Chapter 5: Running the Film Backwards

Night 44: A Day of Checklists

Noor's forty-fourth day was spent almost entirely with checklists. In a week, she and Tomás would go outside the station on a spacewalk to replace a worn part on the outside of the station's frame. A spacewalk usually lasts six or seven hours, and almost every minute of it is planned in advance. Today was a preparation day: checking the two spacesuits, charging their batteries, testing their radios and laying out every tool they would need.

A spacesuit is really a small spaceship shaped like a person. It holds air at a safe pressure, removes the carbon dioxide that the wearer breathes out, keeps the body cool with water flowing through thin tubes in a garment worn underneath, and protects against the fierce sunlight and the deep cold of the shadow. Noor and Tomás worked through each part slowly. Gloves were checked for the tiniest cuts. The helmet lights were tested. Each tool was given a short cord, called a tether, so that nothing could float away into space.

There were also rules for the body. Before going out, the crew would breathe pure oxygen for a long time to remove some of the nitrogen from their blood, because the suit works at a lower pressure than the station. Without this step, bubbles could form in the body, rather like the bubbles that appear when you open a bottle of fizzy water. The ground teams had written every step of this process into the checklist. Tomás read each line aloud, Noor did it and said "done", and he moved to the next line.

In the afternoon, they put on the suits for a practice session inside the station. Getting into a spacesuit takes a long time and a second pair of hands, so Ade helped each of them in turn, connecting hoses and locking the helmet ring into place. Inside the suit, the world became quieter, and the only sounds were the hum of the fan and the voices on the radio. Noor practised moving her stiff gloved fingers around the handles of the tools, and Tomás practised the order in which they would open the hatch, check their tethers and move out along the handrails. A voice from the ground followed every step, asking questions and confirming each answer. When they finally took the suits off again, both of them were sweating and hungry.

By the evening, both suits were ready, hanging in the small room called the airlock like two tall white figures. Noor felt pleasantly tired, the way you feel after a day of careful work that leaves no room for mistakes. Before sleep, she read through the next day's plan, and the plan for the spacewalk once more from the end to the beginning. Tomás had taught her this habit: once you know a procedure forwards, go through it backwards, to see how each step depends on the one before.

Reading the checklist backwards made her think of the night's story. On an earlier night, she had told Sami that the galaxies are moving apart. Tonight she would do what astronomers did next: run the film backwards, to see where it leads.

What Happens When You Rewind

Imagine that you have a film of the universe, recorded over billions of years, and you play it in reverse. In the forward film, the galaxies drift apart as space stretches between them. In the reverse film, they drift together. The farther back you go, the closer they come. Groups of galaxies that are now separated by hundreds of millions of light-years would, in the distant past, have been much closer to their neighbours.

Keep rewinding, and something else happens. The galaxies themselves are made of stars, gas and dust. As the film runs backwards, all of this matter is squeezed into a smaller and smaller space. And when matter is squeezed, it heats up. You know this from a bicycle pump, which grows warm when you pump hard, or from the way air in a tyre gets warmer when it is filled quickly. In the reversed film of the universe, the squeezing goes far beyond anything a pump can do. The universe becomes hotter and hotter.

At some point in the rewind, it becomes too hot for stars and galaxies to exist at all. Earlier still, it becomes too hot even for atoms. Everything is a glowing mixture of tiny particles and light, far hotter than the centre of the Sun. Run the film back to about 13.8 billion years ago, and the universe is so hot and so dense that our present laws of physics can no longer describe it with confidence. That is where the film, as we know it, begins.

This is the basic reasoning behind the idea of a hot beginning. It is not based on guessing what the beginning was like. It is based on taking what we see today, the expansion, and following it carefully backwards using the known laws of physics. Each step can be checked. Does the reasoning predict things we can actually find today? On the next nights, we will see that it does. But in the years after Hubble's discovery, this was not yet clear, and many scientists found the idea of a beginning very uncomfortable.

There is also a limit to how far back the rewind can be trusted. For temperatures and densities that scientists can create or measure on Earth, in laboratories and large machines, the laws of physics are well tested, and following the film back through those conditions is fairly safe. Beyond them, the reasoning becomes less certain, and at the very start it fails completely. So the story of the beginning is told with the greatest confidence for the parts that match conditions we can test, and with more and more care as it approaches the first moment. On later nights, we will meet each of these stages in turn.

