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Cosmology at Small Scales: Ultra-Light Dark Matter and Baryon Cycles in Galaxies

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Cosmology at Small Scales: Ultra-Light Dark Matter and Baryon Cycles in Galaxies Mattia Mina Cosmology at small scales: ultra-light dark matter and baryon cycles in galaxies Thesis submitted for the degree of Philosophiae Doctor Institute of Theoretical Astrophysics Faculty of Mathematics and Natural Sciences 2020 © Mattia Mina, 2020 Series of dissertations submitted to the Faculty of Mathematics and Natural Sciences, University of Oslo No. 2312 ISSN 1501-7710 All rights reserved. No part of this publication may be reproduced or transmitted, in any form or by any means, without permission. Cover: Hanne Baadsgaard Utigard. Print production: Reprosentralen, University of Oslo. To all my beloved Preface This thesis is submitted in partial fulfilment of the requirements for the degree of Philosophiae Doctor at the University of Oslo. The research presented here was conducted at the Institute of Theoretical Astrophysics (ITA), at the University of Oslo, under the supervision of professor David F. Mota and associate professor Hans A. Winther. This thesis represents an effort to contribute to the development of the scientific knowledge related to the history of the Universe. The introductory chapters serve the purpose of placing the work I have done during the last four years in a broader context, in order to make my research better understandable and to set the academic publications accompanying this thesis in a broader perspective. In Chapter 1, I will introduce the recent advances in the field of modern cosmology, which contributed to the development of the standard model of cosmology, the ΛCDM model. I will briefly describe the history of the Universe and I will point to the problems and the open questions within the currently accepted theory describing the evolution of the Universe. I will introduce the theory of general relativity and I will describe how tiny perturbations from a smooth, homogeneous and isotropic Universe have evolved into the cosmic web and into the wide collection of structures we can observe today in the sky. In Chapter 2, I will focus on one of the most puzzling mysteries concerning the Universe as known today: the dark matter. I will describe in detail the cosmological and astrophysical probes used to study the effects of dark matter on the visible component of the Universe, and I will explain the evidences pointing to the existence of such an elusive form of matter. I will explain why the ΛCDM model does not provide an accurate description of the Universe at small scales, I will introduce the small-scale challenges that the standard model of cosmology is facing today, and I will discuss possible solutions within the dark sector and the baryonic physics. In Chapter 3, I will introduce the numerical tools needed to model the structure formation process and, in general, I will describe the common techniques employed in cosmological N-body simulations. I will also explain how the baryonic physics is modelled in hydrodynamic simulations and how relevant astrophysical phenomena are implemented in sub-grid models. In Chapter 4, I will summarise the main investigation points of my research and I will briefly discuss the opportunities that current and future experiments will provide to promote our understanding of the mysterious nature of dark matter and, in general, of the Universe. iii Preface Acknowledgements I wish to thank many people who have a special meaning for me. First, I want to thank my supervisors David F. Mota and Hans A. Winther, who taught me how to freely develop my scientific thoughts. They encouraged me at every step of my Ph.D. and they left me the freedom to work in a way I really enjoyed. Together, we worked on highly engaging and stimulating projects. I also want to thank Sijing Shen, not only as a collaborator in one of my Ph.D. projects, but also as a friend and as an example of seriousness and dedication. Furthermore I would like to thank other scientists and friends I have met in Norway. I am sorry if I am forgetting someone, but here is the list in no particular order: Robert H., Robert W., Marie, Max and Rahel, Lluis and Nina, Clara, Ainar and Marianne, Ranajoy, Bridget, Ata, Monica, and Harald. With many of them I have shared interesting ideas and stimulating conversations about physics, astrophysics and cosmology, and with all of them I have shared my wonderful experience in Oslo. I wish to thank my all long-time friends and the ciuk pist companions, sharing their life and happiness with me. In particular, I wish to thank Carlo, Paolo and Maurizio, who have been my best fiends since I was a child. I wish to thank my parents who, with many sacrifices, have given me the opportunity to study and to embark on this wonderful journey of personal growth, and my whole family who, for better or