Before You Begin: A Note to the Reader

At the end of a working day, many of us step outside for a moment. Perhaps we are walking to the bus, carrying out the rubbish or waiting for a shift to begin. Sometimes, without planning to, we look up. There is the Moon, or a single bright star, or on a lucky night a whole sky of them. And sometimes, in that quiet moment, an old question comes back to us: where did all of this come from?

This book tries to answer that question gently, one evening at a time. It tells the story of how the universe began, as far as people understand it today: how it was once extremely hot and dense, how it has been expanding and cooling for about 13.8 billion years, how the first atoms, the first stars and the first galaxies formed, and how, much later, the Sun, the Earth and the atoms of our own bodies came to be. It also tells, more briefly, how people found these things out. Many of the great discoveries were made by people at work: a woman paid to measure stars on glass plates, two engineers trying to remove a noise from an antenna, a teacher of physics working out his ideas with pencil and paper.

The story is told over fifteen nights, and each chapter is one night. Every chapter begins with a working day of Noor, an astronaut who is spending six months on a space station high above the Earth. Her job is full of schedules, checklists, small repairs and long hours, much like many jobs on the ground. Something in her day leads to a question, and that question leads to the night's story. Before she sleeps, Noor sends a short message to her younger brother, Sami, who drives a delivery truck at night and reads her messages on his break. Noor, Sami and the crew of the station are imagined people, and the station is not a real one, but the life they live is close to the life of real astronauts.

Each chapter ends with a few common misunderstandings: things that many people believe about the universe, or about life in space, that are not quite right. These are not there to make anyone feel foolish. Almost everyone has believed some of them at some time, including many scientists before the facts were known. Looking at a misunderstanding closely is often the easiest way to understand what is really true.

How to Read These Nights

You do not need any science background to read this book. Every new word is explained when it first appears, and the explanations use ordinary things: bread rising in an oven, a passing siren, a glass of water, a long shift that ends at dawn. If a part seems difficult, it is fine to read on. The next night will often make it clearer.

One chapter takes about twenty minutes to read, and the chapters are meant to be read in order, one a night, like a series of bedtime stories for grown-ups. The early nights collect the clues, and the later nights use them. But there is no hurry. If you read two chapters on a quiet weekend and none during a busy week, the universe will wait for you. It has been waiting for a very long time.

The numbers in this book are rounded, and most of them carry the word "about". Astronomers measure many things very precisely, but for a story told at night, a round number is easier to carry. Where something is not yet known, the book says so plainly. There is still a great deal that nobody understands about the beginning of the universe, and some of the most interesting questions are still open.

Finally, you may enjoy looking up now and then as you read. You do not need a telescope or a dark mountain. A window, a doorway or a short walk after work is enough. The light you see on those evenings is part of the same story that this book tells.

Now, the first working day on the station is almost over. Noor is tired, her head feels strange in weightlessness, and the stars are rising outside the window. Let us join her for the first night.

Chapter 1: A Question in the Dark

Night 1: The First Night in the Sleeping Bag

Noor's first working day on the space station began very early, long before it reached the station itself. She woke in a small room on the ground, ate a quiet breakfast, put on her flight suit and climbed with her crewmates into the capsule on top of a tall rocket. A few hours after the engines lit, the capsule joined the station, the hatches opened, and the people who already lived there welcomed the new crew with tired, happy faces.

The rest of the day was full of small, careful jobs. Noor learned where the emergency equipment was kept. She practised the route to her sleeping cabin, pulling herself along handrails because her feet no longer had any use for the floor. She unpacked a bag of clothes, a toothbrush and a notebook, and she tied each item down, because anything left loose in the air would drift away. By the evening her head felt full and heavy, as it often does in the first hours of weightlessness, and her hands were a little clumsy. Tomás, the commander, told her that this was normal. Ade, the scientist, said that on his first evening he had lost three pens in ten minutes.

At the end of the day, Noor went to the small round room with the large window. The window is made of several thick panes, and through it she could see the curve of the Earth sliding past. Blue ocean, white cloud, then the thin bright line of the air at the edge of the planet. As she watched, the station crossed into the night side of the Earth. The ground went dark, and above it the sky was full of stars.

On the dark side of the planet, she could see the lights of towns scattered like spilled sugar across the land, and far away the white flicker of a thunderstorm inside the clouds. Less than an hour later, a thin line of orange and blue appeared at the edge of the Earth, and the Sun rose again. Tomás floated in beside her and said that she would see about sixteen sunrises and sixteen sunsets every twenty-four hours from now on. The station keeps one clock agreed with the teams on the ground, he explained, so that the crew can sleep, eat and work on a regular schedule, even when the Sun rises every hour and a half.

Later, zipped into her sleeping bag, which is tied to the wall of a cabin the size of a narrow cupboard, Noor read a message from her younger brother. Sami drives a delivery truck at night. He reads his messages on his break, at about three in the morning, parked beside a quiet road. His message was short. "Glad you arrived safely. Here is a question for you, since you are closer to the sky than any of us. How did it all begin? Everything, I mean. The whole universe."

