Chapter 6: The Oldest Light

Night 57: A Hiss on the Line

On the morning of day 57, Noor noticed the hiss during the first call with the ground. It was very faint, like the sound of rain far away, and it sat underneath every voice that came through her headset. The teams on the ground could still hear her clearly, and she could still understand them. But the hiss was there, and on a space station, anything that should not be there is worth a look.

The ground asked her to help find the cause. They sent a short checklist, and she worked through it between her other jobs of the day. First she listened with a different headset. The hiss stayed. Then she switched to a different radio panel. The hiss stayed. Then she checked the settings that the ground read out to her, one by one. Everything matched. By lunch, she had ruled out almost everything that was easy to rule out, which is a useful kind of progress, even if it does not feel like progress at the time.

In the afternoon, a specialist on the ground asked her to follow a cable from the panel to the wall and to check one small connector behind a cover. It took her twenty minutes to open the cover, because every screw had to be caught before it could float away. Behind it, the connector looked fine. She pressed it gently into place anyway, as the checklist said. The specialist asked her to say a few words. She counted from one to ten. There was a pause, and then the voice from the ground said, warmly, that the hiss was gone.

Tomás laughed and said that half of all engineering is pressing things gently back into place. Ade said that the other half is believing your instruments when they tell you something strange. Noor wrote the result in the log, closed the cover again and collected the screws from the small bag she had tied to her sleeve.

That evening, when Noor told Sami about the hiss, he wrote back from his truck, parked at a loading bay. He said that his radio in the cab hisses every night between two towns, in the same place, and that he has stopped noticing it. Noor smiled at that. She knew a story about a hiss that two engineers could not stop noticing, and about what it turned out to be. It was the right story for night 57.

A Glowing Fog

On the previous night, we followed the film of the universe backwards. If the galaxies are moving apart today, then long ago everything was closer together. And when matter and light are squeezed together, they become hot. Lemaître pictured a "primeval atom". The supporters of the steady-state idea pictured a universe without any beginning at all. Fred Hoyle, who was on that second side, gave the first idea its famous name in a radio talk. But a name is not evidence. Tonight's story is about the evidence that settled the question.

Let us imagine the young universe, a few hundred thousand years after the beginning. It was much hotter and denser than anything in the universe today. It was so hot that atoms could not hold together. An atom, as we will see in more detail on later nights, is a tiny centre called a nucleus with even tinier particles called electrons around it. In the young universe, the electrons could not stay with their nuclei. Every time an electron settled down, the heat knocked it free again.

Free electrons have an important effect on light. They get in its way. A beam of light in that early universe could not travel far before it bumped into an electron and was sent off in a new direction. Then it bumped into another one, and another. The light was everywhere, but it could not go anywhere in a straight line. The whole universe glowed, but it was not clear. It was like a thick, bright fog.

You have probably seen something similar on a foggy morning. The sun is up, and the air around you is bright. But you cannot see the houses at the end of the street, because the light from them is scattered on its way to your eyes. The young universe was like that everywhere, in every direction. If someone could have stood inside it, they would have seen only a bright, even glow, with nothing behind it.

Then, slowly, the universe kept expanding and cooling. At about 380,000 years after the beginning, it had cooled to a temperature of a few thousand degrees. That is still very hot, about as hot as the surface of some cool stars, but it was cool enough for an important change. The electrons could now settle around the nuclei and stay there. Atoms formed, mostly atoms of hydrogen and helium.

The Moment the Fog Cleared

Once the electrons were inside atoms, they no longer got in the way of light so easily. Suddenly, the light could travel freely in straight lines. The fog cleared. Astronomers sometimes compare this moment to a lamp that has been inside a thick cloud and is suddenly in clear air. The light that was released at that moment set off in every direction across the universe. Some of it has been travelling ever since, for about 13.8 billion years, and some of it is arriving at the Earth tonight.

When that light set off, it was the glow of something with a temperature of a few thousand degrees. Light from such a hot thing has short waves, and some of it would have looked orange or yellow to human eyes. But on the night of the moving galaxies, we saw that space itself has been stretching for billions of years. As space stretched, the waves of this old light stretched with it. They have been stretched by more than a thousand times. Today, the waves are no longer visible light at all. They are much longer waves, the kind called microwaves.