worse, have always supported me. They have always encouraged me to dream big and look ahead, they have taught me healthy values and moral principles, and they have been true inspiration during my entire life. Last but not least, a special thank to my beloved Viviana, who has always supported and even endured me, every day of my life since I met her. She has helped me fully understand that the Universe is such a wonderful place! Mattia Mina Oslo, October 2020 iv List of Papers Paper I Mattia Mina, David F. Mota and Hans A. Winther. “SCALAR: an AMR code to simulate axion-like dark matter models”. Submitted for publication in Astronomy & Astrophysics. arXiv: 1906.12160. Paper II Mattia Mina, David F. Mota and Hans A. Winther. “Solitons in the dark: non- linear structure formation with fuzzy dark matter”. Submitted for publication in Astronomy & Astrophysics. arXiv: 2007.04119. Paper III Mattia Mina, Sijing Shen and Benjamin W. Keller. “The baryon cycle of the Seven Dwarfs with superbubble feedback”. Submitted for publication in Astronomy & Astrophysics. v Contents Preface iii List of Papers v Contents vii List of Figures ix List of Tables xi 1 Introduction 1 1.1 Introduction . 1 1.2 Modern cosmology . 1 1.3 History of the Universe . 3 1.4 Open questions . 5 1.5 General Relativity . 7 1.6 Background evolution . 8 1.7 The perturbed Universe . 9 1.8 Non-linear regime . 11 2 Dark Matter, baryons and the Universe at small-scales 13 2.1 Introduction . 13 2.2 Cosmological small-scale probes . 16 2.3 Small-scale challenges of CDM . 20 2.4 Ultra-light Dark Matter . 22 2.5 Baryonic effects on dark matter . 28 3 Numerical simulations of structure formation 31 3.1 Introduction . 31 3.2 Cosmological N-body simulations . 32 3.3 Hydrodynamic simulations . 33 4 Summary 37 4.1 The numerics behind ultra-light dark matter models . 39 4.2 Structure formation with ultra-light dark matter . 39 4.3 Superbubbles at work . 40 4.4 Future prospects . 40 Bibliography 43 vii Contents Papers 50 I SCALAR: an AMR code to simulate axion-like dark matter models 51 II Solitons in the dark: non-linear structure formation with fuzzy dark matter 71 III The baryon cycle of the Seven Dwarfs with superbubble feedback 89 viii List of Figures 1.1 Chronology of the Universe. 4 2.1 Composite image of the bullet cluster. 13 2.2 Background evolution of a light scalar field. 24 3.1 Hydrodynamic simulation of the dark matter and the gas components. 31 ix List of Tables 1.1 List of the six independent parameters of the ΛCDM model. 4 xi Chapter 1 Introduction Throughout this thesis, I work in natural units, where, c = ~ = kB = 1. In addition, I use a “mostly positive” metric signature (−, +, +, +). 1.1 Introduction Astronomy is, in its broadest sense, the study of any object and physical phenomena originating outside the Earth. From the motion of small celestial bodies, such as comets, planets and stars, to the formation of bigger objects far away from our own planet, such as galaxies and cluster of galaxies, from exotic objects, like black holes and neutron stars, to gravitational waves: these are only some of the research topics included in modern astronomy. Nowadays, astronomy is mostly associated to observations, driving the theoretical understanding in two related research fields: cosmology and astrophysics. Cosmology studies the origin and the evolution of the Universe as a whole, from the Big Bang to the present day, together with the formation and the dynamics of large-scale structures we can observe today in the sky. Astrophysics, instead, aims to develop the theoretical understanding of the formation of medium and small structures. It applies the laws of physics to describe the birth, life and eventually death of planets, stars, black holes and galaxies. Astronomy is perhaps one of the most exciting research fields. With the passion and the desire to discover and explore the unknown, astronomy has a long and glorious history, dating back to ancient societies, such as the mayans, the babylonians, the greeks, and the astronomers of the renaissance. We believe that we understand how the Universe was born and how it has evolved into what it is today, but the Universe is a mysterious place and we are only starting now to unveil its darkest secrets. As for many astronomers, the beautiful sky at night offered me motivation and inspiration for my studies. 1.2 Modern cosmology Modern cosmology gets its theoretical foundations from the theory of general relativity (GR), published by A. Einstein (Einstein, 1917). Until the beginning of the 20th century, gravity was a force and the corresponding gravitational interaction was universally described by Newton law of gravitation, so far successful when describing the motion of planets around the Sun. With the advent of GR, gravity was no longer considered a force, but rather the result of the space-time curvature induced by the presence of a distribution of mass and energy.
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