Noor smiled in the dark. It was a very large question for a very small cabin. But she had fifteen nights in mind, spread across her six months on the station, and she decided to answer it slowly, one story a night. This book follows those nights. If you like, you can read it the same way: one chapter an evening, after your own working day.

The Oldest Question

Sami's question is perhaps the oldest question people have asked. Long before there were telescopes, long before there was writing, people looked up at the night sky and wondered where it had come from. Almost every culture we know of has told stories about the beginning of the world. Some stories began with water, some with an egg, some with darkness, some with a great silence. These stories were told around fires, at the end of long days of work in fields and on the sea, by people who were as curious as we are.

Those stories did many important jobs. They gave people a sense of their place in the world. They explained the seasons and the stars in a way that a child could remember. They carried values from one generation to the next. Science does not try to replace that kind of meaning. It asks a narrower question: what can we learn about the beginning by looking carefully at the universe itself, measuring what we see and checking our ideas again and again?

For most of human history, that narrower question could not really be asked. People could watch the Sun, the Moon and the planets very carefully, and many cultures made excellent calendars from what they saw. But to study the beginning of the whole universe, astronomers needed two things that did not exist until about a century ago: telescopes large enough to see very distant galaxies, and a theory of gravity strong enough to describe the universe as a whole. That theory, Albert Einstein's general theory of relativity, appeared in 1915. The large telescopes came at about the same time. Only then could the old question become a scientific one.

That narrower question has turned out to be very powerful. In the last hundred years or so, astronomers have learned more about the early universe than in all the centuries before. They did it slowly, with patient work. Many of the discoveries in this book were made by people who were simply doing their jobs: measuring stars on photographic plates, fixing noise in an antenna, checking numbers late at night. Their working days added up to a story that none of them could have told alone.

It is also a story with gaps. Some parts of it are known very well and tested in many different ways. Other parts are still open, and scientists argue about them. In these fifteen nights, I will try to make clear which is which. When something is known, I will say so. When something is not yet known, I will say that too. Not knowing is not a failure. It is simply the place where the next working day begins.

The Short Answer

Before the long story, here is the short answer, the one that fits on the back of a card. About 13.8 billion years ago, the universe was extremely hot and extremely dense. Everything that is now spread across the sky, every star and every galaxy, was packed together in a state hotter than the centre of any star. Since then, the universe has been expanding and cooling. As it cooled, the first particles formed, then the first atoms, then the first stars and galaxies, and much later the Sun, the Earth and us.

That is the main idea. Everything else in this book fills it in. The number 13.8 billion years is so large that it is hard to feel. One way to begin is to compare it with a human life. If you lived to be eighty, the age of the universe would be more than one hundred and seventy million of your lifetimes. Another way, which we will use on the last night, is to squeeze the whole history into one working day and see what happens at each hour.

You will often hear this early hot state called the Big Bang. It is a famous name, and we will use it in this book, but it is only a name. It does not mean that there was a loud bang, and it does not mean that something exploded in an empty room. On a later night, we will meet the man who gave it that name, and we will see that he did not believe in the idea himself. For now, it is enough to remember that the Big Bang means the hot, dense early universe and its expansion.

Why should a universe that expands also cool? You may have felt the answer in your own hands. When you use a spray can for a while, the can becomes cold, because the gas inside spreads out as it escapes. When a gas is squeezed, it warms up, which is why a bicycle pump gets warm at the bottom after hard pumping. The universe follows the same rule on a vast scale. As space has stretched over billions of years, everything in it has spread out and cooled, from a heat beyond anything on Earth to the deep cold of space today.

The short answer also leaves out something important. It does not tell us what caused the beginning, or whether there was anything before it. Those questions are among the open ones. The science of the early universe describes what happened after the first moment, and it describes that very well. About the first moment itself, scientists honestly say that they do not yet know.

How Can Anyone Know?

Sami's next question came the following night, and it was a good one. "How can anyone know what happened billions of years ago? Nobody was there." This is the question every careful person should ask. We cannot travel back in time, and we cannot rerun the universe in a laboratory. So how do astronomers know anything at all about its beginning?

The answer is that the past has left clues, much like the clues a mechanic reads when a truck comes into the workshop. A good mechanic can look at worn brakes, a scratch on the paint and the mud in the wheel arches and tell you a lot about where the truck has been, even though she was not in the cab. Astronomers do something similar with the universe. They look at what is here now and ask what history would have produced it.

There are three main clues, and the next several nights will tell each one in turn. The first clue is that the galaxies, the huge families of stars that fill the universe, are moving apart from each other. If they are moving apart now, they must have been closer together in the past. The second clue is a faint glow of light that comes from every direction in the sky, the oldest light there is, left over from a time when the whole universe was hot and glowing. The third clue is the mixture of the lightest kinds of atoms, hydrogen and helium, which are found in just the amounts that a hot early universe would have made.

