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Modelling non-linear structure formation with modified gravity and massive neutrinos

Student thesis: Doctoral Thesis

The observation that distant Type Ia supernovae, the explosive death of massive stars, were fainter than possible in a non-accelerating expanding universe lead to one of the most significant paradigm shifts in cosmology since Einstein's introduction of General Relativity. Yet could the former have been the first signal that the latter's reign as the dominant theory of gravity is approaching its own demise? Within the bounds of General Relativity, the simplest way to explain the late-time accelerating expansion of the Universe required by the supernovae observations is to add a small, positive valued cosmological constant A, whose negative pressure causes the acceleration. Along with cold dark matter, the cosmological constant forms the basic of the current concordance cosmological model. Other additional components of the Universe, under the general label of dark energy, can replace A. However, what if a modification to the theory of gravity could explain the observed acceleration instead? We will see that the quest to answer this question is an uphill path which entails many steps that have been overcome already and many that have yet to be surmounted.

Modified gravity theories exist that can produce accelerating expansion without a cosmological constant, typically through the introduction of a scalar field that couples to matter via the gravitational metric, thus modifying the strength of gravity. In some modified gravity theories, the expansion history of the Universe in the A-cold-darkmatter model can be reproduced almost perfectly. For such theories, the best signal to search for becomes the modification to the strength of gravity. However, some theories have a so-called screening mechanism built in, whereby in high density environments the modifications to the strength of gravity become negligible, making tests of these theories in the confines of the solar system ineffective. Therefore, one of the best regimes to investigate modified gravity is on cosmological scales, where the large-scale clustering of structure in the Universe would be enhanced relative to General Relativity. Indeed, many upcoming galaxy surveys plan to constrain just such an enhancement.

However, this too is not as simple as it might seem. Neutrinos, one of the fundamental particles of the Universe, have been shown to have mass since the observation of accelerating expansion. As a result, massive neutrinos don't cluster on scales smaller than their free-streaming length. This free-streaming effect causes a suppression of structure formation that is dependent on the neutrino masses. Again, placing a constraint on neutrino masses is a key aim of many upcoming galaxy surveys. However, this sets up a potential degeneracy between the enhancement of structure formation due to modified gravity and the suppression due to massive neutrinos. For example, the large-scale structure of a universe with General Relativity and light neutrinos could be statistically similar to that of a universe with a strong modification to gravity and heavier neutrinos. Finding ways to break this degeneracy is vital if upcoming galaxy surveys are to simultaneously constrain modfied gravity and neutrino masses.

In this thesis, we present a code, MG-PICOLA, which is capable of simulating structure formation with the scale-dependent effects of both modified gravity and massive neutrinos. We have included a method to estimate the screening effect for three different mechanisms, which allowed us to build a variety of modified gravity models in to MG-PICOLA. We show that while MG-PICOLA uses a fast, approximate simulation method, its output matches that of full N-body simulations up to quasi-non-linear scales. We next investigate whether redshift-space distortions in the clustering of largescale structure offer a way to break the modified gravity-massive neutrino degeneracy. We do so by including both effects in the Taruya-Nishimichi-Saito model of redshift-space distortions that is implemented in the MG-Copter perturbation theory code, and compare the output of the model to that of MG-PICOLA simulations at the level of the dark matter distribution. We find that our model is capable of capturing the degeneracy breaking potential that us present in the redshift-space dark matter power spectrum multipoles. We also investigate how the degeneracy evolves with redshift and demonstrate again how our model captures the redshift evolution at the level of the dark matter distribution. However, we cannot observe the distribution of dark matter directly, only through galaxies, which act as a biased tracer. Therefore, we extend our MG-Copter-based redshift-space distortion model to include bias and fit it to friends-of-friends halo catalogues produced from MG-PICOLA simulations using a Markov Chain Monte Carlo approach. We demonstrate that the model fits recover a linear bias that is consistent with that estimated from the simulations. Throughout we find that the best-fit parameters are sensitive to the fitting setup only when the modified gravity and neutrino mass parameters in the model don't match those in the simulation, which can help constrain the two effects. We also find that the redshift-space halo power spectrum multipoles have larger degeneracy breaking potential than their dark matter counterparts, and that our extended model is effective at capturing the behaviour seen in the multipoles of the simulated halo catalogues.
Original languageEnglish
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Award dateDec 2019

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