Big History

FOREWORD BY DAVID CHRISTIAN

Lead Senior Editor Helen Fewster Senior DTP Designer (Delhi) Harish Aggarwal Editor Ira Pundeer Senior Editors Peter Frances, Dr. Rob Houston Jacket Design Development Manager Sophia MTT Production Manager Pankaj Sharma Pre-production Manager Balwant Singh Senior Art Editors Amy Child, Phil Gamble, Managing Jackets Editor (Delhi) Saloni Singh Senior DTP Designer Vishal Bhatia Sharon Spencer Producer, pre-production Jacqueline Street-Elkayam DTP Designer Nand Kishore Acharya Picture Research Manager Taiyaba Khatoon Project Art Editors Paul Drislane, Mik Gates, Producer Mary Slater Duncan Turner, Francis Wong Picture Researcher Sakshi Saluja Design Assistant Alex Lloyd Managing Art Editor Michael Duffy Managing Editor Angeles Gavira Guerrero Illustrators Project Editors Camilla Hallinan, Wendy Horobin, Rajeev Doshi (Medi-mation) Andy Szudek Art Director Karen Self Peter Bull (Peter Bull Art Studio) Design Director Phil Ormerod Editor Kaiya Shang Arran Lewis Editorial Assistant Francesco Piscitelli Publisher Liz Wheeler Dominic Clifford US Editors Margaret Parrish, Christy Lusiak Publishing Director Jonathan Metcalf Jason Pickersgill (Acute Graphics) Picture Researcher Liz Moore DK DELHI Mark Clifton Cartography Ron and Dee Blakey, Ed Merritt, Senior Managing Art Editor Arunesh Talapatra Simon Mumford Art Editors Roshni Kapur, Meenal Goel Jacket Designers Mark Cavanagh, Suhita Dharamjit Managing Editor Chitra Subramanyam Jacket Editor Claire Gell First American Edition, 2016 Published in the United States by DK Publishing 345 Hudson Street, New York, New York 10014 Copyright © 2016 Dorling Kindersley Limited DK, a Division of Penguin Random House, LLC 16 17 18 19 10 9 8 7 6 5 4 3 2 1 001–287353–October/2016 All rights reserved. Without limiting the rights under the copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, photocopying, recording, or otherwise), without the prior written permission of the copyright owner. Published in Great Britain by Dorling Kindersley Limited. A catalog record for this book is available from the Library of Congress. ISBN: 978-1-4654-5443-0 Printed and bound in China DK books are available at special discounts when purchased in bulk for sales promotions, premiums, fund-raising, or educational use. For details contact: DK Publishing Special Markets, 345 Hudson Street, New York, New York 10014 or [email protected] A WORLD OF IDEAS: SEE ALL THERE IS TO KNOW www.dk.com Big History Institute Consultants Contributors Macquarie University was founded with a unique David Christian, Director, Big History Institute Introduction – Elise Bohan purpose: to bring minds together unhindered by Professor David Christian (DPhil Oxford) is the founder of Big History. David Threshold 1 – Robert Dinwiddie tradition. Created to challenge the education is a Distinguished Professor at Macquarie University, co-founder with Bill Gates of Threshold 2 – Jack Challoner establishment—Macquarie has a rich track record the Big History Project, host of one of the 11 classic TED Talks with over 6 million Thresholds 3 and 4 – Colin Stuart of innovation—Big History is such an innovation. views, and instructor in the world’s first Big History MOOC on COURSERA®. Threshold 5 – Derek Harvey The Big History Institute builds upon the pioneering Threshold 6 – Rebecca Wragg-Sykes role that Macquarie University has played in the David has given hundreds of presentations internationally, including Davos Threshold 7 – Peter Chrisp evolution of the new field of Big History. It brings World Economic Forum in 2012, 2014, and 2015. He is a member of the Australian Threshold 8 – Ben Hubbard together a community of scholars and students Academy of the Humanities and the Royal Holland Society of Sciences and Timelines of world history – Philip from both the sciences and the humanities that Humanities, and a member of the editorial boards of the Journal of Global History Parker pursues research questions across disciplinary and The Cambridge World History. boundaries and discovers new ways of thinking. The Big History Institute is a global hub for Andrew McKenna, Executive Director, Big History Institute educators, members of the public, and partners Andrew McKenna (BCom LLB UNSW, MIntRel Macquarie) coordinates Big History from the research, government, nonprofit, and as an integrated initiative encompassing research, teaching, and outreach. Andrew business sectors. leads the strategic growth and development of the Big History Institute globally. Tracy Sullivan, Education Leader, Big History Institute Tracy Sullivan (BA GradDipEd UWS, MA Macquarie) was on the curriculum development team for Big History Project, and oversees implementation in Australian schools. Tracy coordinates educational initiatives to support the growth and development of Big History globally.

CONTENTS 8 FOREWORD BY DAVID CHRISTIAN 10 INTRODUCTION 1 THE BIG BANG 16 GOLDILOCKS CONDITIONS 28 THE ATOM AND THE UNIVERSE 18 ORIGIN STORIES 30 THE UNIVERSE GETS BIGGER 20 THE NEBRA SKY DISK 32 THE EXPANDING UNIVERSE 22 ASTRONOMY BEGINS 34 THE BIG BANG 24 EARTH ORBITS THE SUN 36 RECREATING THE BIG BANG 26 SEEING THE LIGHT 38 BEYOND THE BIG BANG 2 STARS ARE BORN 42 GOLDILOCKS CONDITIONS 44 THE FIRST STARS 46 THE PUZZLE OF GRAVITY 48 THE FIRST GALAXIES 50 HUBBLE EXTREME DEEP FIELD 3 ELEMENTS ARE FORGED 54 GOLDILOCKS CONDITIONS 56 THE LIFE CYCLE OF A STAR 58 HOW NEW ELEMENTS FORM INSIDE STARS 60 WHEN GIANT STARS EXPLODE 62 MAKING SENSE OF THE ELEMENTS 4 PLANETS FORM 66 GOLDILOCKS CONDITIONS 78 EARTH COOLS 90 CONTINENTS DRIFT 68 OUR SUN IGNITES 80 EARTH SETTLES INTO LAYERS 92 HOW EARTH’S CRUST MOVES 70 THE PLANETS FORM 82 THE MOON’S ROLE 94 OCEAN FLOOR 72 THE IMILAC METEORITE  84 THE CONTINENTS ARE BORN 74 THE SUN TAKES CONTROL 86 DATING EARTH 76 HOW WE FIND SOLAR SYSTEMS 88 ZIRCON CRYSTAL

5 LIFE EMERGES 98 GOLDILOCKS CONDITIONS 114 LIFE DISCOVERS SUNLIGHT 130 ANIMALS GAIN A BACKBONE 100 STORY OF LIFE 116 OXYGEN FILLS THE AIR 132 RISE OF THE VERTEBRATES 102 LIFE’S INGREDIENTS FORM 118 COMPLEX CELLS EVOLVE 134 JAWS CREATE TOP PREDATORS 104 THE GENETIC CODE 120 SEX MIXES GENES 136 PLANTS MOVE ONTO LAND 106 LIFE BEGINS 122 CELLS BEGIN TO BUILD BODIES 138 WENLOCK LIMESTONE 108 HOW LIFE EVOLVES 124 MALES AND FEMALES DIVERGE 140 ANIMALS INVADE LAND 110 HISTORY OF EVOLUTION 126 ANIMALS GET A BRAIN 142 REINVENTING THE WING 112 MICROBES APPEAR 128 ANIMAL LIFE EXPLODES 144 THE FIRST SEEDS 6 HUMANS EVOLVE 180 GOLDILOCKS CONDITIONS 196 ANCIENT DNA 214 THE INVENTION OF CLOTHING 182 THE PRIMATE FAMILY 198 THE FIRST HOMO SAPIENS 216 HUMANS HARNESS FIRE 184 HOMININS EVOLVE 200 BRINGING UP BABIES 218 BURIAL PRACTICES 186 APES BEGIN TO WALK UPRIGHT 202 HOW LANGUAGE EVOLVED 220 HUMANS BECOME DOMINANT 188 GROWING A LARGER BRAIN 204 COLLECTIVE LEARNING 190 THE NEANDERTHALS 206 THE BIRTH OF CREATIVITY 192 KEBRA NEANDERTHAL 210 HUNTER-GATHERERS EMERGE 194 EARLY HUMANS DISPERSE 212 PALEOLITHIC ART 7 CIVILIZATIONS DEVELOP 224 GOLDILOCKS CONDITIONS 238 POLLEN GRAINS 252 POPULATION STARTS TO RISE 226 CLIMATE CHANGES 240 FARMERS DOMESTICATE 254 THE FENTON VASE 256 EARLY SETTLEMENTS THE LANDSCAPE ANIMALS 258 SOCIETY GETS ORGANIZED 228 FORAGERS BECOME FARMERS 242 FARMING SPREADS 260 RULERS EMERGE 230 AFFLUENT FORAGERS 244 MEASURING TIME 262 LAW, ORDER, AND JUSTICE 232 HUNTERS BEGIN TO GROW FOOD 246 NEW USES FOR ANIMALS 264 THE WRITTEN WORD 234 FARMING BEGINS 248 INNOVATIONS INCREASE YIELDS 266 WRITING DEVELOPS 236 WILD PLANTS BECOME CROPS 250 SURPLUS BECOMES POWER 8 INDUSTRY RISES 302 GOLDILOCKS CONDITIONS 318 EQUALITY AND FREEDOM 336 ROAD TO GLOBALIZATION 304 THE INDUSTRIAL REVOLUTION 320 NATIONALISM EMERGES 338 ENGINES SHRINK THE WORLD 306 COAL FUELS INDUSTRY 322 THE INDUSTRIAL ECONOMY 340 NEWS TRAVELS FASTER 308 STEAM POWER DRIVES CHANGE 324 THE WORLD OPENS TO TRADE 342 SOCIAL NETWORKS EXPAND 310 THE PROCESS OF 326 WAR DRIVES INNOVATION 344 GROWTH AND CONSUMPTION 328 COLONIAL EMPIRES GROW 346 FINDING THE ENERGY INDUSTRIALIZATION 330 SOCIETY TRANSFORMS 348 NUCLEAR OPTIONS 312 INDUSTRY GOES GLOBAL 332 EDUCATION EXPANDS 350 ENTERING THE ANTHROPOCENE 314 GOVERNMENTS EVOLVE 334 MEDICAL ADVANCES 352 CLIMATE CHANGE 316 CONSUMERISM TAKES OFF

146 SHELLED EGGS ARE BORN 160 THE PLANET BLOSSOMS 174 ICE CORES 148 HOW COAL FORMED 162 MASS EXTINCTIONS 176 EARTH FREEZES 150 LIZARD IN AMBER 164 PLANTS RECRUIT INSECTS 152 THE LAND DRIES OUT 166 THE RISE OF MAMMALS 154 REPTILES DIVERSIFY 168 GRASSLANDS ADVANCE 156 BIRDS TAKE TO THE AIR 170 EVOLUTION TRANSFORMS LIFE 158 CONTINENTS SHIFT 172 HOW WE CLASSIFY LIFE AND LIFE DIVIDES 268 WATERING THE DESERT 282 ÖTZI THE ICEMAN 296 EAST MEETS WEST 270 CITY-STATES EMERGE 284 CONFLICT LEADS TO WAR 298 TRADE GOES GLOBAL 272 FARMING IMPACTS THE 286 AGE OF EMPIRES 288 HOW EMPIRES RISE AND FALL ENVIRONMENT 290 MAKING MONEY 274 BELIEF SYSTEMS 292 UNHEALTHY DEVELOPMENTS 276 GRAVE GOODS 294 TRADE NETWORKS DEVELOP 278 CLOTHING SHOWS STATUS 280 USING METALS 354 ELEMENTS UNDER THREAT 360 TIMELINES OF WORLD HISTORY 392 PHILOSOPHY AND FAITH 356 THE QUEST FOR SUSTAINABILITY 362 PREHISTORIC WORLD, 394 EARLY MODERN WORLD, 1450–1750 358 WHERE NEXT? 402 INVENTION AND DISCOVERY 8 MYA–3000 BCE 404 WORLD OF EMPIRES, 1750–1914 367 CALENDAR SYSTEMS 411 ASTRONOMY AND SPACE 368 ANCIENT WORLD, 3000–700 BCE 412 MODERN WORLD, 1914 ONWARD 374 CULTURE AND CREATIVITY 422 MILESTONES IN MEDICINE 376 CLASSICAL WORLD, 700 BCE – 600CE 424 INDEX AND 383 GREAT BUILDINGS 384 MEDIEVAL WORLD, 600–145O ACKNOWLEDGMENTS

