Chapter 4: The Galaxies Are Moving Apart

Night 31: Catching a Cargo Ship

On the thirty-first day of her stay, Noor caught a spaceship with a robotic arm. The cargo ship had been launched two days earlier and had spent the time slowly closing the distance to the station, firing its small engines again and again under the careful watch of the teams on the ground. By the afternoon it was close enough to see through the windows: a white cylinder with wide solar panels, moving almost exactly as fast as the station, so that it seemed to hang still in the black sky above the bright Earth.

Noor sat at the robotics workstation, a small desk with screens and two hand controllers, one for each hand. Tomás sat beside her, reading the checklist aloud. The cargo ship stopped about ten metres away and waited. On the ground, the teams confirmed that everything was ready. Then Noor moved the long arm forward, very slowly. One hand controlled the direction, the other the turning of the arm's end. On the screen, she could see a camera view from the tip of the arm, with a small fitting on the cargo ship growing larger and larger.

The work required calm hands and steady breathing. Both the station and the cargo ship were travelling at about 28,000 kilometres per hour around the Earth, but relative to each other they were almost still, like two cars moving side by side on a motorway. When the fitting was in the centre of the screen, Noor pressed the trigger. The arm's end closed around the fitting with a soft, firm grip. "Capture," she said. On the ground, someone began to clap.

The rest of the day was spent connecting the cargo ship to the station and checking that the hatch could be opened safely. When it finally opened, the first thing Noor noticed was the smell. It smelled of new plastic, of packaging and, faintly, of apples. Somewhere among the bags of equipment, spare parts, clothing and food, the ground teams had packed a small box of fresh fruit. That evening, the crew shared apples by the large window, holding them carefully so that no drops of juice escaped.

Watching the cargo ship from the window earlier, Noor had thought about how hard it is to tell whether a distant object is moving towards you or away from you. Up close, the robotic arm's camera made it easy. But how could anyone tell how a galaxy is moving, when it is millions of light-years away and looks like a still smudge on a photograph? That was tonight's story, and it began with a rainbow.

Light Has Fingerprints

When sunlight passes through a glass prism, or through drops of rain, it spreads out into a band of colours: red, orange, yellow, green, blue and violet. That band is called a spectrum. Every colour in it is light of a slightly different wavelength, which means that its waves are spaced a slightly different distance apart. Red light has longer waves. Blue and violet light have shorter waves. The spectrum is simply light sorted by the length of its waves, like a pile of letters sorted by size.

In the 1800s, scientists found that if they spread light out carefully with a good instrument, the spectrum was not smooth. It was crossed by thin dark lines, like the gaps in a barcode. They soon learned what the lines meant. Each kind of atom, such as hydrogen, sodium or iron, absorbs and gives out light only at certain exact wavelengths. When light passes through a gas, the atoms in that gas remove their own particular wavelengths and leave dark lines in exactly those places.

You may have seen a simple example of these fingerprints without knowing it. Some older street lamps glow with a deep orange-yellow light. They are filled with a gas containing sodium, and sodium atoms, when they are made to glow, give out light mainly at one particular yellow wavelength. If you looked at the light of such a lamp through a good prism, you would see almost no rainbow at all, only a bright yellow line. Every kind of atom has its own set of lines like this, as personal as a signature. A hot gas shows them as bright lines, and a cooler gas in front of a bright source shows them as dark lines in the same places.

This was a wonderful discovery, because it meant that light carries fingerprints. By looking at the pattern of lines in the light of a star, astronomers could tell which kinds of atoms were present in it, without ever going there. The pattern of hydrogen is always the same, whether the light comes from a lamp in a laboratory or from a star far across the galaxy. Astronomers learned to recognise these patterns as easily as you recognise the handwriting of a friend.

But astronomers soon noticed something else. In the light of some stars and nebulae, the familiar patterns of lines were all there, but they were not quite in the right places. The whole pattern was shifted a little, either towards the red end of the spectrum or towards the blue end. The fingerprint was the same, but it had slid along the band of colours. That small shift turned out to carry an enormous message.

A Passing Siren

You have heard something like this shift many times, with sound instead of light. Think of an ambulance passing you on the street with its siren on. As it comes towards you, the siren sounds higher. As it passes and moves away, the sound suddenly drops lower. The siren itself has not changed. The driver has not touched any switch. What changes is the spacing of the sound waves as they reach your ears.