Microwaves are a kind of radio wave with fairly short waves. They are the same family of waves that a microwave oven uses to warm food, but the old light of the universe is extremely weak. It does not warm anything that you could feel. Its temperature today is only about 2.7 degrees above absolute zero. Absolute zero is the coldest possible temperature, about minus 273 degrees Celsius, so this old light is at about minus 270 degrees.

Astronomers call this light the cosmic microwave background. "Cosmic" means that it belongs to the whole universe. "Microwave" tells us what kind of waves it is today. "Background" means that it is behind everything else, like the back wall of a stage. In this book, we will mostly call it simply the oldest light. It is the oldest light that we can ever see, because before it, the universe was a fog that light could not cross.

There is something remarkable about this. The oldest light does not come from one star or one galaxy. It does not come from one direction. It comes from every direction at once, from the whole sky, day and night. As you read this page, a very faint part of it is passing through the room around you. It is like standing inside the fading echo of a great hall, where the sound comes from all the walls together.

The People Who Found It

The first people to think carefully about this old light were working on a different question. In 1948, the physicist George Gamow and his younger colleagues Ralph Alpher and Robert Herman were trying to understand how the lightest elements could have been made in a hot, dense early universe. That is the story of our next night. But while they were working on it, Alpher and Herman realized something. If the early universe really had been so hot, a faint glow of that heat should still fill space today. They estimated that this glow would now be only a few degrees above absolute zero.

Think for a moment about how these three people worked. They had no telescope that could see the early universe and no computers like the ones we use today. They worked with pencils, paper, simple calculating machines and long conversations. Gamow was known for his playful humour and for writing friendly books that explained physics to ordinary readers. Alpher and Herman did much of the patient, careful calculation. Their estimate, about five degrees, was not exactly right, but it was close to the true value, which was a remarkable result for a first attempt.

Their prediction did not attract much attention at the time. Measuring such a faint glow seemed far beyond the instruments of the day, and many scientists were not yet convinced that the universe had a hot beginning at all. The idea was mostly forgotten for some years. This happens more often in science than people think. A good idea can wait quietly in old papers until the right instrument, or the right person, comes along.

In the early 1960s, two radio engineers began working with a large horn-shaped antenna that had been built to receive signals bounced off satellites. They wanted to use it for careful measurements of faint radio waves from space. To do that, they first had to understand every bit of noise that the antenna and its receiver produced by themselves. So they measured, and checked, and measured again. And they found a faint hiss that they could not explain. It was the same in every direction they pointed the antenna, at every time of day and in every season.

They did what good engineers do. They searched for the cause in their own equipment. They checked the receiver and the connections. They even found that a pair of pigeons had been nesting inside the horn and had left droppings on its inner surface. The pigeons were taken away, and the engineers cleaned the horn carefully. The hiss remained. It was not coming from the antenna, and it was not coming from anywhere nearby. It was coming from the sky.

At about the same time, a group of physicists not far away had been building an instrument to look for exactly such a glow, because they had worked out the idea again for themselves. When the two groups heard about each other's work, they understood what the hiss was. In 1965, the results were published. The engineers had found the oldest light. For many astronomers, this was the moment when the steady-state idea lost its strongest support, because a universe without a hot beginning had no natural way to fill all of space with such a glow.

A Map of Tiny Differences

After 1965, the oldest light became one of the most studied things in the whole sky. At first, it seemed perfectly smooth. It had the same temperature in every direction, as far as anyone could measure. That was a puzzle in its own way. If the young universe had been perfectly smooth, how did it later become lumpy, with galaxies here and empty space there? There had to be some small differences somewhere, if only someone could measure them.

Measuring them from the ground was very hard, because the air itself glows a little in microwaves and the water in the air gets in the way. So scientists built satellites to measure the oldest light from above the air. A satellite launched in 1989 measured the glow with great care, and in 1992 its team announced that they had found the differences. They were tiny, about one part in 100,000. Some parts of the sky were very slightly warmer and some very slightly cooler than the average.