FOREWORD Today, more and more schools and universities are teaching Big History, and it’s a story we all need to know. I vividly remember a globe map of the world sitting in a In the book you are holding in your hands, you will find classroom when I was a child. I also remember a geography a beautifully illustrated account of this story, a sort of class, taught in a school in Somerset in England, where we globe in words and pictures that links knowledge from learned how to draw sections through the earth, showing many different disciplines. Big History shows how our the various layers of soil beneath our feet, and how they world developed, threshold by threshold, from a very connected to other parts of England. For me, the most simple early universe, to the emergence of stars and exciting thing in school was always the sudden connections, chemistry, and on to a cosmos that contained places realizing that layers of chalk beneath our feet were made like our earth on which life itself could emerge. from the remains of billions of tiny organisms—called coccolithophores—that had lived millions of years ago, and And you’ll also see the strange role played by our own that the same remains could also be found in layers of chalk species, humans, in this huge story. We appear at the in other parts of England and other countries much farther very end of the story, but our impact has been so colossal away. What was Somerset like when the coccolithophores that we are beginning to change the planet. We have done were alive? For that matter, where was Somerset back then? something else that is perhaps even more astonishing: from That’s a question I couldn’t even ask when I was at school our tiny vantage point in the vast universe, we have figured because at that time scientists didn’t know for sure that the out how that universe was created, how it evolved, and how continents moved around the surface of the earth. it became as it is today. That is an amazing achievement, and in this book you will explore the discoveries that For me, the globe in the corner of my classroom was a key allowed us to piece together this story. This is the world to all this knowledge. It helped me see the place of Somerset globe that we need today, early in the 21st century, as in Britain, of Britain in Europe—so that’s where the Vikings we try to manage the huge challenges of maintaining came from!—and of Europe in the world. Big History is like our beautiful planet and keeping it in good condition the globe, but it’s much bigger: it includes all the observable for those who will come after us. universe and all observable time, so it reaches back in time for 13.8 billion years to the astonishing moment of the Big DAVID CHRISTIAN Bang, when an entire universe was smaller than an atom. Big History includes the story of stars and galaxies, of new FOUNDER OF BIG HISTORY elements from carbon—the magical molecule that made life possible—to uranium, whose radioactivity enabled us not DIRECTOR, BIG HISTORY INSTITUTE just to make bombs, but also to figure out when our earth was formed. It is like a map of all of space and time. And CO-FOUNDER OF THE BIG HISTORY PROJECT once you start exploring that map, you will be able, eventually, to say: “So that’s what I’m a tiny part of! That’s my place in the grand scheme of things! So what’s next?”

Big History provides a framework for understanding literally all of history, ever, from the Big Bang to the present day. So often subjects in science and history are taught one at a time—physics in one class, the rise of civilization in another—but Big History breaks down those barriers. Today, whenever I learn something new about biology or history or just about any other subject, I try to fit it into the framework I got from Big History. No other course has had as big an impact on how I think about the world. BILL GATES, WWW.GATESNOTES.COM CO-FOUNDER OF THE BIG HISTORY PROJECT

WHAT IS BIG HISTORY? BIG HISTORY IS THE STORY OF cosmos? Throw a dart at any point in the history of the universe and it will HOW YOU AND I CAME TO BE. land on a page of the Big History story. No matter how obscure this page, or It is a modern origin story for a modern how far removed it may seem from the age. This grand evolutionary epic rouses world we know, it will invariably our curiosity, confronts our ingrained describe a fragment of this grand intuitions, and marries science, reason, scientific narrative, in which all events and empiricism with vivid and dynamic and all chapters are connected. storytelling. Best of all, Big History provides the scope and scientific In this volume we traverse the stars, foundations to help us ponder some of the galaxies, the cells inside your body, the most exciting and enduring questions and the complex interactions between all about life, the universe, and everything. living and nonliving things. We stretch our minds to the limits of human These universally compelling understanding in order to see reality questions include: How did life on Earth from many angles, and on many scales. evolve? What makes humans unique? What is truly remarkable about looking Are we alone in the universe? Why do at the world from such an expansive we look and think and behave the way perspective is that we begin to engage we do? And what does the future hold with many facets of the natural world for our species, our planet, and the that we often miss, or take for granted. How often do we think about the fact that every atom inside each of our

HOW OFTEN DO WE naked eye. It is also important to remember that Big History is not a static THINK ABOUT THE tale that proclaims how things are and will be for all time. It is a provisional FACT THAT EVERY narrative that is constantly being updated as our knowledge about the natural world ATOM INSIDE EACH grows, and as our needs as a species evolve. OF OUR BODIES WAS From a cosmic perspective, we see that humans are a novel species that MADE INSIDE A appear on the scene very late in this evolutionary history. We were not there DYING STAR? at the beginning, and we are almost certainly not the species with whom the bodies was made inside a dying star? evolutionary buck stops. Yet Big History is Or that ancient celestial implosions gave still very much a human story, written by rise to the kinds of chemistry that makes humans, for humans. At a certain point in life possible? How frequently do we this tale we choose to focus on our species zoom out far enough in our historical and our corner of the galaxy, because musings to see connections that from our point of view, this is where the transcend the actions of kings, armies, action and the meaning is. politicians, and peasants? In the grand scheme of space and time, Our minds do not instinctively follow humanity may seem like little more than a the threads of our evolutionary history cosmic footnote. But when we look closely to the point where all national, tribal, at our blue planet we see that our species and species boundaries fall away. But is responsible for some very remarkable when we allow ourselves to explore things that no other species has achieved beyond these domains we come face in the 3–4 billion years that life has to face with a single family tree, which existed on Earth. As far as we know, Homo shows that every one of us shares a sapiens is the first and only species to common ancestor with every living represent the universe becoming self-aware. organism on the planet: from worms, Humans are now the dominant force to fish, to reptiles, to chimpanzees, to altering the planetary biosphere, a bird singing on the other side of the and we have kicked the pace of terrestrial world, and the strangers who sleep evolution into a dramatic new gear. through its refrain. BIG HISTORY HELPS US TO QUESTION EVERYTHING WE SEE, AND EVERYTHING WE THINK WE KNOW. Big History helps us question everything As you explore this remarkable our accumulated knowledge and we see, and everything we think we know. narrative, you will discover that our experiences to new generations via In the process, we discover that the species has been so successful in DNA, we have developed the means universe is far stranger than we often expanding and colonizing the globe, in to transmit this information culturally. imagine, and that the shape of history large part because of our capacity for Such a radical innovation in information is molded by forces that are often what big historians call collective sharing was made possible by the surprising, and hard to see with the learning. Although we cannot impart human invention of symbolic language.

At first this meant sharing ideas through WITH THE ABILITY TO BUILD UPON the oral tradition. But eventually we EXISTING INFORMATION OVER MANY developed writing, which reduced the GENERATIONS, HUMANS LEARNED error rate in the transmission of EVER FASTER, AND KNOWLEDGE AND information and left humans in INNOVATION PROLIFERATED. possession of a tool resembling a crude external hard drive. For the first time we giant evolutionary leap in a relatively technological arms race leave us could store large bodies of information short period of time. We have liberated or enslaved? And how long without having to use the limited transitioned from our initial role as will most of us continue to exist as fully memory capacity of our brains. one of evolution’s many simple players, biological beings, unenhanced by to a fledgling director engaged in the technological modifications? With the ability to build upon existing task of consciously shaping the trajectory information over many generations, of evolution on Earth. While this is a These are the kinds of questions humans learned ever faster, and very exciting role, it also presents that the Big History story prompts us knowledge and innovation proliferated. immense challenges. to consider. There is no doubt that in While many civilizations collapsed and terms of its scope, content, and method, some discoveries were lost for centuries, It is sobering to look back at our Big History is a truly modern story, fit the overall trend was a feedback loop extensive family tree and recall that for the needs of a modern age. of accelerating cultural change: the 99 percent of species that have ever lived invention of ever faster and more are now extinct. In light of this, Like all origin stories of previous ages, accurate methods of information it is natural and beneficial to consider this narrative is designed to help orient sharing generated rapid bursts of whether our species will be able to live us with where we come from, what we innovation, and vice versa. sustainably and prosperously for many are, and where we might be going. But years into the future. And if we can unlike ancient origin stories that were While the oral tradition persisted achieve this, how might it be possible? built upon myth and intuition, this for tens of thousands of years, it only evolutionary epic relies on the theories took a few hundred years for humans Will we reduce our consumption of modern science to help us get to grips to transition from the age of the of energy and live more simply? with the world around us. printing press to the digital world of Or will we harness our immense today. If the pace of cultural evolution collective brainpower to engineer For most of us, thinking about things continues at such a rate we may see more sophisticated ways of producing that are very big, very small, and very the emergence of a new evolutionary clean energy and sustainable products old does not come naturally. But paradigm in mere decades. and services? Will our modern pursuing big ideas and chasing the answers to profound universal questions Because of our astounding capacity for collective learning and cultural development, humans have made a BIG HISTORY IS A TRULY MODERN ORIGIN STORY, FIT FOR THE NEEDS OF A MODERN AGE.

does! We cannot help wanting to know complex in various pockets of cosmic what else is out there: whether it be order. This story helps us to see that our among the stars, inside black holes, planet and our species emerged among or in the mysterious workings of our a rare set of goldilocks conditions, where brains, our DNA, or the remarkable the balance and stability of elements was bacterial ecosystems that live on, “just right” to sustain life. around, and inside us. Once you explore this book and The Big History story helps to get a feel for the big picture it presents, facilitate our exploration of these and we hope you will be left pondering other exciting domains. It allows us many new and rousing questions. As to focus on an array of subjects and you sit, poised to embark on this journey historical moments and encourages of discovery, there is one question in us to ponder the nature of reality on particular that we hope you will consider. many different scales. We learn to relate the details to the big picture, What role will you play in and observe how broad trends can determining how events unfold in contextualize local phenomena and the next threshold of this great events. By exploring the viewpoints cosmic drama? of both the generalist and the specialist, we are able to think more “BENEATH THE AWESOME carefully and creatively about cause and effect, and devise more innovative DIVERSITY AND COMPLEXITY OF responses and solutions to the many challenges we face in the world today. MODERN KNOWLEDGE, THERE Big History’s unified perspective IS AN UNDERLYING UNITY AND also helps us to see the present in dynamic terms, and shows us that we COHERENCE, ENSURING THAT are not only the successors of previous evolutionary thresholds, but also the DIFFERENT TIMESCALES REALLY possible progenitors of those to come. DO HAVE SOMETHING TO SAY Our story is divided into eight thresholds of increasing complexity, TO EACH OTHER.” DAVID CHRISTIAN, BIG HISTORIAN each of which highlight some of the key transitionary phases in this cosmic evolutionary history. As we move from threshold to threshold, you will see how profoundly each stage is connected, and how matter and information in the universe grow denser and more

THRESHOLD

THE BIG BANG What are the origins of our universe? It is a question that has captivated our species, probably since we emerged. Centuries of observation, investigation, and scientific endeavor have led us to the Big Bang theory—but that too leaves questions unanswered, and our quest for further explanation continues.

GOLDILOCKS CONDITIONS The universe formed in the Big Bang. We do not know if anything existed before it, and we only have a glimpse of what happened in the fraction of a second immediately afterward. But over the next 380,000 years, the universe expanded and cooled, and the fundamental forces and forms of matter that we know today emerged. What changed? Suddenly, space, time, energy, and matter came into existence in the Big Bang. Before the Big Bang We don’t know what existed before the Big Bang. There might have been nothing. But there are other possibilities. For example, one alternative theory proposes a multiverse—a vast realm from which universes keep appearing.