When the ambulance moves towards you, each new wave of sound starts a little closer to you than the one before, so the waves arrive bunched together. Bunched waves sound higher. When the ambulance moves away, each new wave starts a little farther away, so the waves arrive stretched apart. Stretched waves sound lower. The faster the ambulance moves, the bigger the change. A trained ear, or a good instrument, could even tell how fast it was going from the change in pitch alone.

Light behaves in a similar way. When a source of light moves towards us, its waves arrive a little bunched, so its light is shifted slightly towards the blue end of the spectrum. When a source moves away from us, its waves arrive stretched, so its light is shifted towards the red end. The shift towards red is called a redshift. The bigger the redshift, the faster the source is moving away. The dark lines in the spectrum make it possible to measure this precisely, because astronomers know exactly where each line should be when the source is not moving.

With galaxies, as we will see shortly, the stretching of light comes mostly from the stretching of space itself, not just from galaxies speeding through space like ambulances. But the siren is a good way to begin. The important point is simple: by looking at how far the fingerprint has slid, astronomers can tell whether a distant galaxy is moving towards us or away from us, and roughly how fast.

These shifts are usually tiny. For the stars in our own galaxy, which move at speeds of tens of kilometres per second relative to the Sun, the lines move only a very small distance along the spectrum, and measuring them takes good instruments and great care. Astronomers use the same method to find planets around other stars, by measuring the small back-and-forth movement a planet's pull gives to its star. For galaxies far away, as Slipher was about to discover, the shifts can be much larger.

The People Who Noticed

The first person to measure the movements of the spiral nebulae was Vesto Slipher, who began this work in 1912 at an observatory with a good telescope and a very patient attitude. The light of a spiral nebula is faint, and spreading it into a spectrum makes it fainter still. Slipher sometimes had to keep a single photographic plate exposed over several nights, adding the faint light little by little, to get one usable spectrum.

His first measurement, of the Andromeda nebula, showed a blueshift: Andromeda is moving towards us. But as he measured more and more spiral nebulae during the 1910s, he found that most of them showed redshifts, and some were moving away at hundreds of kilometres per second. This was very strange. If the spiral nebulae were small clouds inside our galaxy, why were most of them rushing away? At that time, nobody yet knew that they were separate galaxies, and nobody had a good explanation.

Meanwhile, the theory needed to explain them was already waiting. In 1915, Albert Einstein had published his general theory of relativity, a new description of gravity in which space and time can bend and stretch. In 1917, he applied it to the universe as a whole. Like almost everyone at the time, he believed that the universe was still, neither growing nor shrinking. His equations did not naturally give a still universe, so he added an extra term to them to hold it still.

Others looked at the same equations and saw something different. In 1922, a mathematician and physicist named Alexander Friedmann showed that the equations allowed a universe that grows or shrinks over time. In 1927, a priest who taught physics, Georges Lemaître, worked out the same kind of solution independently. He went further: he connected it with the measured redshifts of galaxies and suggested that the universe is expanding. His paper appeared in a journal that few astronomers read, and at first it received little attention.

Then, in 1929, Edwin Hubble, who had already shown that the spiral nebulae were galaxies, compared their distances with their redshifts. He found a clear pattern. The farther away a galaxy is, the faster it is moving away from us. A galaxy twice as far away is moving away about twice as fast. Today, astronomers often call this rule the Hubble–Lemaître law, to recognise both men. After the expansion of the universe became accepted, Einstein gave up the extra term he had added to keep the universe still.

Hubble's numbers were not perfect. Because the distances he used were too small, his value for the speed of the expansion was several times too large, and it took decades of further work by many astronomers to correct it. But the pattern itself, more distance and more speed together, has been confirmed again and again with better instruments. This is common in science. The first measurement shows that something is there, and later measurements, often by other people, show exactly how much. On a later night, we will see that Einstein's extra term returned many years later, in a surprising new role.

Bread Rising in the Oven

What does it mean that farther galaxies move away faster? The simplest picture is one that bakers know well. Imagine a ball of bread dough with raisins mixed all through it. As the dough rises in a warm place, every part of it swells. Every raisin moves away from every other raisin. No raisin is pushing the others. They are simply being carried apart by the dough between them as it grows.