The satellite did not take a picture in one moment, as a camera does. It scanned the whole sky again and again, for years, and its team added up the measurements so that the tiny differences could rise above the noise of the instrument. In that way, the team faced the same problem as the two engineers with their horn antenna. Before they could trust what the sky was telling them, they had to understand every bit of noise that their own equipment made.

To imagine how small this is, think of a large, calm lake. If the lake were perfectly still, its surface would be completely flat. Differences of one part in 100,000 are like ripples so small that you could never see them with your eyes. Yet these tiny ripples matter enormously. Where the young universe was very slightly denser, gravity pulled a little more strongly. Over hundreds of millions of years, those places gathered more and more matter. The tiny ripples in the oldest light are the seeds of the galaxies, including our own.

Later satellites made sharper and sharper maps of the whole sky. The maps look like a speckled egg, with spots of slightly warmer and slightly cooler light. By studying the sizes and patterns of these spots, astronomers can measure many things about the universe: how old it is, how much ordinary matter it contains and how much of the strange matter and energy that we will meet on later nights. The age of about 13.8 billion years that this book uses comes largely from such maps.

So the oldest light is not only proof of a hot beginning. It is also a kind of baby photograph of the universe, taken when it was about 380,000 years old. Compared with its age today, that is like a photograph of a person taken in the first day of their life. The face is not yet the face of the adult, but the shape is already there.

A Message to Sami

Before sleep, Noor floated by the large window for a few minutes. The station was over the night side of the Earth, and the sky above the thin line of the atmosphere was black and full of stars. She knew that the oldest light was there too, between the stars and behind them, invisible to her eyes but filling every direction. The window, the station and her own hands were bathed in it, very faintly.

Earlier in the evening, Ade had shown Tomás and Noor one of the maps of the oldest light on a screen in the galley. Tomás said that it looked like the skin of an orange seen very close. Ade said that every galaxy they could see from the window had grown from small differences like those spots, enlarged by gravity over billions of years. Nobody spoke for a while after that, which on a busy station is a kind of compliment.

She wrote to Sami: "The hiss in your radio is probably a nearby mast or the engine. But in radio hiss, a tiny part can come from the oldest light in the universe. In a way, you may have been listening to a faint trace of the beginning on your night drives." Sami answered an hour later, on his break, that he would never again be annoyed by that hiss between the two towns.

Perhaps, after your own working day, you have a sound like that in your life: the hum of a fridge, the noise of traffic, the hiss of an old radio. Most of it, of course, comes from ordinary things nearby. But the oldest light is there too, every evening, coming from every side. It was released long before there were stars, long before the Earth, and it has been travelling all that time to reach the place where you are sitting now.

Common Misunderstandings

Many people think that the oldest light comes from one place, far away, where the Big Bang happened. It is natural to picture it that way, because we usually think of light as coming from a lamp or a star. But the oldest light comes from every direction equally, because the hot fog filled the whole universe, not one spot inside it. Wherever we look, we see the place in the distance where the fog cleared for the light that is reaching us now. An astronomer in a galaxy very far away would see the same glow all around them, coming from their own distant surroundings.

Many people also think that, with a big enough telescope, we could look past the oldest light and see the beginning itself. With ordinary light, and with any other kind of wave like it, we cannot. Before about 380,000 years, the universe was a fog that light could not cross, so no light from earlier times can reach us in a straight line. It is like trying to see the inside of a cloud from the outside. Scientists learn about earlier times in other ways: from the amounts of the lightest elements, which is the story of our next night, and from the patterns in the oldest light itself.

Finally, some people have heard that the oldest light was "only the snow on old television sets". The truth is a little gentler than that. Old television sets showed a moving pattern of dots, called snow, when they were not tuned to a station. Most of that snow came from the set itself, from other electrical equipment and from radio sources nearby. But a small part of it did come from the oldest light. So anyone who ever sat in front of an untuned television was, without knowing it, watching a tiny echo of the young universe. On the next night, we will go back even further, to the first few minutes, and see what the hot fog was made of.