Particles of matter Matter and and antimatter form antimatter annihilate from mass-energy each other ENERGY AND Energy As the universe cools, MATTER IN AN As the universe quarks are bound INTERCHANGEABLE cools, matter and together by gluons FORM CALLED antimatter stop to form protons MASS-ENERGY and neutrons returning to energy AN INCONCEIVABLY HOT, SMALL, DENSE SINGLE, UNIFIED Protons and neutrons SUPERFORCE combine to form the UNIVERSE first atomic nuclei SHORT-LIVED (of hydrogen, helium, INFLATION EXPANDS and lithium) THE UNIVERSE MORE RAPIDLY Superforce Electrons combine separates into with nuclei to form gravity and Grand Unified Theory the first atoms (GUT) forces GUT forces Free separate into strong electrons nuclear force and electroweak force Electroweak force separates into electromagnetic force and weak interaction

BIG IDEAS THE EARLIEST ASTRONOMERS ORIGIN STORIES At points in history that vary according to the culture, but typically from about Nearly all human cultures and religious traditions have nurtured origin 4000 BCE in Europe and the Middle East, stories—symbolic accounts that describe how the world came about. it seems that humans began to tire of merely These stories or narratives were most often passed from one generation gazing at, and devising stories about, objects to the next in the form of folk tales or ballads, and sometimes through such as the stars, sun, and moon. Instead writing or pictures. some individuals began making detailed recordings of celestial phenomena. These investigations were carried out for a variety of mostly practical reasons. An ability to identify a few stars, and to understand sky movements, proved useful for navigation. It was also realized that the sky is a sort of O rigin stories are extremely varied in Ranginui (Rangi) and Papatuanuku (Papa), ASTRONOMERS IN CHINA detail, but they tend to include some the Sky Father and Earth Mother. Rangi common themes. Often they tell how the and Papa remain physically cleaved together RECORDED OBSERVATIONS OF universe acquired order from an original until pushed apart by their six offspring to state of either darkness or deep chaos. create the separate realms of Earth and sky. MORE THAN 1,600 SOLAR In several versions, including the Old Many stories also account for the creation of Testament’s Book of Genesis, this order is celestial bodies such as the sun and moon. ECLIPSES FROM 750 BCE imposed by a supreme being or deity. In For example, in a story from China, the first some stories, creation is a cyclical process. living being, Pangu, hatches from a cosmic ONWARD For example, in Hindu thought, order is egg. Half the shell lies under him as the generated only to be destroyed and then Earth; the rest arcs above him as the sky. clock that could be used, for example, to regenerated. Many stories begin with Each day for thousands of years he grows, tell farmers when to sow crops or to give Earth. In some, people and gods emerge gradually pushing Earth and sky apart until warning of important natural events. In from the Earth. In others, an animal they reach their correct places. But then ancient Egypt, for example, the rising of the dives into a boundless primeval ocean Pangu disintegrates. His arms and legs bright star Sirius around the same time as and retrieves a portion of Earth from become mountains, his breath the wind, the sun heralded the annual flooding of the which the cosmos is created. his eyes turn into the sun and moon. Often Nile. A final reason for studying the heavens was to predict solar eclipses. Chinese ORIGINS OF THE SKY, SUN, MORE THAN 100 DISTINCT astronomers are thought to have attempted AND MOON ORIGIN STORIES HAVE BEEN this as long ago as 2500 BCE, but it was not Many origin stories describe how the sky IDENTIFIED FROM VARIOUS until the 1st century BCE that the ancient was created along with Earth, often by Greeks reached the level of astronomical splitting off from another primeval object. PEOPLES AND CULTURES sophistication needed to do it accurately. In a common form of the Maori creation ACROSS THE WORLD Successful eclipse prediction had little myth, the universe is created from nothing specific practical use but it did confer on the by a supreme being, Io. He also creates celestial objects originate as physical predictor very significant mystical powers representations of gods. For example, an and, as a result, considerable peer respect. WE HAVE INHERITED FROM OUR origin story from ancient Egypt begins with FOREFATHERS THE KEEN LONGING Nun, the primeval ocean, from which the In some early cultures, accurate FOR UNIFIED, ALL-EMBRACING god Amen rises. He takes the alternative observation not only had practical uses but KNOWLEDGE. name Re and breeds more gods. While his was also intertwined with religion. Some tears become mankind, Amen-Re retires to of the most sophisticated observations before Erwin Schrödinger, Austrian theoretical physicist, 1887–1961 the heavens, to reign eternally as the sun. the invention of the telescope were made by the Maya, who colonized parts of Central Origin stories such as these developed America between 250 and 900 CE. They because early humans needed to find an made accurate calculations of the length of explanation for their own existence and for the solar year, compiled precise tables of the everything that they saw around them. The positions of Venus and the moon, and were cultures that fostered these stories regarded able to predict eclipses. They used their them as true, and for their adherents they calendar to time the sowing and harvesting usually carried great importance and of crops. But they also saw a link between emotional power. But such perceptions the cycles they observed and the place of were based on faith and not on accurate their gods in the natural order. Specific observations or scientific reasoning. events in the night sky were seen as representing particular deities. The Maya also practiced a form of astrology, drawing a connection between cycles in the sky and the everyday life and concerns of the individual. 18 THRESHOLD 1

A MODERN NARRATIVE ▶ Brahma the creator According to some older forms of Hinduism, the god Big History is a modern-day origin story. Brahma, who is usually depicted with Part of this story is an account of how the four heads, was born from a golden egg universe formed provided by the Big Bang and created Earth and everything in it. theory of cosmology. The theory describes the formation of a universe with a beginning and a structure. Modern cosmology as a whole also contains an account of a universe that changes over time, as matter and energy take on different forms, new particles come into existence, space itself expands, and structures such as stars and galaxies emerge. The Big Bang theory, as part of the Big History narrative, shares some other features with traditional origin stories. For example, in common with several of the stories, it proposes that everything—all matter, energy, space, and time—originated from nothing. Big Bang theory and the traditional stories also set out to answer many of the same questions—including how did the universe begin? The theory does not give a complete account of how the universe came to be the way it is now. For example, it does not explain the origin of life or the evolution of humans. But it does form part of the larger framework of Big History that attempts to answer these and other questions. However, in one crucial respect, Big Bang theory, like Big History in general, differs from traditional origin stories in that it seeks to provide a literal and accurate account of the universe’s origins. It represents the current state of scientific thinking, arrived at after many centuries of both gradual change and sudden leaps forward. Like other scientific theories in Big History, the theory also makes predictions that can be tested against evidence, allowing it to be refined or even disproved and overturned. Some questions remain unanswered by Big Bang theory. But, at least for now, it offers the most convincing account of when and how the universe began. THERE WAS NEITHER NONEXISTENCE NOR EXISTENCE THEN; THERE WAS NEITHER THE REALM OF SPACE NOR THE SKY WHICH IS BEYOND. The Rig Veda, a collection of Sanskrit hymns, 2nd millennium BCE ORIGIN STORIES 19

HARD EVIDENCE THE NEBRA Small disks may SKY DISK denote stars, but During the European Bronze Age, people developed their knowledge of most appear to astronomy and put it to practical uses. The Nebra Sky Disk is a key piece be decorative, as of evidence for observation of the sky at this time. Analysis of the disk’s they do not match materials also reveals information about metalworking and trade. known star patterns The Bronze Age in Europe began around used to indicate times for sowing and Large gold 3200 BCE. Dug up near Nebra in central harvesting crops and to coordinate the disk probably Germany in 1999, the 3,600-year-old Nebra solar and lunar calendars. Alternatively, Sky Disk depicts the sun, moon, and 32 the objects on the disk may illustrate a represents stars, including possibly the Pleiades star significant astronomical event—a solar the sun cluster. It is the oldest known portrayal of eclipse on April 16, 1699 BCE. On that such a variety of sky objects. The disk also date, the sun, as it was eclipsed by the moon, reveals that its owners had measured the was close in the sky both to the Pleiades angle between the rising and setting points and to a tight grouping of three planets— of the sun at the summer and winter Mercury, Venus, and Mars. solstices—the days of greatest and least daylight each year. Whatever its exact use, the Nebra Sky Disk provides clear evidence that some There are two schools of thought as to Bronze Age people had made detailed sky what the disk was used for or represents. observations and also developed tools to Some archaeologists think that it was an help them mark the passage of time and astronomical clock, which could have been the seasons. Pleiades Arcs added, one of them covering Sun or full moon two stars Waxing moon Sunboat added or partially eclipsed sun PHASE 1 PHASE 2 PHASE 3 ▲ Phases in construction If the disk was held horizontally, its Holes were punched The disk was made in three phases, edge would represent the horizon into the rim after significantly separated in time, suggesting it underwent some repurposing. The addition other additions for an of the sunboat indicates that it may have taken unknown purpose on religious significance. Sunset point at 82° 82° Sunrise point Metal sources summer solstice at summer solstice The disk’s copper came ▶ The golden arcs from the Austrian Alps. Its The two arcs on the disk span Sunrise point at tin—used with copper to make 82°, the angle between the winter solstice bronze—and its original gold were points on the horizon where from Cornwall, England. The gold in the sun sets (or rises) at the the arcs and sunboat came from the summer and winter solstices Carpathian Mountains in eastern for the location where the disk Europe. Evidently there were was found. well-established trade routes across Europe at the time. Sunset point at winter solstice Gold nugget 20 THRESHOLD 1

The Pleiades A group of stars on the disk may represent the Pleiades star cluster, of which the brightest stars could have been seen with the naked eye by Bronze Age skygazers. In central Europe, the Pleiades would have been a prominent evening feature in the southeastern sky around harvest time. Stars and dust in the Pleiades Gold arc, with hundreds Golden arcs span the angle of tiny protrusions, may between the setting (or represent a sunboat and oars rising) points of the sun at summer and winter solstices THE NEBRA SKY DISK 21 The Nebra hoard The disk was buried with other objects, including two swords made of bronze with copper and gold inlays, a chisel, two axeheads, and two armbands, collectively called the Nebra hoard. It is not known why the disk was placed with these objects. The hoard was buried in around 1600 bce, but the disk could be older. When first examined by archaeologists, it was suspected to be an elaborate fake, but corrosion tests, excavation of the burial site, and examination of the other artifacts pointed to its authenticity. Bronze Age sword from the Nebra hoard Gold crescent may signify either a crescent moon or the sun during a solar eclipse Blue-green patina, caused by oxidation of disk’s copper content, was probably an intentional decorative feature The sunboat The arc of gold at the bottom of the Nebra sky disk is thought to be a sunboat—the means by which some ancient people imagined the sun was conveyed from its setting point in the west to its rising point in the east during the night. The hairlike protrusions around the edge of the arc might represent oars. If the arc is indeed a sunboat, it would be the earliest known representation of one.

13.8 BYA THE UNIVERSE FORMS 10-36 SECONDS INFLATION 10-32 SECONDS INFLATION 10-12 SECONDS SEPARATION OF FUNDAMENTAL IN THE BIG BANG AFTER BIG BANG BEGINS AFTER BIG BANG ENDS AFTER BIG BANG FORCES IS COMPLETED ASTRONOMY BEGINS For most of human history, people were too busy surviving to spend much time thinking about the world’s underlying nature and origins. But from around 1000 BCE, a few began to try answering key questions about the universe without recourse to supernatural explanations. These thinkers—initially concentrated in number of indivisible particles called atoms. Mediterranean lands, especially Greece— Finally, in the 4th century BCE the influential realized that to understand the world it scholar Aristotle added a fifth element, is necessary to know its nature, and that ether, to Empedocles’ four. Although natural phenomena should have logical Aristotle was skeptical of the idea of atoms, explanations. Although they did not always it is remarkable that the concepts of both find the correct answers, this leap marked atoms and elements had been proposed the start of a 3,000-year journey that has more than 2,000 years before either was led in the modern world to such key theories proved to exist. key as the Big Bang model of the universe. EARTH’S SHAPE AND SIZE THE NATURE OF MATTER Among the many other ideas that Aristotle The fundamental questions of what the gave his views on was the concept that Earth world is made of, and where matter came is a sphere. Earlier Greek scholars, such as from, are some of the oldest. In the 6th Pythagoras, had already argued this, but century BCE, Greek philosophers such as Aristotle was the first to summarize the Thales and Anaximenes proposed that all substances were modifications of more THE IDEA THAT EARTH IS FLAT intrinsic substances, the main candidates WAS STILL THE PREVAILING being water, air, earth, and fire. In the VIEW IN CHINA UP TO THE 5th century BCE, Empedocles claimed that everything was a mixture of all four of EARLY 17TH CENTURY these substances, or elements. His near- contemporary Democritus developed the idea that the universe is made of an infinite Sun’s rays Tower’s main points of evidence. Chief among them ▲ Earth-centered universe shadow was that travelers to southern lands could see This 17th-century illustration by Andreas Tower in stars that could not be seen by those living Cellarius depicts Aristotle and Ptolemy’s model. Alexandria 500 miles (800km) farther north—explainable only if Earth’s Working out from the center, the Moon, Mercury, from Alexandria surface is curved. In 240 BCE, by comparing Venus, the Sun, Mars, Jupiter, Saturn, and the Height of tower to Syene how the sun’s rays reach Earth at Syene and stars move in circular orbits around Earth. Alexandria, the mathematician Eratosthenes Length of was able to estimate Earth’s circumference. east to west, whereas Earth itself did not shadow He came up with a figure of about 25,000 seem to move. An alternative view, put miles (40,000km)—close to the true value forward by the astronomer Aristarchus, was Angle at Earth’s 7° Well at Syene known today. that the sun is at the center and that Earth center is equal 7° orbits it, but this idea did not gain much ◀ Estimating Earth’s circumference EARTH AND THE SUN credence. In 150 CE, Claudius Ptolemy— to angle at which When the sun was overhead above Aristotle thought that Earth was at the center an eminent Greek scholar living in shadow is cast in a well at Syene, it cast a shadow at of the universe and that the sun, planets, and Alexandria—published a book called an angle of about 7° in Alexandria. stars move around it. This seemed like Alexandria Dividing this angle into 360°, then common sense given that every night various multiplying the result by the distance celestial objects (and during the day, the sun) Center of between the two places gave an could be seen moving across the sky from Earth estimate of Earth’s circumference of about 25,000 miles (40,000km). 22 THRESHOLD 1