Any one of these clues, by itself, might be explained in some other way. What convinced most scientists was that all three point to the same history, and that later measurements, made with better and better instruments, kept agreeing with it. That is how much of science works. It is not one dramatic proof, but many patient checks that all fit together.

One kind of check is especially convincing. It is when an idea predicts something that nobody has seen yet, and then, later, someone finds it. If a mechanic says, "Because of this wear, I expect to find a loose bolt behind that panel," and the bolt is there, you trust the mechanic more. In the story of the early universe, this has happened more than once. The oldest light, for example, was predicted in 1948 and found by accident in 1965, as we will see on a later night.

There is a fourth helper that makes all of this possible, and it is light itself. Light takes time to travel. When we look at something very far away, we see it as it was when the light left it. That means a telescope is also a kind of time machine, though it can only look and never visit. Light will be the subject of the next night's story.

A Book of Fifteen Nights

Here is the shape of the journey ahead. On the next nights, we will follow light as a messenger, and learn how astronomers discovered that the universe is far larger than anyone had imagined. We will see how they found that the galaxies are moving apart, and what it means to run that film backwards. We will meet the oldest light, and learn what happened in the first few minutes, when the lightest atoms were made, and even earlier, before there were any atoms at all.

After that, the story moves forward again. We will watch the first stars switch on after a long darkness, and see how galaxies grew. We will learn how the atoms in your own body were made inside stars, and how the Sun and the Earth were born. Then we will turn to the great unknowns: the strange push called dark energy, and the questions at the very edge of what we can see. On the last night, Noor will pack her bags for the journey home, and we will look at the whole story at once.

Each night has the same simple shape. First, we follow Noor through part of a working day on the station. Then comes the night's story. Then we return to the station for a few quiet minutes before sleep. At the end of each chapter, there is a short section about common misunderstandings: ideas that many people hold, often for good reasons, that are not quite right. Clearing them away is one of the gentlest ways to learn.

You do not need any science background to follow this book. Every new word will be explained when it first appears. If a number feels too large to imagine, that is all right. Astronomers cannot truly imagine these numbers either. They use them carefully, round them when they can and keep their sense of wonder.

A Message to Sami

Noor's first answer to her brother was short, because she was very tired. She wrote: "The short version: about 13.8 billion years ago, everything was hot and packed tight, and it has been spreading out and cooling ever since. The long version will take fifteen nights. Drive safely."

Before she closed the message, Ade floated past her cabin on his way to bed and noticed the notebook in her hand. When she told him about Sami's question, he laughed softly and said that it was the best kind of question, because nobody could answer it in one night. "Fifteen nights may not be enough," he said. "But it is a good start."

Then she turned off the small light in her cabin. Weightlessness makes sleep strange at first. There is no pillow pressing up, no mattress below, and her arms floated in front of her until she tucked them into the bag. Outside, the station moved on, once around the Earth in about ninety minutes, into the day side and out again. Somewhere far below, at about three in the morning, Sami would park his truck, open his phone and read her words.

Perhaps you are reading this at the end of your own long day. If you can, step outside for a minute before bed, or look out of a window, and find one star. You do not need to know its name. Just notice that its light has travelled a very long way to reach you, and that you are part of the same universe as that star, with the same long history behind you. That is where the story begins.

Common Misunderstandings

Many people think that astronauts float because there is no gravity in space. It is an easy mistake to make, because the pictures look exactly like that. But at the height of the station, about 400 kilometres above the ground, the Earth's gravity is still about nine-tenths as strong as it is on the surface. If you could build a tower that high and stand on top of it, you would still feel nearly nine-tenths of your normal weight.

The real reason for floating is that the station and everyone inside it are falling. They are falling all the time, towards the Earth. But the station is also moving sideways very fast, at about 28,000 kilometres per hour. As it falls, the curved surface of the Earth falls away beneath it, so it never comes any closer. It keeps falling around the planet instead of into it. Because Noor, her crewmates and every pen and toothbrush fall together at the same rate, nothing presses on anything else. That is the feeling we call weightlessness.

Many people also think that the Big Bang was an explosion that happened at one point in empty space, like a firework in a dark sky, with everything flying outwards from a centre. This is probably the most common misunderstanding about the beginning of the universe. As far as we can tell, there was no empty space outside waiting to be filled, and there was no centre. The early hot, dense state was everywhere at once, and it is space itself that has been stretching ever since. This idea is not easy, and we will come back to it slowly on the nights ahead.

Finally, some people believe that scientists claim to know everything about the beginning of the universe, and others believe that scientists know almost nothing and are only guessing. Neither is true. Some parts of the story, such as the expansion and the oldest light, are measured with great care and checked in many ways. Other parts, such as the very first moment and what dark matter and dark energy really are, are honestly unknown. A good way to read this book is to keep both in mind: confidence where the evidence is strong, and an open mind where it is not. On the next night of this book, we will begin with the messenger that carries all this evidence to us: light.