COSMOLOGICAL ANOMALIES SHED LIGHT ON THE DARK SECTOR by Tanvi Karwal A dissertation submitted to The Johns Hopkins University in conformity with the requirements for the degree of Doctor of Philosophy. Baltimore, Maryland June, 2019 c 2019 Tanvi Karwal ⃝ All rights reserved Abstract Little is known about dark energy and dark matter; but their simple descriptions in the ΛCDM model of cosmology fit numerous datasets well. Recently however, tensions have emerged between the results of different datasets, as have certain unex- pected results. These anomalies may indicate new physics beyond ΛCDM; a revision of how we describe the dark sector. My work uses cosmological anomalies to explore the dark sector. My research resolves perhaps the most exciting tension in cosmology, the Hubble tension, with early dark energy (EDE). I developed two physical models for EDE and find that these models not only solve the Hubble tension, but also fit mostcosmolog- ical datasets well. No other solution to the Hubble tension proposed thus far can do both - fully solve the tension while still fitting early and late-time measurements of the Universe. The model that succeeds ΛCDM should solve not only the Hubble tension but ii ABSTRACT also other cosmological tensions such as the S8 anomaly. My research investigates a decaying dark matter model to address both tensions simultaneously but finds the constraints from late-universe observations too stringent to permit a full resolution. I am also building a phenomenological tool to model the dark sector in a widely- used cosmological code. This tool, generalised dark matter, is a powerful formalism capable of emulating the effects of a wide variety of dark matter and dark energy models, simplifying placing constraints on different fundamentally-motivated models. The formalism and two immediate applications of it are discussed. My work also shows that a curious cosmic-ray (CR) measurement likely has an astrophysical explanation rather than one motivated by dark matter. More flux in CR positrons is observed at high energies than expected, indicating new sources for high-energy CR positrons. My work demonstrates that nearby pulsars could source these positrons and it develops a new analysis technique to distinguish dark matter and pulsar sources for CR data. The aforementioned anomalies and many others have persisted in cosmology for years. It may be time to look beyond the standard model of cosmology for new physics and new descriptions of the dark sector. iii ABSTRACT Readers Marc Kamionkowski (Primary Advisor) Adam Riess Petar Maksimovic Sabine Stanley Susan Kassin iv Acknowledgments Science is not accomplished in isolation and I am grateful for the encouragement and guidance I have received from many. I am infinitely grateful to my advisor Marc Kamionkowski. Your advising is largely responsible for moulding me into the researcher I am today. Hopefully, we can agree that that is a compliment. Thank you for being kind and understanding, even when you have to remind me how old the Earth is. Besides striving to be as brilliant as you, I also aspire to be as unperturbed as you. I am extremely grateful for your support, presenting me with every opportunity for growth and of course, your endless anecdotes. I hope to return as a collaborator someday. My closest collaborators Tristan Smith, Vivian Poulin and `academic uncle' Ilias Cholis, I have learnt more from you than you tried to teach me. Thank you for trusting my scientific judgement as well as pointing out its fallacies. We have accomplished some spectacular science together and I look forward to more science to come. v ACKNOWLEDGMENTS I am grateful also to others who mentored me, whether knowingly or not. Julian Mu~noz,Patrick Breysse, Ibou Bah and Kim Boddy, you are magnanimous scientists and I deeply appreciate the time you spent on me when you had no obligation to. Kim, I want to grow up to be you. Roller-coasters are more fun when you have people to scream with, and I am grateful for the many graduate students I shared this ride with. In particular, Dan Pfeffer, Kim Berghaus and Kirsten Hall, you are my cornerstones. Dan, I relyonyou far more than I care to admit, you are a bouncy-castle of support. Kim, thank you for telling me to solve my problems. You have weird approaches to motivational speeches, but they work wonderfully. Kirsten, I found a soul-sister in you and I appreciate your calming and validating words, as well as your love of art and cake. You guys give me the warm-and-fuzzies. On a similar note, thank you Kelley Key for keeping all us graduate students in check and always having zombie-movie recommendations for me. I felt truly taken care of at this department, in large part thanks to you. I must also thank Naitri Jadeja, mostly because she forced me to by interminably whining, but also because she kept me sane during the many years of graduate school. Finally, I thank my walking birthplace Anita Karwal, the previous tenant and book-thief Janvi Karwal, and black-belt embarrasser Atul Karwal. You guys make me believe I can do anything. vi Dedication To my loving parents and sister. I wrote a PhD thesis just to cure your insomnia. vii Contents Abstract ii Acknowledgments v List of Tables xii List of Figures xiii 1 Introduction 1 2 The Hubble Tension 10 2.1 Early Dark Energy at the Background Level . 