fit observations, rather than being genuinely predictive. Through the calibration of multiple star formation and feedback parameters within zoom simulations using the Enzo code, I obtain a set of parameter values able to reproduce the observational constraint of baryon makeup of haloes of masses between 1010 and 1012M⊙. Comparing two different star formation setups, I show that feedback is self-consistent with a higher feedback energy injection associated with a low star formation efficiency, and vice versa. I also explore the reproducibility of the simulation results and conclude that operational differences in the implementation of the numerical code can create approximately 10% - 25% deviation in the stellar and baryon mass respectively. Using the best star formation and feedback prescription from the zoom simulations, I attempt a cosmological box simulation in a standard cold dark matter cosmology beyond z = 0. Comparing with previous work, I find a similar elongation in the general evolution of the distribution of temperature and density of the gas into the future. I then extend the analysis to the evolution of the halo mass function, the equation of state of the intergalactic medium and star formation rate into the future to determine the level of convergence with various resolutions. Interestingly, I identify a turn-around in the cosmic star formation, deviating from the extrapolation obtained from observations. There is also a cross over in the fraction of baryon in the form of stars and in gas around the period of 'freeze out' in both the zoom and box simulations. Lastly, I present results from simulations of counter-factual simulations, exploring the effects of different values of Λ on structure formation and evolution. These simulations allow the study of anthropic explanations for the small observed value of Λ, considering the relative frequency of observers within a multiverse. It appears that a distinct peak in the star formation rate may be due to the presence of Λ. For a higher value of Λ, the peak in star formation is of a lower value, and it occurs earlier in the evolution of the universe. Given these differences in the star formation rate, the UV background must be calculated self consistently in these models. This corrected UV background will provide a more realistic representation of the counter-factual universe. I show that almost 54% of possible observers reside in universes with a value of Λ similar to ours, implying that anthropic principle can explain why Λ is of such a small value.