Karim A. Malik David R. Matravers How Cosmologists Explain the Universe to Friends and Family Astronomers’ Universe Series Editor Martin Beech Campion College The University of Regina Regina, Saskatchewan, Canada TheAstronomers’UniverseSeriesisaimedatthesamepeopleasthePractical Astronomy Series—in general, active amateur astronomers. However, it is also appropriate to a wider audience of astronomically-informed readers. Because optical astronomy is a science that is rather at the mercy of the weather, all amateur astronomers inevitably have periods when observing is impossible. At such times they tend to read books about astronomy and related subjects. When researching this market, it is quite surprising to discover how few books there are that are of direct appeal to “armchair astronomers”. There are many “popular science” books about matters cosmological, but because of their general audience these all start from the beginning, covering and re-covering the basics. At the other end of the spectrum there are professional books that are highly mathematical and technical, not intended for enjoyable reading. The Astronomers’ Universe Series begins by assuming an appropriate level of knowledge. Basic information about the distance, the solar system, galaxies, etc. is not part of these books, which can take a basic understanding of this as their starting point. The series is differentiated from popular science series (such as Springer’s Copernicus books) by a strong design image which will attract active amateur astronomers, and will also appeal to “armchair astronomers” (or cosmologists) and other readers who already have the necessary background knowledge. Thebookshavetobecarefullywritten,structuredandeditedsoastobeaimed at these scientifically-aware readers: they will have a background knowledge of astronomy and probably cosmology but many of them will not have formally studied science (amateur astronomers come from almost all walks of life) and will be discouraged by mathematical treatments. The content will therefore mostly be descriptive, with only essential mathematics included. More information about this series at http://www.springer.com/series/6960 Karim A. Malik • David R. Matravers How Cosmologists Explain the Universe to Friends and Family Karim A. Malik David R. Matravers School of Physics and Astronomy Institute of Cosmology and Gravitation Queen Mary University of London University of Portsmouth London, UK Portsmouth, UK ISSN 1614-659X ISSN 2197-6651 (electronic) Astronomers’ Universe ISBN 978-3-030-32733-0 ISBN 978-3-030-32734-7 (eBook) https://doi.org/10.1007/978-3-030-32734-7 © Springer Nature Switzerland AG 2019 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Foreground image: This map shows the oldest light in our universe, as detected with the greatest precision yet by the Planck mission. The ancient light, called the cosmic microwave background, was imprinted on the sky when the universe was 370,000 years old. Background: Planck’s Cosmic Microwave Background Map (alt. color scheme). Credit: ESA and the Planck Collaboration This Springer imprint is published by the registered company Springer Nature Switzerland AG. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland To our parents. Preface Discussing our work over the years with friends, family members, and other non-cosmologists, we frequently get asked “what is it you do exactly”towhich we usually say “well, we study cosmology.” The conversation then usually takes one of two turns: either we give a single sentence explanation along the lines of “we study the universe and its origins,” in which case our counterpart often feels short changed and rightly so, or we try to explain in some detail, to pay justice to the topic we spend our lives researching, but our counterpart gets overwhelmed by the rather complicated subject and loses interest. Both outcomes were and are really frustrating for us and our friends. We don’t want to describe our work and main research interest with a short sentence, but we also don’t want to lecture our friends and watch their eyes glaze over when talking about equations and observational data. We would have liked to recommend a nontechnical book, and there are many commendable ones, but we could find none that would explain in sufficient detail while at the same time being accessible to a person without a Mathematics or Physics background. The books we saw available were either too simplistic, making things unnecessarily mysterious, or too complicated, expecting too much prior knowledge. This book aims at filling this gap. We started 6 years ago, thinking “how hard can it be?”, but it turned out to be quite difficult. To explain modern cosmology without using equations, and hence make it accessible for somebody without a physics background, but without oversimplifying the subject, is rather difficult. This book aims to provide an up-to-date overview of modern cosmology, in particular the evolution of the large-scale structure, the distribution of galaxies and clusters of galaxies, in the universe. Cosmology is a complex area vii viii Preface of research, without question. But nevertheless, it is well within the grasp of a “layperson,” if this person is prepared to keep an open mind and happy to spend some time letting some of the concepts sink in. Tohelp the reader, we try to explain most technical concepts within the book and also added a glossary attheendwherewedefineandbrieflyexplainthemostfrequentlyoccurring technical terms. The book is aimed at the interested layperson with little or no physics background but an interest in modern cosmology. In the brief introduction, we try and give a rounded, complete picture of our current understanding of cosmology based on the latest research and in particular the structure of the universe on very, very large scales. We hope that this is not too overwhelming but instead entices the reader to find out more! Besides, we have to start somewhere. We are aware that therefore the introduction will be rather challenging, but we ask the reader to persist, as most or all of the topics will then be discussed in detail and put into context. After the introduction, we provide the reader with an overview of the scientific method, how observations are made and what kind of observations are there. We then discuss the constituents of the universe, including dark matter and dark energy, and provide an overview of the forces shaping the universe, in particular gravity. We thus take the reader on a tour back in time from the present day to the very beginning and discuss the beginning of the universe, a period called inflation, which sets the scene, or the initial conditions, for the following evolution. We end with a concluding chapter. London, UK Karim A. Malik Westbourne, UK David R. Matravers August 2019 Contents 1 Introduction 1 1.1 The Cosmological Standard Model 2 1.1.1 From Inflation to Radiation Domination 4 1.1.2 From Radiation to Matter Domination 5 1.1.3 From Matter Domination to the Present Day 6 1.1.4 What Constitutes the Universe 7 1.2 Two Pictures of the Universe 9 2HowDoesScienceWork? 15 2.1 Introduction 15 2.2 The Scientific Method 16 2.3 Examples 20 2.3.1 Colour Dependent Gravity 20 2.3.2 The Perihelion Shift in the Orbit of Mercury 21 2.4 Changing Theory in Physics 23 2.5 Cosmological Simulations 25 3WhatObservationsDoWeUse? 31 3.1 Introduction 31 3.2 What Reaches Us 32 3.2.1 Different Forms of Light: Electromagnetic Radiation 32 3.2.2 Non-standard Messengers: Neutrinos and Gravitational Waves 35 ix x Contents 3.3 How Observations Are Made 39 3.3.1 Optical Telescopes 40 3.3.2 Radio Telescopes 44 3.3.3 Microwave Telescopes 46 3.3.4 Examples for Non-standard Messenger Observatories 49 3.4 Future Experiments and Telescopes 54 3.4.1 SKA 54 3.4.2 Euclid 54 3.4.3 Extremely Large Telescope—ELT 55 3.5 Closing Comments 56 4WhatDoWeObserve? 57 4.1 Distance Units in Cosmology 57 4.2 Things We Can Observe Directly 59 4.3 How to Measure Distances in Cosmology 63 4.4 Putting Things into Perspective 70 4.5 An Even Larger Map: The Cosmic Microwave Background 77 5 What is the Universe Made Of? 81 5.1 Familiar Types of Matter, What Is Normal Stuff Made Up Of? 81 5.1.1 Matter on the Smallest Scales: Up to the Size of Atoms 81 5.1.2 Matter on Larger Scales: Many Particles 84 5.1.3 Radiation 86 5.1.4 Waves and Particles