10-6 SECONDS THE FIRST PROTONS AND 3 MINUTES THE FIRST ATOMIC 380,000 YEARS THE UNIVERSE BECOMES 13.6 BYA THE FIRST AFTER BIG BANG NEUTRONS FORM AFTER BIG BANG NUCLEI FORM AFTER BIG BANG TRANSPARENT STARS FORM A STATIONARY OR A ▲ Ulugh Beg Working at his SPINNING EARTH? observatory at Samarkand, Ulugh Beg Linked to the issue of what is and other astronomers at the center of the universe, determined matters the question of whether or not such as the tilt of Earth rotates was debated for Earth’s spin axis and around 2,000 years up to the an accurate value for 17th century CE. The prevailing the length of the year. view was that Earth does not spin, as this fit best with the idea of an Earth-centered universe. However, there were opponents to this view, including Greek philosopher and astronomer Heraclides Ponticus in the 4th century BCE, as well as an Indian astronomer, Aryabhata, and Persian astronomers (Al-Sijzi and Al-Biruni) between the 5th and 15th centuries CE. Each proposed that Earth rotates and that the stars’ apparent movement is just a relative motion caused by Earth’s spin. But it was not until the Copernican Revolution (see pp.24–25) that Earth’s rotation became accepted as fact, and it was not until the 19th century that it was categorically proved. the Almagest, which affirmed the Uzbekistan, during the 15th century) made THE SIZE AND AGE OF THE UNIVERSE prevailing view that Earth is at the center. major contributions to knowledge of the solar Ptolemy’s detailed model fit with all known system and in particular to cataloging A final popular subject for speculation observations but in order to do so contained star positions. among early philosophers was the complex modifications to Aristotle’s original question of whether the universe is finite ideas. For about the next 14 centuries, the (limited) or infinite, both in extent and in Earth-centered view of Aristotle and time. Aristotle proposed that the universe Ptolemy totally dominated astronomical is infinite in time (so it has always existed) theory, and it was adopted throughout but finite in extent—he believed that all the Europe by medieval Christianity. During stars were at a fixed distance, embedded this time, Islamic astronomers such as in a crystal sphere, beyond which was Ulugh Beg (who worked from a great nothing. The mathematician Archimedes observatory in Samarkand, in what is now made a reasoned estimate of the distance to the fixed stars and realized it was vast— at least what we would now call 2 light years—but stopped short of claiming it to be infinite. In the 6th century CE, Egyptian philosopher John Philoponus opposed the prevailing Aristotelian view by arguing that the universe is finite in time. It was not until the 20th century that scientists began to find answers to these questions. IN POSITION EARTH LIES IN THE MIDDLE OF THE HEAVENS VERY MUCH LIKE ITS CENTER. Claudius Ptolemy, astronomer and geographer, 90–168 CE ASTRONOMY BEGINS 23

T o the people of medieval Europe up to an invisible, distant sphere that rotated including the sun and stars, rose up in the the mid-16th century, the question of rapidly, approximately daily, around Earth. east, moved across the sky, and then set how the universe is organized had been The sun, moon, and planets also revolved below the western horizon. answered centuries before by Ptolemy, in around Earth, attached to other invisible his modifications to ideas first asserted by spheres. For most people, this explanation DOUBTS ABOUT GEOCENTRISM Aristotle (see pp.22–23). According to seemed reasonable—after all, looking up at The geocentric model of the universe did Ptolemy, Earth stood still at the center of the the sky at night, it did seem that Earth was not satisfy everyone, however. A serious universe. Stars were “fixed” or embedded in quite still, while all other objects in the sky, doubt focused on what it predicted about the planets. According to the original BIG IDEAS EARTH ORBITS Aristotelian version of geocentrism, the THE SUN planets rotated around Earth in perfect ▼ The solar system circles, each at its own steady speed. But in miniature In the 16th and early 17th centuries, the prevailing view of an Earth- if this was true, the planets should move This model of the solar centered, or geocentric, universe, as first put forward by the Greek across the sky with unvarying speed and system, called an scholars Aristotle and Ptolemy, was challenged by a simpler sun- brightness because they were always the armillary sphere, is a centered, heliocentric, model. This single idea eventually led to the same distance from Earth—and this wasn’t Copernican version, scientific revolution, a whole new way of thinking about the universe. what was observed. Some planets, such as showing the sun at the Mars, varied hugely in brightness over time, center and the planets and when their movements were compared revolving around it. with those of the outer sphere of fixed stars, the planets sometimes reversed direction— a behavior called retrograde motion. To deal with these problems, Ptolemy had modified the Aristotelian model. For example, he had planets attached not to COPERNICUS WAS A DOCTOR, CLERIC, DIPLOMAT, AND ECONOMIST AS WELL AS AN ASTRONOMER the spheres themselves, but to circles called epicycles attached to the spheres. To some astronomers, these modifications looked like “fixes” to fit the model to observational data. From time to time, they suggested alternative ideas, such as that Earth orbits the sun. But supporters of geocentrism had what seemed like an excellent reason for ruling this out. They argued that if Earth moves, the stars should be seen shifting a little relative to each other over the course of a year—but no such shifts could be detected and so, they answered, Earth cannot move. THE COPERNICAN MODEL In the face of these arguments—and the power of the Catholic Church, which supported the established view—for centuries there was little opposition to the idea of a geocentric universe. However, around 1545, rumors began circulating in Europe that a new and convincing challenge—in the form of a sun-centered theory of the universe—had appeared in a book,

CHURCH REACTION I THINK THAT IN THE DISCUSSION OF NATURAL In 1616, the Roman Catholic Church banned De Revolutionibus—a ban that was PROBLEMS WE OUGHT TO BEGIN NOT WITH THE enforced for more than 200 years. This probably came about as a result of a SCRIPTURES, BUT WITH EXPERIMENTS AND dispute the Church was having with the astronomer Galileo Galilei, a champion DEMONSTRATIONS. of the Copernican theory who had made discoveries that supported heliocentrism. In particular, in about 1610, Galileo had Galileo Galilei, astronomer, 1564–1642 discovered moons circling Jupiter, proving that some celestial objects do not orbit De Revolutionibus Orbium Coelestium (“On moving freely in orbits. Brahe had observed Earth. The dispute with Galileo caused the Revolutions of the Celestial Spheres”), comets apparently passing through the De Revolutionibus to undergo intense scrutiny by a Polish scholar, Nicolaus Copernicus. spheres, which convinced him that they by the Church, and because some of its ideas Copernicus based his theory on several did not actually exist. He also observed a assumptions. The first was that Earth supernova, contradicting a long-held idea spins on its axis, and it is this rotation that no change takes place in the heavens. that accounts for most of the daily motion Another shortcoming in Copernicus’s GALILEO NAMED JUPITER’S of the stars, planets, moon, and sun across theory was his belief that all celestial objects MOONS THE MEDICEAN STARS the sky. Copernicus considered that it was must move in circles, which forced him to AFTER THE MEDICI FAMILY inconceivable that thousands of stars were retain some of Ptolemy’s “fixes.” But in the rotating rapidly around Earth. Instead, he 1620s, the work of the German astronomer proposed that their apparent motion was Johannes Kepler showed that orbits were an illusion caused by Earth’s spin. He elliptical, not circular. By removing most discounted an objection that this would of the remaining “fixes,” this simplified and seemed to go against biblical statements, it create catastrophic winds, pointing out improved the heliocentric model. In the late was banned. In 1633, Galileo himself was that Earth’s atmosphere was part of the 17th century, Isaac Newton expanded on eventually put on trial and forced to recant planet and so part of the motion. Kepler’s work, and with his laws of motion his views. Copernicus’s core assumption was that and newly introduced force of gravity (see the sun, not Earth, is at or near the center pp.46–47), Newton was able to explain THE SCIENTIFIC REVOLUTION of the universe, and that the planets— exactly why celestial objects move in the Banned by the Catholic Church and viewed including Earth, just another planet—circle way they do. His work Principia effectively ambivalently at first by astronomers, the the sun at differing speeds. This system removed the last doubts about heliocentrism. Copernican theory took time to catch on. could explain, in a simpler way, the motions These improvements in the Copernican More than 150 years passed before some of and variation in brightness of the planets theory took place against the backdrop of its basic assumptions were shown to be true without recourse to any of Ptolemy’s “fixes.” A third important assumption was that the stars are much farther from Earth and the AT REST, HOWEVER, IN sun than had previously been accepted. This explained why the relative positions of the stars as seen from Earth appeared THE MIDDLE OF EVERYTHING to remain fixed over the course of a year. THE THEORY DEVELOPS IS THE SUN Nicolaus Copernicus, astronomer and mathematician, 1473–1543 De Revolutionibus was published when other important advances in cosmology. In Copernicus was dying, and it was a century the early 17th century, the development of beyond dispute. But what was important or more before his theory became widely telescopes helped establish that stars are far about the theory was that it established accepted. One problem was that his model more distant than planets and exist in vast cosmology as a science and represented a contained misconceptions that had to be numbers. It was even suggested that the serious blow to some old, traditional ideas corrected by later astronomers. Copernicus universe could be infinite. Kepler, however, about how the universe works, many dating clung to the idea that all movements of pointed out that it cannot be infinite, static, from Aristotle. As such, it is often viewed as celestial bodies occur with the objects and eternal, otherwise the night sky would ushering in the scientific revolution—a series embedded in invisible spheres. In 1576, look uniformly bright due to there being a of advances between the 16th and 18th the English astronomer Thomas Digges star emitting light from every direction. centuries that transformed views of nature suggested modifying the Copernican system and society in the early modern world. by removing the outermost sphere, in which stars are embedded, and replacing it with a star-filled unbound space. In the 1580s, the Danish astronomer Tycho Brahe banished the rest of the spheres in favor of planets EARTH ORBITS THE SUN 25