The Primeval Atom

The first person to take the rewind seriously was Georges Lemaître, the priest and physics teacher who had already suggested, in 1927, that the universe is expanding. In 1931, he went further. If the universe is expanding, he wrote, then in the far past it must have been very small and dense. He proposed that the universe began from a single, extremely dense state, which he called the "primeval atom". He imagined that this first state broke apart, like a very large atom splitting into smaller pieces, and that from those pieces the universe grew.

Lemaître's details were not quite the ones accepted today. He did not have the knowledge of particles and atoms that came later, and his picture of the first state was different from the modern picture. But his main idea was the right one: the universe has a history, and at the start of that history it was in a very dense state, quite unlike today. He was the first to say clearly that the expansion points back to a beginning.

It is worth noticing who Lemaître was. He was a priest who also worked as a scientist and teacher. Some people at the time thought that his idea of a beginning might have come from his religion. Lemaître himself was careful to keep the two apart. He saw the beginning of the universe as a question for physics, to be judged by evidence, and he did not use it to prove or disprove any belief. That is a good example for anyone thinking about these questions today. Science can describe how the universe changed. Questions of meaning and faith are a different kind of question, and each person approaches them in their own way.

At first, many scientists did not like the idea of a beginning. A universe that had always existed, without any start, seemed simpler and more natural to them. Others objected that the age of the universe calculated from Hubble's early measurements was too short, shorter even than the age of the Earth as measured by geologists. That problem came from Hubble's distances being too small, and it disappeared when they were corrected. But for some years it made the idea of a beginning seem doubtful.

A Universe Without a Beginning

In 1948, a group of scientists offered a rival idea. They accepted that the galaxies are moving apart. But they suggested that the universe had no beginning and would have no end. As the galaxies spread out, they said, new matter appears very slowly in the empty space between them, so slowly that nobody could ever detect it directly, perhaps a single atom here and there over enormous periods of time. From this new matter, new galaxies form, filling the gaps. In this picture, the universe always looks the same, on average, at every time. It was called the steady-state theory.

The steady-state idea was not foolish. It was clever, it was carefully worked out and it made clear predictions, which is exactly what a good scientific idea should do. Its supporters liked that it did not need a mysterious first moment. One of its best-known supporters was Fred Hoyle, an astronomer who was also a popular writer and a lively speaker on radio. On a later night we will meet him again, because he also made great contributions to understanding how the elements are made in stars.

It was Hoyle who gave the rival idea its famous name. In a radio talk in 1949, he described the idea of a hot beginning as a "Big Bang". Many people have said that he meant the name as a joke against the idea he disagreed with. Hoyle himself later said that he had only wanted a vivid picture for his listeners. Whatever he intended, the name stuck, and it is now used everywhere, even by scientists, for the idea that he hoped would turn out to be wrong.

So, by the early 1950s, there were two serious pictures of the universe. In one, the universe began in a hot, dense state and has been changing ever since. In the other, the universe has no beginning and always looks the same. Both agreed with Hubble's expansion. To choose between them, scientists needed something more: a clue that one picture predicted and the other did not.

It is easy, looking back, to divide the people in this story into winners and losers. But that is not how science really works. The steady-state theory forced the supporters of a hot beginning to sharpen their ideas and to look harder for evidence. Questions raised by its supporters led to useful work, including Hoyle's own great work on the elements. A rival idea, carefully argued, is not an enemy. It is a kind of colleague who asks the difficult questions that everyone else would rather avoid, much like the coworker in any team who always asks, "But have we checked?"

Settling It With Evidence

The two pictures made different predictions about the past. In the steady-state picture, the universe has always looked the same, so very distant galaxies, seen as they were long ago, should look much like nearby ones. In the hot-beginning picture, the universe has changed over time, so the distant, early universe should look different from the universe today.

One test came from radio astronomy, which grew quickly after the 1940s. Radio telescopes can detect galaxies that give out strong radio waves. When radio astronomers in the 1950s and early 1960s counted these sources, they found that there seemed to be more of them far away, which means long ago, than nearby. The universe in the past did not look the same as the universe today. This was a problem for the steady-state picture, although its supporters argued about how to read the counts.