Now pick any raisin and imagine you are standing on it. As the dough rises, the raisins close to you move away slowly, because there is only a little dough between you and them. The raisins far from you move away much faster, because there is a lot of dough between you and them, and all of it is swelling at once. That is exactly the pattern Hubble found. And it does not matter which raisin you choose. From every raisin, all the others seem to move away, and the farther ones faster.

In the universe, the dough is space itself. The galaxies, or rather groups of galaxies, are the raisins. They are not flying through space away from a central point. Instead, the space between them is stretching, so the distances between them grow. And as light travels through stretching space, its waves are stretched too. That is the main reason why the light of distant galaxies arrives with a redshift. The longer the light has travelled through stretching space, the more its waves have been pulled out towards the red.

Every picture has its limits, and the bread does too. A ball of dough has an edge and a centre, and it sits inside a larger oven. As far as we know, the universe has no edge and no centre, and it is not expanding into anything outside it. So the raisin picture is best used for one idea only: everything moving apart from everything else, with the farther things moving apart faster. For that idea, it works beautifully.

There is one more thing the bread can show. If you could watch the rising dough from one raisin, you could measure how fast your neighbours were moving away and how far they were. From those two numbers, you could work out how long ago all the raisins were squeezed together in a small, tight ball, before the dough began to rise. Astronomers did exactly this with the galaxies. The speed of the expansion and the distances of the galaxies together give a rough age for the universe. The careful modern value, using many other clues as well, is about 13.8 billion years.

This discovery changed the question of the beginning completely. If the galaxies are moving apart now, then yesterday they were a little closer together, last year closer still and long ago much closer. A universe that is growing is a universe with a history. For the first time, the question "How did it all begin?" was not only a question for stories and philosophy. It had become a question that could be studied with telescopes and measurements.

A Message to Sami

After the apples, Noor wrote to her brother. "Today I caught a cargo ship with a robotic arm. Tonight's story: the light from far galaxies is stretched towards red, like a siren dropping in pitch as an ambulance drives away. The farther the galaxy, the more stretched the light. Space itself is stretching, like rising bread with raisins in it. There was fresh fruit in the cargo, by the way. I thought of you."

Sami's reply came after his shift. "I hear sirens every night," he wrote. "I will never hear them the same way again." He added that one of the people he delivered to, an old baker who started work at four in the morning, had laughed when he told her about the raisin bread. She had said that she had been making models of the universe for forty years without knowing it.

Noor laughed when she read it, and read it aloud to Tomás and Ade at breakfast. Tomás said that the best explanations always seemed to come from people who work with their hands. Ade said that he wanted the baker's opinion on the next night's story, which was about running the whole film of the universe backwards.

Perhaps tonight, if you hear a car or a train pass by, you can listen for the change in its sound as it goes from coming to going. It is a small everyday thing. But the same idea, applied with great care to the light of distant galaxies, showed that we live in a universe that is growing, and that it must once have been very different.

Common Misunderstandings

Many people think that if all the galaxies are moving away from us, then we must be at the centre of the universe. It certainly looks that way from here. But remember the raisins. From every raisin in the rising dough, all the other raisins appear to move away. An astronomer living in a galaxy billions of light-years from us would see exactly the same thing: all the distant galaxies moving away from her, the farther ones faster. There is no special centre. Every place sees itself as the middle, and none of them is.

Another common belief is that the galaxies are flying through space away from one point, like pieces from an explosion. This picture is very natural, but it is not the best description. The galaxies do move a little through space, pulled by their neighbours. But the main reason the distances grow is that space itself is stretching between them. That is also why there is no point in space where the expansion began. The stretching is happening everywhere at once, and it has been since the beginning.

Some people wonder whether everything is expanding, including the Earth, the station and their own bodies. If space is stretching, are we stretching too? The answer is no. The stretching matters only across the enormous empty distances between groups of galaxies. Inside an atom, a person, a planet, a solar system or a galaxy, the forces that hold things together are far stronger than the gentle stretching of space, so these things keep their size. Your body, your kitchen table and Noor's robotic arm are not growing. And not every galaxy moves away from us. Our close neighbour Andromeda is moving towards us, pulled by gravity, just as Slipher found in his very first measurement. The expansion is a pattern of the universe on the largest scale, not a rule for every object in it. On the next night, we will follow that pattern backwards in time, and see where it leads.