W Fraunhofer’s (12mi)lllioanmg rHefelrescctihnelgctoelemsplceotpees,a14780f9.t spectroscope chaesatmrstiocFhrnnaelaoosbtuiyummSncJane’pheroksessosefsctedutepoparrperlhodicokstvefn(rctoesaeuotsnrb,apmstFr.merosihi.raLnenpaueyttvniteehhorhenlonetk)fpelneirdo,,new1wsn8hi1on4, VnsiaodtMraweironekcustnlissufoiisenewtrede,nb1rbs7utyoa6loCn0budhses–agmro1al7obela8ssjxet0ilceysts.s,, Ritmeef1SJlplae7eemtrs3vcrocea0tevForinssnaeip–srSlgdtise1ahtotHst7bsroepoe6yoarlrbedu0tfa,sslsVscemcee.tyeoridi,cnvrp1raaeueo7lbsr2bre.e1aurfv.rillaAoetticbidpotysainnrJis,omagpibhmhanogepleriricocqvauilnalgity. 1800 Newton’s reflector 1700 FFriirsestdtarmri’csbdehpeaimsBacstreoueaatmsrnlheslcaeoemexsld,s,et1fmhtno8oetar3nodsm8etfe.aaeaTbnrabyhdsuyiasrrsditnagrs. Firpshatosuattsrloleotogonowrpboafeiepmnrgmhsyey,faea1innin8ne3atne9ntr,droeebcfojfeirccditeisnngt., dreIesbscnsapFhudiatghirteecrnoeoarlsNmersnaftisvcroiweatceoattwoifohliimdrcpatreaoskpbaercnbarieptpna,oredrrcg1rorrorat6rsrabidtace6cwiltuoeaf8aibcolnmll.e.lean,eTc,dc,dthkibenyg s, 1i8th45a.5ft (1.5m) wide mirror, New record for the largest TIMELINES telescope set in 1686 by Christiaan Huygens, who builds a 310ft (95m) long aerial, tubeless refractor. Telescopic sight Refractor 111⁄2ft (3.5m) long SEEING THE and micrometer built by Johannes Hevelius, LIGHT invented by William 1647. Hevelius subsequently Gascoigne, 1638, builds even longer telescopes facilitating more and uses them to make the accurate plotting first accurate map of the moon. and measurement of celestial objects. Gatehltilieleselseoocsbobcsipuoneeiptrl,odve1sacCl6taaoi0aton2e9tnf0hr.lsSiot-cphmoltiorcmawowdCweieethohrhbufityrmhceh. PtaeLtleiepnsptceofriplsehed,e1yf6o. 0r 8a,rbeyfrHacatnisng The telescope, and a lesser-known instrument, bMuailstsbivyeWreilfllieacmtoPra, rwson the spectroscope, are the main tools with which VISIBLE LIGHT VISIBLE LIGHT astronomers have expanded knowledge of the TELESCOPE TELESCOPE universe and its beginnings. TECHNOLOGY ASTRONOMY The first telescopes were designed to gather visible light only, and within 100 years had split into two main types—refractors and 1600 reflectors. In the 19th century, the spectroscope—which can be used to study the composition and motions of celestial objects—was invented. During the 20th century, bigger optical telescopes came along, followed by radio telescopes. Innovations since the 1970s have included launching telescopes into space and, in the case of radio telescopes, arranging them on the ground in arrays. 26 THRESHOLD 1

JamTeelsesWcoeplbaebuinSscpdhau,ce2e0to18. FoTiprhhmseautearsaasKadtdaetaetamelesitl3opceeaoo3ntksrifstficvineTtp3noecg(eh6p1Hholete0esnphra,esmiotewaccgilc)nmomoatswudipgiietrr,yeiionbrd1tocsbtu9easraetld9m’elwsagl3jneunisii.nrdcnhsdrset,aofpbroeyr. HiaHtthcsiipcpefpiuprparsaoartrstccoeiootbimssoseSmneraasavtsokaeuftelrilsseoittmhnaeisregms,nh1.atl9yks8eo9sf. 2000 maTbphtaphCicesekodfCgsirfmrorsootsiEumcmmxnmiapscdpplioBcraiarrasceodcemrwki,a(ag1Catdr9ivOooe9eunBb3n.yEd). SPACE TELESCOPES TashteoHniushbibnlgeiSmpaagceesToefloebscjeocptes ipnlaocuerdgianlaoxrybiatn, 1d9b9e0y.oItnhda, sanpdeeimrepdrdoeveed meapsiunrteo smepantcseoafntdhtie umen,ivperrosvei’sdiangge.JersAeTCryeuno.lsoeIstripmnPahhgdetieoceAnialapnlzmenrtseiasitaiotocdLcs1ononriFaa9oocpybidn6rwotBeoHedse7narattal,eRaefnevtpbiwtcsogretoyucmthbitrebsloJheeisodhaetpraehUc,csteCuor1ee,knWsradaN9lgBiiyytmniv6r.neisiBeglgo4bcwsreBu,ooarslbeinlnivdrtlydaelygdraeoeni.ofdd, apbshstsHoaottrFeriaionprrs’rtgsisbrytVoyasmepnDpepHailgrlhteheideaacE,coneslnptthmeanehroeetostrniwou.i,nmgfadticrewm1vseetasnet8sdlthbipa7tseedubhksir2any.ernnogfktnetGTlhdiuahhbsisneeytneosafsauvliifnln’gKiiashtrtasmcteesh.pommfieoft,csttep1rd8hue6mr1.e cie1Yaoevn8srve9tmrko5hlp.eluvesseTesteRhdepdieiedrfflniraoalanrdricngrstaeecWtossiortusevirrcaseereorfcionrefhsa,isticnstht, ieuosrch TwhoerkVteorgyeLthaergr etoAfrorramy (imVLage s, bAe)g,iangsroopuepraotfio2n7sraindNioetweleMsceoxpiceos,t1h9a8t 0. Milky Way. teilnFeisWrcsohtpepteaha,terobanrnubam,iodlItalliEliobkicndyeetoTdohswmGwiiesasariri,lhnsnetone1sctarti9saeoaHhlec3lndnlR-es7eosudsiesr.koapbfbeiRytdoreeboeomainrbmnlirraaeneegeplrsutlpgvtaohsoaaefteecleltaoes1hut.xc9theniihe2teidiesv0ediesoHssircsuotsoototefaovsngsiieksdhacreeeleoyarxswotxphauieaanrsntd—dthineg. ory,pAhircizpolantae,sb, ytaCkleyndaebTooumt baawuegehk, who BEYOND VISIBLE apart. LIGHT Spectra of nebulae, stars, and 1900 galaxies are studied by William and Margaret Huggins, 1860s. Pmlautkeosdihisscdoivsecroevde,r1y9af3t0,eratc tohmepLaroinwgellphOobtsoegrrvaat They measure the redshifts of stars, showing how fast they are moving.

13.8 BYA THE UNIVERSE FORMS 10-36 SECONDS INFLATION 10-32 SECONDS INFLATION 10-12 SECONDS SEPARATION OF FUNDAMENTAL IN THE BIG BANG AFTER BIG BANG BEGINS AFTER BIG BANG ENDS AFTER BIG BANG FORCES IS COMPLETED THE ATOM AND THE UNIVERSE From the early 19th century to the late 1920s, a series of breakthroughs ◀ Henrietta Leavitt occurred in the physical sciences. They transformed our understanding Over 20 years, Leavitt of the workings and structure of the world at both infinitesimally small studied 1,777 variable scales and at the very largest, raising the possibility of an infinite cosmos. stars at the Harvard College Observatory These discoveries paved the way for the time, the German theoretical physicist before stumbling upon advances of the 1930s to the 1950s, from Albert Einstein showed that matter and her key discovery. the realization that the universe is energy have an equivalence. Simultaneously, expanding to the development of ideas a new field of physics, quantum theory, was on how energy and matter interact at proposing (among other things) that light the subatomic level. Through the coming can behave either as a wave or as a stream together of ideas in cosmology and particle of particles. By the late 1920s, it was known physics, these breakthroughs eventually led to the development of the Big Bang theory. PROBING MATTER AND ENERGY WHAT WE OBSERVE AS MATERIAL BODIES AND FORCES ARE NOTHING BUT SHAPES The idea that matter consists of atoms AND VARIATIONS IN THE STRUCTURE was first suggested by the ancient Greek, OF SPACE. Democritus (see p.22). In the early 1800s, an Englishman, John Dalton, revived the Erwin Schrödinger, Austrian theoretical physicist, 1887–1961 idea. Dalton regarded atoms as indivisible, but around the turn of the 20th century, experiments by scientists such as the New Zealander Ernest Rutherford proved that they have a substructure. Around the same ▶ Understanding Each atom is Electrons Positively Positively Electrons orbiting the atom a single, scattered like charged charged in a ring (or rings) From around raisins in a plum ground nucleus 1800 until the mid- indivisible entity material 1920s, a step-by-step pudding evolution occurred in the understanding of atomic structure. Later, from the late 1920s, physicists found that atomic nuclei have a substructure. An atom is Thomson’s plum pudding (1904) The Nagaoka’s Saturnian model (1904) like a tiny, discoverer of the electron, British physicist Japanese physicist Hantaro Nagaoka solid ball J.J. Thomson, suggests a “plum-pudding” proposes an atom has a central nucleus, model, with negatively charged electrons around which the electrons orbit in one Dalton’s atom (1803) English chemist embedded in a positively charged sphere. or more rings, like the rings of Saturn. John Dalton pictures atoms as extremely small spheres, like tiny billiard balls, that 28 THRESHOLD 1 have no internal structure and cannot be subdivided, created, or destroyed.

10-6 SECONDS THE FIRST PROTONS AND 3 MINUTES THE FIRST ATOMIC 380,000 YEARS THE UNIVERSE BECOMES 13.6 BYA THE FIRST AFTER BIG BANG NEUTRONS FORM AFTER BIG BANG NUCLEI FORM AFTER BIG BANG TRANSPARENT STARS FORM that atomic nuclei consist of protons American named Henrietta Leavitt. Her Galaxy appears slightly and neutrons and are held together breakthrough concerned a class of star redder to observer by a newly detected force, the strong called Cepheid variables, which cyclically force. Also discovered at this time was vary in brightness. Leavitt found a link Galaxy moving Redshifted ◀ Redshift antimatter—subatomic particles that are between the cycle period and brightness away from spectrum line When an object such as identical to their matter equivalents except of these stars, meaning that if both could observer a galaxy is receding at for opposite electrical change—and that the be measured, a good estimate could be high speed, light waves coming together of matter and antimatter made of their distance from Earth. Within Wavelength from it appear stretched. can annihilate both, producing pure energy. a few years, it became apparent that some is stretched This causes features in stars are tens of thousands of light-years the galaxy’s spectrum, THE DISTANCES TO STARS away, while some vaguely spiral-shaped Line in original such as prominent lines, During roughly the same period, great nebulous patches in the sky, known at spectrum to shift toward the red advances were made in understanding the time as “spiral nebulae,” seemed (long wavelength) end. the true scale of the cosmos. In 1838, the to be millions of light-years away. REDSHIFT This is a redshift. German astronomer Friedrich Bessell made the first reliable measurement of the distance SHIFTING NEBULAE Galaxy appears slightly to a star other than the sun, using a method Between 1912 and 1917, the American bluer to observer called stellar parallax. The star, although astronomer Vesto Slipher studied several one of the closest to the sun, seemed at the “spiral nebulae” and realized that many Galaxy were moving away from Earth at high approaching A LIGHT-YEAR—THE DISTANCE speed, while a few were approaching observer LIGHT TRAVELS THROUGH Earth. He found this out by measuring SPACE IN A YEAR—IS ABOUT a property of the light from the nebulae Blueshifted Wavelength ◀ Blueshift called redshift or blueshift. It seemed spectrum line is squashed Light waves from a 6 TRILLION MILES odd that the nebulae were moving at rapidly approaching such speed relative to the rest of the object appear squashed, (9.5 TRILLION KILOMETERS) galaxy. Partly prompted by Slipher’s shifting features in the findings, in 1920 a formal debate was spectrum toward the time almost unimaginably far off—what held in Washington, DC on whether these blue (short wavelength) would now be called 10.3 light-years away. nebulae might be separate galaxies outside end. This is a blueshift. It was 1912 before a system was discovered our own. The debate was inconclusive. for estimating the distance to many more But within a few years, the answer had been BLUESHIFT remote stars. The discoverer was an found—by another American astronomer named Edwin Hubble (see pp.30–31). Cloud density varies with probability that it Empty Electrons gain space or lose energy contains an electron on moving between orbits Tiny, very dense, Electrons moving Nucleus as in Electron orbit Cloud region Nucleus as positively charged around randomly Rutherford containing in Rutherford model electrons model nucleus Rutherford and the nucleus (1911) Bohr’s electron orbits (1913) Danish Schrödinger’s electron cloud model Rutherford proves experimentally that physicist Niels Bohr proposes that (1926) According to Austrian physicist an atom’s nucleus is much smaller and electrons can move in spherical orbits, Erwin Schrödinger’s model, the locations of denser than previously thought—and at fixed distances from the nucleus, electrons in an atom are never certain and that much of an atom is empty space. and can “jump” between orbits. can be stated only in terms of probabilities. THE ATOM AND THE UNIVERSE 29