In 1963, another discovery added to the problem. Astronomers found a new kind of object, extremely bright and extremely far away, which came to be called a quasar. Today we know that quasars are the brilliant centres of distant galaxies, where enormous amounts of gas are falling towards giant black holes. What mattered for the argument was where they were found. Quasars are seen at great distances, which means long ago, but there are no quasars of that kind close to us. The universe of long ago had them, and the universe of today does not. Once again, the past looked different from the present.

The hot-beginning picture also made a much more striking prediction. If the young universe was very hot, it would have been filled with glowing light, like the inside of a furnace. As the universe expanded and cooled, that light would not disappear. It would be stretched and weakened, but it should still be there today, filling all of space, coming from every direction. In 1948, some physicists predicted this leftover light. If anyone found it, it would be very hard for the steady-state picture to explain.

For more than fifteen years after the prediction, nobody went looking for it in a serious way. The instruments of the time were not quite sensitive enough, and many scientists did not take the prediction very seriously. Then, in 1965, it was found, almost by accident, by two engineers who were trying to solve a practical problem with an antenna. That discovery, the oldest light in the universe, is the story of the next night of this book.

Notice how this argument was settled. It was not settled by who had the cleverest words or the most famous supporters. It was settled because the two ideas made different predictions, and people went out and measured. This is perhaps the most important lesson in the whole story of the beginning. Science is not a contest of opinions. It is a way of letting the universe itself decide.

Before Sleep

Noor wrote to Sami before turning off her light. "Tonight's story: if the galaxies are moving apart now, they were closer in the past, and further back everything was squeezed together and very hot. A priest who taught physics called the first state the primeval atom. A rival idea said the universe had no beginning at all. The name Big Bang was given by a man who did not believe in it. The argument was settled by evidence, which I will tell you about next time."

Sami wrote back during his break that night. He said that at his depot, when something went wrong with a delivery, the drivers always argued about whose fault it was, and the manager always said the same thing: "Stop arguing and check the tracking." He thought that the astronomers had done the same thing. Noor smiled and agreed. Then she thought about the spacewalk again, and about how much depends, in any kind of work, on checking instead of assuming.

Before she slept, Tomás knocked gently on the wall of her cabin to say goodnight. He mentioned that he had once worked with an engineer who used to say that a checklist is a conversation with all the people who made mistakes before you. Noor liked that. In a way, she thought, the whole history of science was a checklist of that kind, written by people who had been wrong, noticed it and passed on what they had learned.

If you have a minute this evening, think of something in your own working day that you understood better by going through it backwards: a mistake you traced to its cause, a job you planned from the end. That kind of thinking is exactly what astronomers did with the universe. They took the expansion they could see today and asked, patiently, what must have come before.

Common Misunderstandings

Many people imagine that the universe began as a tiny point floating in a vast empty space, like a bright dot in the middle of a dark room, and then grew outwards to fill the room. This is not what the idea of a hot beginning says. As far as we can tell, there was no room. There was no empty space outside the early universe, waiting to be filled. The early universe was hot and dense everywhere, and space itself has been stretching ever since. If the universe is infinite today, as it may be, then it was probably infinite then too, only much denser. What was very small was the part that has since become the universe we can see.

Many people also take the name Big Bang at face value, and picture a loud explosion with a centre and a sound. But there was no bang in the ordinary sense. Sound needs air or another material to travel through, and there was no listener and no outside from which to hear it. There was no centre either, as the raisins in the rising bread showed on the previous night. The name came from a radio talk by a man who preferred a different idea. It is a useful label, and we use it in this book, but it describes the beginning about as well as a nickname describes a person.

A third common misunderstanding is that the Big Bang theory explains what caused the beginning of the universe. It does not. The theory describes how the universe has changed from a very hot, dense early state to the universe we see today, and it describes that very well. But about the very first moment, and about what came before it, if anything did, the theory says nothing reliable. Our present laws of physics stop working when we try to apply them there. Scientists honestly say that this part is not yet known. What they can test is what happened after, and the first great test came in 1965, from a faint hiss in an antenna, which is where the next night's story begins.