12 2.1.1 Model . 13 2.1.2 Method . 18 Fisher Matrices . 19 Planck Data . 22 2.1.3 Constraints on the EDE . 24 Fixing τ = τPl .............................. 25 Fixing τ = τPl + 2στ;Pl .......................... 26 viii CONTENTS Fixing τ = τPl + 5στ;Pl .......................... 29 2.1.4 Conclusions . 29 2.2 Ultra-Light Axion-like Fields as Early Dark Energy . 37 2.2.1 The cosmological dynamics of ULAs . 40 Background dynamics . 40 Perturbed dynamics in the fluid formalism . 43 Initial conditions of perturbations . 44 2.2.2 Current constraints to ULAs . 46 2.2.3 Implications for cosmological tensions . 49 The Hubble tension . 49 EDGES exotic 21 cm measurement . 53 2.2.4 Conclusions . 56 2.3 A Resolution of the Hubble Tension with Early Dark Energy . 57 3 Combining Cosmological Tensions 70 3.1 Decaying dark matter model . 73 3.1.1 Background dynamics . 74 3.1.2 Calculation of ∆Neff ........................... 76 3.1.3 Effects on observables . 77 3.2 Methodology . 80 3.3 Results . 81 3.4 Discussion and conclusions . 89 4 Generalised Dark Matter 92 ix CONTENTS 4.1 GDM Dynamics . 94 4.1.1 Background dynamics . 94 4.1.2 Perturbation evolution . 94 4.1.3 Effect on the CMB . 95 4.2 Applications of the GDM formalism . 100 4.2.1 Principal component analysis with GDM . 100 4.2.2 The CMB low multipole tension . 101 5 The Cosmic-Ray Positron Excess 103 5.1 The CR positron excess and possible explanations . 104 5.2 The pulsar explanation of the CR positron excess . 106 5.2.1 Data . 108 5.2.2 Astrophysical uncertainties surrounding the pulsar explanation . 108 Neutron star birth distribution . 109 Neutron star spin-down distribution properties . 112 Injection properties of CR positrons . 115 CR propagation through the ISM and heliosphere . 119 Pulsar simulations . 120 5.2.3 Differentiating pulsars and dark matter with CRs . 122 Power-spectrum analysis of the CR positron spectrum . 122 Results . 124 Projections for future data . 126 5.3 Conclusions . 128 x CONTENTS Bibliography 131 Vita 166 xi List of Tables 2.1 Fiducial values of ΛCDM parameters used in the Fisher-matrix analysis . 24 2.2 The best-fit χ2-values per experiment for various cosmologies . 68 2.3 Best-fit parameter values for various cosmologies . 69 3.1 Priors on MCMC parameters . 82 3.2 Best-fit parameter values for ΛCDM and DDM for various datasets . 88 3.3 The best-fit χ2-values per experiment for ΛCDM and DDM . 89 5.1 Spin-down power distribution properties for pulsar simulations . 116 5.2 CR propagation model parameters . 120 5.3 Potential to observe features in the CR e+ spectrum for various experiments 128 xii List of Figures 2.1 Background evolution of early dark energy . 15 2.2 Shifts in the CMB TT spectrum due to EDE . 17 2.3 Partial derivatives of the TT power spectrum with respect to ΛCDM and EDE parameters . 21 2.4 Results for EDE and the Hubble parameter fixing τ = τPl .......... 27 2.5 Results for EDE and the Hubble parameter fixing τ = τPl + 2στ;Pl ..... 28 2.6 Results for EDE and the Hubble parameter fixing τ = τPl + 5στ;Pl ..... 30 2.7 Correlations between the Hubble parameter and EDE for various zc .... 31 2.8 The best-fit CMB TT spectra for ΛCDM and ΛCDM+EDE models . 33 2.9 The maximum allowed fractional energy density in EDE . 35 2.10 Background evolution of the ULA EDE . 42 2.11 Cosmological constraints on the density of ULA EDE today . 47 2.12 Posteriors for ULA EDE and ΛCDM parameters . 48 2.13 Effect of ULA EDE on the Hubble parameter . 52 2.14 Effect of ULA EDE on the EDGES tension . 54 xiii LIST OF FIGURES 2.15 Posteriors for ΛCDM and EDE cosmologies showing the EDE solution to the Hubble tension . 61 2.16 Variation of physical scales due to EDE . 62 2.17 CMB TT residuals of the EDE cosmology compared to ΛCDM . 67 3.1 Effect of DDM on observables . 78 3.2 Posteriors for ΛCDM and DDM for Planck data . 83 3.3 Posteriors for ΛCDM and DDM for Planck + local H0 ............ 84 3.4 Posteriors for ΛCDM and DDM for Planck + local H0 + supernovae + BAO 85 3.5 Diminishing the H0 and S8 tensions simultaneously .