13.8 BYA THE UNIVERSE FORMS 10-36 SECONDS INFLATION 10-32 SECONDS INFLATION 10-12 SECONDS SEPARATION OF FUNDAMENTAL IN THE BIG BANG AFTER BIG BANG BEGINS AFTER BIG BANG ENDS AFTER BIG BANG FORCES IS COMPLETED THE UNIVERSE GETS BIGGER During the 1920s, two key breakthroughs led to a revolution in understanding the size and nature of the universe. Both were the result of discoveries made by the astronomer Edwin Hubble. ▼ Photographic In 1919, Hubble arrived at Mount Wilson to observe a class of stars called Cepheid evidence Observatory in California at age 30. His variables in some of the nebulae, including These two (negative) arrival coincided with the completion of what today is called the Andromeda Galaxy. photographic plates what was then the largest telescope in the Cepheid variables are stars whose distances were used by Hubble to world, a reflector with a 100in (2.5m) can be estimated by measuring their average identify a specific wide mirror, called the Hooker Telescope. brightness and the lengths of their cycles of Cepheid variable star in brightness variation. As a result of his the Andromeda Galaxy. ENDING THE GALAXY DEBATE observations, in 1924 Hubble was able to Studies on this star were At that time, the prevailing view was that announce that the Andromeda nebula and crucial in confirming the universe consisted of just the Milky Way other spiral nebulae were far too distant to that the Andromeda Galaxy, although in 1920 a famous debate be part of the Milky Way and so must be Galaxy is outside (see p.29) had considered whether or not galaxies outside our own. Almost overnight, the Milky Way. some vaguely spiral-shaped nebulae—fuzzy, the universe had become a much bigger star-containing objects—in the night sky place than anyone had previously imagined. might be collections of stars outside our own galaxy. Hubble, who had been studying RECEDING GALAXIES these nebulae, already strongly suspected Hubble next studied a phenomenon that that they were outside our galaxy. In had already been noted by an astronomer 1922–23, he used the Hooker Telescope called Vesto Slipher: many of the spiral galaxies had large “redshifts” in their Hubble’s note VAR! indicates he had found spectra, meaning that they were moving away from Earth at high speed a star that varied in (see p.29). Again by observing Cepheid brightness between variables, Hubble began measuring the distances to these galaxies and the two plates compared the distances to their redshifts. He noticed something date on which remarkable: the more distant a galaxy plate was taken was, the greater was its recessional velocity—a relationship that became known as Hubble’s Law. Hubble published his results in 1929. Although he himself was initially skeptical, to other astronomers it was clear that only one conclusion could be drawn—the whole universe must be expanding! THE HISTORY OF ASTRONOMY IS A HISTORY OF RECEDING HORIZONS. Edwin Hubble, American astronomer, 1889–1953 30 THRESHOLD 1

10-6 SECONDS THE FIRST PROTONS AND 3 MINUTES THE FIRST ATOMIC 380,000 YEARS THE UNIVERSE BECOMES 13.6 BYA THE FIRST AFTER BIG BANG NEUTRONS FORM AFTER BIG BANG NUCLEI FORM AFTER BIG BANG TRANSPARENT STARS FORM The world’s largest telescope Completed in 1917, the Hooker Telescope was the world’s largest telescope for about 30 years. Its glass mirrors, which had to be cast to an accuracy of a few millionths of an inch, had to be kept cool to prevent them from warming up and becoming distorted. THE UNIVERSE GETS BIGGER 31

13.8 BYA THE UNIVERSE FORMS 10-36 SECONDS INFLATION 10-32 SECONDS INFLATION 10-12 SECONDS SEPARATION OF FUNDAMENTAL IN THE BIG BANG AFTER BIG BANG BEGINS AFTER BIG BANG ENDS AFTER BIG BANG FORCES IS COMPLETED THE EXPANDING UNIVERSE Edwin Hubble’s work showed that many galaxies are receding from us a “primeval atom” as he called it—that at a rate proportional to their distance. It was soon deduced that the disintegrated in an explosion, giving rise universe must be expanding, but astronomers still had to understand to space and time and the expansion of the nature of this expansion and what the universe is expanding from. the universe. By 1933, Einstein (who had by now abandoned his cosmological constant) By the beginning of the 1930s, scientists it became clear to many astronomers that was in full agreement with Lemaître’s theory, were also starting to address a question the universe must indeed be expanding, calling it “the most beautiful and satisfactory that philosophers had been pondering for although neither Hubble nor Einstein was explanation of creation to which I have several millennia—has our universe always convinced at first. Despite this, for many ever listened.” existed or did it have a beginning? Physicists, years credit for the discovery was given to mathematicians, and astronomers were now Hubble, but today most experts agree it Simple physics dictates that the universe in a position to try answering this question. should be equally shared with Lemaître. compressed into a tiny point would be extremely hot. During the 1940s, Russian- THE RADIUS OF SPACE BEGAN AT ZERO; THE FIRST STAGES American physicist George Gamow, and OF THE EXPANSION CONSISTED OF A RAPID EXPANSION colleagues, worked out details of what might DETERMINED BY THE MASS OF THE INITIAL ATOM. have happened during the exceedingly hot first few moments of a Lemaître-style Georges Lemaître, astronomer, 1894–1966 universe. This included working out how the nuclei of light elements, such as helium, ▼ Georges Lemaître EINSTEIN’S POSSIBLE UNIVERSES DISCOVERING THE BIG BANG might have been forged, starting with just Arguably the first If the universe is expanding, and the clock protons and neutrons. The work showed person to propose The story of how scientists came to realize is run backward, then the farther back in that a “hot” early universe, evolving into that the universe is that the universe is expanding began in 1915 time you look, the denser the universe what is observed today, is at least expanding, Lemaître was with the publication of Albert Einstein’s becomes. But, as Lemaître reasoned, one theoretically feasible. In a 1949 radio a priest and physicist as general theory of relativity. This theory can only go so far before the universe is interview, the British astronomer Fred Hoyle well as an astronomer. is a description of how gravity works at the crushed into an infinitely dense point. So coined the term “Big Bang” for the model largest scales, and it defines what possible in 1931, he suggested that the universe was of the universe Lemaître and Gamow universes can exist. Part of Einstein’s theory initially a single, extremely dense particle— had been developing. At last, Lemaître’s consists of a set of equations that have to be startling hypothesis had a name, which solved to give a description of the long-term, ▶ Expanding space has stuck ever since. large-scale behavior of the universe. The universe’s expansion is most accurately thought of in terms of space itself expanding Early galaxy clusters Einstein’s initial solution to his equations and carrying objects with it—called were closer together suggested that the universe is contracting, but cosmological expansion—rather than he could not believe this, so he introduced a galaxies and galaxy clusters moving away than they are today “fix”—an expansion-inducing factor called from each other “through” space. the cosmological constant—into his theory to Free gas and allow for a static universe. In 1927, the At the beginning of dust not yet time, all matter was absorbed into Belgian astronomer Georges Lemaître, concentrated in a tiny who had studied Einstein’s equations particle –Lemaître’s galaxies and heard of Hubble’s measurements of galaxy distances, proposed that the “primeval atom”— whole of space is expanding—but his that exploded hypothesis failed to attract widespread attention. After Hubble released his findings about receding galaxies in 1929, 32 THRESHOLD 1

10-6 SECONDS THE FIRST PROTONS AND 3 MINUTES THE FIRST ATOMIC 380,000 YEARS THE UNIVERSE BECOMES 13.6 BYA THE FIRST AFTER BIG BANG NEUTRONS FORM AFTER BIG BANG NUCLEI FORM AFTER BIG BANG TRANSPARENT STARS FORM RECESSIONAL VELOCITY miles (km) per second 12,500 Each disk represents On a local scale, (20,000) a galaxy gravity dominates over expansion, holding galaxy 9,300 A galaxy’s velocity (15,000) is estimated from clusters together measurements of 6,200 its redshift Over time, the space (10,000) between galaxies and The slope of the galaxy clusters empties 3,100 line gives a value out as free gas and dust (5,000) for the Hubble are pulled into galaxies Constant In the 1930s, it A galaxy’s distance is was assumed that the estimated by taking rate of expansion was measurements from some of its variable stars at or near a uniform rate—with just slight 0 30 60 90 120 slowing due to gravity DISTANCE FROM EARTH millions of light-years ▲ The Hubble Constant If the velocities of a number of galaxies are plotted against their distances, a “best fit” line can be drawn close to all the plotted points. The slope of the line is an estimate of the Hubble Constant, itself a measure of the rate of the universe’s expansion. All galaxy clusters are gradually moving away from each other—there is no center to the expansion Some galaxies gradually develop spiral shapes THE EXPANDING UNIVERSE 33

TIMELINES HYDROGEN ATOM THE BIG BANG (2,47,0900°0C°)F dlii1ga0mh0te-myteeirlalriosn HELIUM-4 HELIUM-3 Since the 1930s, when the Big Bang theory was first ATOM ATOM proposed, physicists and cosmologists have been 380,000 YEARS LITHIUM 7 testing and developing the theory and filling in the NUCLEUS 8 FIRST ATOMS DEUTERIUM details of the first moments of the universe. 18,000°F Electrons combine with ATOM (10,000°C) protons to form hydrogen Part of the work to improve the Big Bang theory has been carried atoms and with other Photons can now move out by experiments in which high-energy particles are collided 1000 YEARS nuclei to form deuterium freely without colliding to recreate Big Bang-like conditions (see pp.36–37), and part has (heavy hydrogen), helium, with free electrons been purely theoretical, involving the formulation of equations and lithium atoms. As and models. During the experimental side of this journey, many electrons are now bound new subatomic particles have been discovered. Another focus up in atoms, they no longer of research has been the fundamental forces that govern particle interfere with photons, interactions. It has been known since the 1930s that there are four which are free to travel of these forces: gravity, the electromagnetic force, the strong force, through space as radiation, and the weak interaction. During the Big Bang, it is theorized that and the universe becomes these forces were initially unified. Then, as the universe cooled, transparent. they split off, possibly triggering new phases of the Big Bang. Gradually, physicists have fitted all the known particles and the DEUTERIUM forces into a scheme called the Standard Model of particle physics. NUCLEUS One important change to the original theory was made in the (110800,0,00000°°CF) 1980s by the American physicist Alan Guth. He proposed that at a very early stage a part of the universe underwent an extremely 1 YEAR HELIUM-4 HYDROGEN fast expansion called cosmic inflation. Guth’s idea helped explain NUCLEUS NUCLEUS some aspects of the universe today, including why at the largest scales matter and energy seem to be distributed very smoothly. (FREE PROTON) The reality of cosmic inflation is now widely accepted. Up Down There are six types of A proton is Two down 1.8 mmiilllliioonn°°FC) HELIUM-3 quark quark quark. Up and down made of two up quarks and one (1 NUCLEUS quarks are the most quarks and one up quark, plus stable and common. down quark gluons, make 3 MINUTES plus gluons. Neutron up a neutron. Proton Electron This tiny subatomic ▲ Composite particles particle has a negative These particles are composed of other smaller electrical charge. particles. Scores of different composite particles have been identified, but protons and neutrons Gluon By carrying the strong are the only stable types. nuclear force, gluons hold quarks together. Photon A photon is a tiny For each of the six 1 DAY packet of light or types of quark there is other electromagnetic a corresponding type 1(180mmililliloionn°F°C) radiation. called an antiquark. 7 FIRST NUCLEI This particle is Up Down Collisions between protons and associated with a antiquark antiquark neutrons begin forming the nuclei field that gives mass of helium-4 and small amounts of Higgs boson to other particles. other nuclei, such as helium-3 and lithium-7. All the neutrons are ▲ Fundamental particles Positron A positron is the positively mopped up by these reactions, These particles are not, so far as is charged equivalent of the but many free protons remain. known, made of smaller particles. electron. Some, such as quarks, are building blocks of matter. Others, like gluons Two up This consists (10018mi0llimoillni°oCn)1°Fhour and photons, are force-carrier particles. antiquarks of two down 60 (s11e.cb8iollbiniloldisno°nC)°F and one down antiquarks Anti- antiquark, plus and one up proton gluons, form Anti- antiquark, an antiproton. neutron plus gluons. ▲ Antiparticles These are particles with the same mass but an opposite electric charge to their equivalent particles. 34 THRESHOLD 1

(1 1b.i8lliboinllitornilltiorinLll°iCoITnA) °HFTIOUMM-7 6(d12i,a00m0me0itkleemsr ) in (11.mi8llimoillnitorinlltiroillni°oCn)°F 62(11,0m0ii0lnlimodniialekmsme)ter 18(100,000,000t0ritllriiollnio°Fn°C) HIGGS BOSON 1 THE BIG BANG 3 INFLATION BEGINS 1tr8i,l0lio0n0°F Space, time, and an intense The universe undergoes (t1r0ill,i0o0n0°C) 2 GRAND UNIFIED ERA burst of energy appear a short period of extreme This era begins when gravity suddenly and simultaneously. inflation, during which splits off from the other The state of the universe a fantastic amount of mass- fundamental forces. During during the first 10-43 energy comes into existence. this short time, matter and seconds—the Planck Era— Around this time, the strong energy are in a fluidly is uncertain, but it is force splits off from the interchangeable form inconceivably hot and two remaining fundamental called mass-energy. the four fundamental forces. The universe at forces are unified. this point is dominated FORCES by photons (packets of DIAMETER electromagnetic energy). Gravity separates out PARTICLES 10−12 SECONDS 10−43 SECONDS TEMPERATURE 62 million miles Pairs of quarks and 10−36 SECONDS ofSuottrrcoensgenpuacraleteasr (100 million km) antiquarks form and then immediately in diameter annihilate each other (10k.6m)mdiilaemeter 5 FINAL SEPARATION t(rt11irl,,il08lilo0i0on00n°C°F) 10−32 SECONDS The weak interaction separates from the electromagnetic force, and the fundamental forces and laws of physics become as they are today. Residue of particles (101t8ritlrliilloino tnritlrliilloino°n°C)F 4 INFLATION ENDS (110800ttrriilllliioonn°°CF) results from slight As inflation ends, a excess of particles seething mass of particles over antiparticles and antiparticles, such as quark-antiquark pairs, form spontaneously from Quarks are bound into energy and then annihilate heavier particles, such back to energy. The sea as protons, by gluons of particles is sometimes referred to as a quark-gluon 6(120b0illbioiilnnliodmniailkmemset)er plasma. The temperature of the universe at this stage is still many trillion trillion degrees. fpopuarroAnitiminnvtctoielruewstm-heaaotenfhtrtiaeeesrpnnafcerrorotegiomecly.lzeoeredepotauoitrtshTehe 621 billion miles 10−6 SECONDS18(1t0riltlrioillnio°Fn°C) (1 trillion km) in diameter 6 FIRST PROTONS AND NEUTRONS The universe has cooled sufficiently that quarks start becoming bound together by gluons into composite particles, such as protons and neutrons, and antiquarks form into antiprotons and antineutrons. 18 billion°F 1 SECOND (10 billion°C) THE BIG BANG 35

13.8 BYA THE UNIVERSE FORMS 10-36 SECONDS INFLATION 10-32 SECONDS INFLATION 10-12 SECONDS SEPARATION OF FUNDAMENTAL IN THE BIG BANG AFTER BIG BANG BEGINS AFTER BIG BANG ENDS AFTER BIG BANG FORCES IS COMPLETED Physics on a grand scale The large, barrel-shaped machine undergoing a refit here is a part of the LHC called the electromagnetic calorimeter. It measures the energies of electrons and photons to a high degree of accuracy. 36 THRESHOLD 1

10-6 SECONDS THE FIRST PROTONS AND 3 MINUTES THE FIRST ATOMIC 380,000 YEARS THE UNIVERSE BECOMES 13.6 BYA THE FIRST AFTER BIG BANG NEUTRONS FORM AFTER BIG BANG NUCLEI FORM AFTER BIG BANG TRANSPARENT STARS FORM RECREATING THE BIG BANG For years, researchers at the European Organization for Nuclear Research (CERN) have used the world’s largest particle accelerator—the Large Hadron Collider (LHC)—to smash particles together at extreme speeds to recreate conditions that existed shortly after the Big Bang. The LHC is the largest, most sophisticated In 2012, a long sought-after, high-mass, ▲ Seeking the scientific instrument ever built. Located extremely short-lived particle called the Higgs boson underground on the French-Swiss border, Higgs boson was detected. Its existence This computer graphic it accelerates two beams of high-energy confirmed the presence of an energy field, shows a particle particles, moving in opposite directions, the Higgs field, that imparts mass to collision recorded through pipes connected in a ring with a particles passing through it. The significance during the search for the circumference of almost 17 miles (27km). of this for the Big Bang is that it explains Higgs boson. It displays From time to time, the beams are made how in the first moments of the universe features that could be to collide, and the results—which typically particles such as quarks acquired mass, expected from the include the appearance of short-lived, exotic causing them to slow down and combine decay of a Higgs boson particles—are recorded by detectors around to form composite particles, such as protons into two other bosons. the ring. The purpose of the LHC is to and neutrons. One of these decays to a study the range of subatomic particles pair of electrons (green that can exist and the laws governing Other notable successes include the lines) and the other to a their interactions. detection in 2014 of a pentaquark (consisting pair of particles called of four quarks and an antiquark). This muons (red lines). Physicists hope these experiments will discovery may allow scientists to study in refine their ideas about what happened in more detail the strong force that holds the Big Bang and help them to investigate quarks together. some poorly understood cosmic phenomena. The Big Bang-type conditions are recreated only in miniature—so there is no chance the experiments could trigger a new Big Bang and the appearance of a new universe. NEW DISCOVERIES One success of the LHC has been to create a quark-gluon plasma, a maelstrom of free quarks and gluons (see p.34) that is thought to have existed for up to a microsecond (a millionth of a second) after the start of the Big Bang. This was achieved in 2015 by colliding protons with lead nuclei, creating minuscule fireballs in which everything broke down momentarily into quarks and gluons. WE HAVE MADE THE DISCOVERY OF A NEW PARTICLE— A COMPLETELY NEW PARTICLE—WHICH IS MOST PROBABLY VERY DIFFERENT FROM ALL THE OTHER PARTICLES. Rolf-Dieter Heuer, Director of CERN, 1948–, on the discovery of the Higgs boson RECREATING THE BIG BANG 37

13.8 BYA THE UNIVERSE FORMS 10-36 SECONDS INFLATION 10-32 SECONDS INFLATION 10-12 SECONDS SEPARATION OF FUNDAMENTAL IN THE BIG BANG AFTER BIG BANG BEGINS AFTER BIG BANG ENDS AFTER BIG BANG FORCES IS COMPLETED BEYOND THE Red-orange spots BIG BANG These have a temperature just 32°F (0.0002°C) higher than the Although the Big Bang model is now accepted by the vast majority of astronomers, additional evidence is continually being sought to average CMB temperature support it. There are also some problems with the theory that need to be addressed and some aspects that have yet to be understood. All-sky projection The map is a projection of measurements collected across the whole sky ▼ Dark matter A general point in favor of the Big Bang from the sky called the cosmic In this image of a galaxy model is that an important assumption on microwave background (CMB). Early cluster over 7 billion which it is based, the cosmological principle supporters of the Big Bang theory light-years from Earth, (see opposite page), has so far held true. The predicted that this radiation should called El Gordo (“The model also works within the framework of exist, and in 1964 it was detected Fat One”), the blue general relativity (see p.32), which is today by two American radio haze indicates the considered a pillar of cosmology. However, astronomers. The CMB arose distribution of dark these facts do not necessarily mean the Big soon after the Big Bang, when matter—hard-to-detect Bang theory is correct. To be sure of its photons (small packets of radiant matter that appears to validity, specific positive evidence is energy) were freed from interacting bind galaxy clusters needed—but there is no shortage of this. with matter and began to travel together gravitationally. unhindered through space. The pink haze indicates SPECIFIC EVIDENCE X-ray emissions. The most important positive evidence Further strong evidence comes from for the Big Bang is an extremely faint but observations of deep space, looking back uniform thermal radiation coming billions of years in time. Such observations have revealed objects called quasars (the highly energetic centers of galaxies) that no ▲ The cosmic longer seem to exist today. Furthermore, microwave background the most distant galaxies—that is, galaxies The strength of the CMB measured as they existed 10–13 billion years ago— by the Planck spacecraft is shown here look different from closer, modern galaxies. as a temperature variation. Although These observations suggest the universe is the CMB is uniform across the sky, a of a finite age and has evolved over time finely graded scale has been used to rather than been static and unchanging. show tiny variations as colored spots. One other important piece of evidence UNANSWERED QUESTIONS comes from the predominance and One major problem in cosmology in general proportions of the chemical elements is to shed light on the nature of “dark matter” hydrogen and helium in the universe. and how it may have arisen in the Big Bang. The ratios of these two elements in their Dark matter is an unknown substance that different forms (called isotopes) agree very emits no light, heat, radio waves, nor any closely with what is predicted by the Big other kind of radiation—making it extremely Bang theory. hard to detect—but it does interact with other matter. Another challenge is to understand WE CAN TRACE THINGS BACK TO THE EARLIER STAGES “dark energy.” In 1998, it was discovered OF THE BIG BANG, BUT WE STILL DON’T KNOW WHAT that the expansion of the universe has been BANGED AND WHY IT BANGED. THAT’S A CHALLENGE accelerating over the past 6 billion years. The FOR 21ST-CENTURY SCIENCE. reason for the acceleration is not known, but the mysterious phenomenon of dark energy Martin Rees, British cosmologist, 1942– has been proposed as the cause. Very little is known about it at present, but if dark energy exists, it must permeate the whole universe. 38 THRESHOLD 1

10-6 SECONDS THE FIRST PROTONS AND 3 MINUTES THE FIRST ATOMIC 380,000 YEARS THE UNIVERSE BECOMES 13.6 BYA THE FIRST AFTER BIG BANG NEUTRONS FORM AFTER BIG BANG NUCLEI FORM AFTER BIG BANG TRANSPARENT STARS FORM Dark blue spots These areas have a temperature just 32°F (0.0002°C) lower than the average CMB temperature Temperature anomaly Unexpectedly, one hemisphere, centered around this region, is slightly hotter on average than the other Other unanswered questions include why an In an area of sky several Large cold area excess of matter over antimatter appeared billion light years across, no This might be due to during the universe’s first few moments— structure can be detected in a huge void in space without it, no atoms could ever have the distribution of galaxies around 6–10 billion formed—and what caused the cosmic light-years away inflation that produced the smooth distribution of matter that we see in the In a much smaller area of sky, universe today. The final question is “what galaxies can be seen to be triggered the Big Bang?” and this, of course, grouped into clusters rather may never be answered. than evenly distributed ▶ The cosmological principle 5 BILLION LIGHT- 150 MILLION LIGHT- 4 MILLION LIGHT- This principle states that when viewed on a YEARS ACROSS YEARS ACROSS YEARS ACROSS sufficiently large scale, the universe is uniform, although on small scales there are clear variations BEYOND THE BIG BANG 39 in the distribution of objects such as galaxies. It follows from the cosmological principle that the universe has no center and no edges.

THRESHOLD

STARS ARE BORN With space, time, matter, and energy in place after the Big Bang, new powerhouses start to appear—stars. These form as matter is packed tighter and tighter together under the influence of gravity. The extremely high temperatures that result cause atoms to fuse together, releasing a huge amount of energy and opening the door to a new level of complexity in the universe.

GOLDILOCKS CONDITIONS The early universe was shaped by two ingredients, both of which TGCilrnuaymvivtpaysr,iowafthimoicnahtstpienurltglhsroemwdaetntesrity o emerged while it was less than a second old. Gravity acted on tiny f mattotegredthenesrer and hotter variations in the density of matter, setting into motion processes that led to the formation of the first stars and galaxies and, ultimately, a far more complex universe. The nuclei of hydrogen atoms What changed? The early universe consisted of hydrogen and helium—and another form of matter called dark matter. It was also completely without light. Under the influence of gravity, matter began to clump together. As it did so, it heated up until nuclear fusion reactions began, forming the first stars and lighting up the universe. Over time, the new stars clustered together into galaxies. The nuclei of helium atoms Dark matter

Star’s materials Star explodes Today, interstellar are blasted as supernova space is filled into space with plasma Clouds become Plasma bubbles hotter and denser around stars grow GRAVITY PULLS Hydrogen and A star uses fuel and merge MATTER TOWARD helium nuclei fuse quickly, creating UV radiation new elements in interacts with atoms AREAS OF HIGH inside clouds around stars, forming DENSITY, FORMING Fusion releases the process a charged gas called vast amounts CLOUDS of radiation a plasma MATTER TAKES THE FIRST Stars emit SIMPLE FORMS— STARS FORM powerful ATOMIC NUCLEI UV radiation (PROTONS AND Newly formed stars Supermassive NEUTRONS) AND cluster around black holes form at centers of DARK MATTER concentrations of large galaxies dark matter First galaxies form Matter falling Huge amounts as clusters of stars toward a black hole of matter gravitate heats up, releasing are attracted to toward centers each other vast amounts of galaxies of radiation Early galaxies gain order and structure Galaxies collide Mergers inject and merge mass and energy

13.6 BYA THE FIRST STARS FORM THE FIRST STARS TYPICAL FIRST- GENERATION STAR THE SUN For its first 200 million years, the universe was a dark place. But things changed dramatically when clouds of gas collapsed to form the first stars. Inside, new chemical elements formed, and at the ends of their short lives the stars exploded, dispersing the elements into space. The Big Bang During the Epoch of Recombination, although it was also dark, for there were no HOW STARS FORM 13.8 BYA 380,000 years after the Big Bang (see p.34), sources of light. It was a time cosmologists positively charged hydrogen and helium refer to as the Cosmic Dark Ages. Amid the Tiny variations in the density of the dark The universe is an nuclei combined with negatively charged dark soup of neutral gas was even darker matter and the hydrogen and helium gases opaque plasma of electrons to form neutral (uncharged) atoms. stuff: dark matter. Scientists have little idea caused vast clouds of gas to collapse under positively charged Until this point, collisions with free electrons about the nature of dark matter, although the influence of gravity to form huge had prevented photons of light from moving they do know there is lots of it and that it is spherical clumps of matter. This would hydrogen and any distance in a straight line. Now the affected by gravity but doesn’t interact with have happened without dark matter but helium nuclei universe became transparent to light, light or any other form of radiation. much more slowly—so slowly that no stars would have formed to this day. Epoch of Recombination 380,000 years after The enormous energy liberated in the Big Bang collapse heated the balls of gas. At the increasing densities deep inside the balls of Cosmic Dark Ages gas and as a result of the high temperatures 13.796 to 13.4 BYA The first stars Early star form 13.6 BYA forming inside gas cloud NEUTRAL HYDROGEN AND HELIUM ATOMS Hydrogen and helium gases begin to clump together to form clouds Filament of dark matter ▶ Lighting up the early universe The first stars formed about 200 million years after the Big Bang from clouds of hydrogen and helium gas. The intense ultraviolet light produced by these stars reionized the space around them, leaving behind charged atoms (or ions) and free electrons—and defining the character of interstellar space we see today.

13.45 BYA REIONIZATION OF 13.4 BYA THE FIRST GALAXIES THE UNIVERSE BEGINS START TO FORM ▲ The size of early stars intense radiation hit neutral hydrogen and ◀ Early light helium atoms still in space, its energy This is an artist’s According to astrophysicists’ best models, separated the electrons from their nuclei— impression of CR7, a just as they had been before the Epoch small, bright galaxy. At most early stars were much larger than the of Recombination. This “reionization” 12.7 billion light years created a plasma bubble of hydrogen away, CR7 appears as sun and hundreds of times as massive. ions, helium ions, and free electrons in it was about a billion the space around each star. Interstellar years after the Big Bang. at their cores, hydrogen and helium nuclei space today is an extremely tenuous plasma It represents the best collided, and some of them joined together, that was created by this reionization, and evidence so far of or fused. This nuclear fusion resulted in the nearly all radiation can pass through it. first-generation stars. production of more helium nuclei from the hydrogen nuclei, and new, heavier SHORT LIVES generation of stars probably only lived for a elements—including boron, carbon, and The first stars were large and massive: few million years, compared to several billion oxygen—from the helium nuclei (see probably dozens of times the diameter of years for an average star in later generations. pp.58–59). the sun and with hundreds of times as much As the hydrogen and helium “fuel” began mass. Such stars burn out quickly. The first to dwindle at the cores of the stars, they The nuclear fusion inside the collapsing cooled, enabling the collapse to begin again, balls of gas released a huge amount of FIRST-GENERATION STARS LIVED eventually causing the stars to explode as energy, enough to heat the gas to incredibly ONLY A FEW MILLION YEARS supernovas (see pp.60–61). The explosions high temperatures. That made the gas threw a cocktail of new elements and the expand, buoying it up against further BEFORE EXPLODING AS remaining unfused hydrogen and helium out collapse. The high temperature also into space. This cocktail formed the made the balls of gas glow brightly— VIOLENT SUPERNOVAS ingredients of a second generation of stars. and become the first stars. The extremely hot first stars emitted large amounts of powerful ultraviolet radiation that had far-reaching effects. When the Stars form in clusters that The first stars Reionization starts coincide with concentrations explode as 13.45 BYA of dark matter supernovas 13.5 BYA Ultraviolet radiation from hot star creates bubble of ionized Clusters of stars Dwarf galaxies As reionization plasma are drawn together combine to form continues, bubbles into dwarf galaxies larger galaxies, of plasma merge, 13.4 BYA including spiral and eventually the galaxies universe is filled with plasma

13.6 BYA THE FIRST 13.5 BYA THE FIRST STARS EXPLODE STARS FORM AS SUPERNOVAS THE PUZZLE ▼ Isaac Newton OF GRAVITY In the late 1680s, Newton published Gravity, or gravitation, plays a crucial role in the formation of stars and both his Universal Law of Gravitation— planets because it causes matter to clump together. The modern theory the first scientific theory of gravity—and of gravity, Einstein’s general theory of relativity, accurately explains its his three laws of motion. effects. Nevertheless, the true nature of gravity remains a mystery. The ancient Greek philosopher Aristotle that in the absence of air resistance, all two objects. According to Newton’s law, the supposed that Earth is at the center of the falling objects would accelerate downward force depends on the masses of the objects and that everything has a natural tendency at the same rate. English scientist Isaac and the distance between their centers. Newton made sense of Galileo’s prediction to move toward it. According to Aristotle, with his Universal Law of Gravitation. By combining his law of gravitation heavier things have more of this with his laws of motion, Newton was able to tendency and so fall faster. Although NEWTON’S GRAVITY account for the motions of any object under Aristotle’s simple notion was Newton realized that what makes things fall the influence of gravity—from projectiles superficially supported by to the ground here on Earth also keeps the on Earth to planets in space. His theory was moon in orbit. He proposed that gravity is accepted for over 200 years—and scientists observations, experiments by Italian a force and derived an equation that could still use his equation in most situations scientist Galileo Galilei in the 17th century predict the strength of the force between any where they need to calculate the effects showed that he was wrong. Galileo’s of gravity. However, in the 19th century, experiments led him to predict, correctly, Gravitational Gravitational Star hardly moves force on force on star is because of its large mass planet equal to force on planet Under the influence of gravity, the planet falls toward the star, following a curved orbital path STAR PLANET Without gravity, the NEWTON HIMSELF WAS BETTER AWARE OF THE WEAKNESSES planet would follow a IN HIS INTELLECTUAL EDIFICE THAN THE GENERATIONS OF LEARNED SCIENTISTS WHICH FOLLOWED HIM. straight line path Albert Einstein, German physicist, 1879–1955 ▲ Newton’s theory In Newton’s theory, a 46 THRESHOLD 2 star and planet exert an attractive force on each other. Both are subject to an equal force, but the effect on the planet is more obvious because it has a lower mass.

13.45 BYA REIONIZATION OF 13.4 BYA THE FIRST GALAXIES THE UNIVERSE BEGINS START TO FORM calculations of the orbit of planet Mercury, distortion. Objects traveling freely through ◀ Gravitational waves at odds with observations, showed Newton’s distorted spacetime follow curved paths. The first gravitational theory to be flawed. In 1915, German So projectiles and planets are simply waves ever detected physicist Albert Einstein proposed a radical following the equivalent of straight line resulted from the new theory of gravitation—the general theory paths, but in distorted spacetime. A force merger of two black of relativity—that could accurately predict is needed to change an object’s path. For holes. Here, the waves the orbit of Mercury. And according to example, the ground pushes upward on a are represented as Einstein’s theory, gravity is not a force at all. person’s feet, which stops the person from ripples in a two- following a path that would take him or her dimensional sheet of EINSTEIN’S GRAVITY “freefalling” toward the center of Earth. For a spacetime. These General relativity is an extension of special star, the expansion of the hot gas of which it ripples were detected relativity, a theory Einstein published in is made provides the force necessary to keep by sensitive equipment 1905. Special relativity was an attempt to it from collapsing—expansion that lasts as on Earth. reconcile Newton’s laws of motion with the long as the star produces heat (see pp.56–57). theory of electromagnetism, developed in Despite the success of general relativity, the the 1860s. To do that, Einstein had to EINSTEIN’S PREDICTIONS theory is at odds with quantum mechanics, abandon the idea that space and time are The general theory of relativity has been an equally well-tested cornerstone of absolute: people in motion relative to each tested many times, to extremely high modern science. Quantum mechanics other measure distances and intervals of precision. It has also made several important accurately describes the behavior of matter time differently—the differences only predictions, such as the idea that light must at the atomic and subatomic scales, while become significant at extremely high relative also follow the curved paths of distorted gravity accurately describes the behavior speeds. One of the direct consequences of spacetime. The result is a phenomenon of matter at much larger scales—but the two special relativity was the realization that called gravitational lensing, which is evident theories are incompatible. The search for time is a dimension, just like the three in the distorted views of distant galaxies a quantum theory of gravity is a major dimensions of space, and that all four exist whose light has been bent as it passed close concern of modern physics, and it is likely in a four-dimensional grid called spacetime; to nearby galaxies. Another key prediction is that Einstein’s theory of gravity will be objects therefore move through spacetime, the existence of gravitational waves: ripples reinterpreted or superseded as part of not space. in spacetime emanating at the speed of light a grand theory that can describe the from any very energetic event. In 2015, behavior of matter at all scales. One In order to generalize special relativity scientists detected the first hard evidence thing is certain: the puzzle of gravity to include gravity, Einstein realized that of the existence of gravitational waves, is not yet solved. objects with mass distort spacetime. The produced by the merging of two black holes. more massive an object, the greater the Curved sheet Lines called geodesics Massive objects indent the ▼ Einstein’s theory represents distorted represent the shortest sheet—the more massive The best way to illustrate the curves of spacetime distances between points the object, the deeper distorted four-dimensional spacetime is as in spacetime the indentation a two-dimensional sheet. A massive object creates an indentation in the sheet, causing objects nearby to follow curved paths. PLANET STAR There is no force on the planet; instead, it Planet’s path is The distortion of spacetime freely follows a curved path, because it is a circular or causes distance between the shortest distance around the star elliptical orbit adjacent geodesics to increase THE PUZZLE OF GRAVITY 47

13.6 BYA THE FIRST 13.5 BYA THE FIRST STARS EXPLODE STARS FORM AS SUPERNOVAS THE FIRST GALAXIES A galaxy is a vast congregation of stars orbiting a common center. The first galaxies began to form soon after the first stars, around clumps of dark matter. Mutual gravitational attraction caused these small galaxies to merge, each merger sparking new flurries of star birth. ▼ Galaxy evolution Dark matter was crucial in the creation various scales. The clumping process drove GROWING GALAXIES In the absence of of the first galaxies, just as it was for the the formation of individual stars as the direct observations, formation of the first stars (see pp.44–45). concentrations of matter began to rotate As matter fell toward matter, the dark astrophysicists Slight variations in the density of dark and heat up, eventually resulting in nuclear matter haloes grew in size, and so did the construct simulations matter in the early universe caused the dark fusion (see pp.56–57). At a larger scale, the galaxies. Like water draining down a plug to test their theories matter and ordinary matter—in the form same process also produced clusters of stars. hole, much of the matter began to spin as of how the first galaxies of hydrogen and helium gas—to clump Each star cluster, plus its surrounding gas, it fell, so that it went into orbit around the formed. The images together. The dark matter formed a network was attracted to neighboring clusters, and most dense, central part of the halo. As a below are snapshots of sinuous filaments and nodes, or haloes, at the universe’s first galaxies were born. result, galaxies that began as irregularly from one of those shaped masses began to gain order and simulations. DARK MATTER ORDINARY MATTER GALAXY 0.6 billion years old Dark matter clumps together 1.0 billion years old The dark matter has further due to gravity, attracting hydrogen and helium gas collapsed in on itself, increasing the gravitational (red in the bottom image) into its filaments and attraction on stars and gas. A small, irregularly nodes. Clusters of stars (blue dots) form where shaped galaxy forms, its stars orbiting a point the density is particularly high. at its center. 48 THRESHOLD 2