Chapter 13: The Mystery of Dark Energy

Night 150: A Gentle Push

On mission day 150, the station was going to raise itself a little higher above the Earth. This is called a reboost, and it happens from time to time. At about 400 kilometres up, there is almost no air, but there is not quite none. A very thin trace of the atmosphere still reaches that high. Every second, the station runs into a great many of those thinly scattered air particles, and together they slow it down by a tiny amount. Over weeks and months, the station slowly sinks lower. So, every so often, its engines fire to push it back up.

Noor found it a quietly surprising fact. Even here, far above the clouds and the weather, the station was not completely free from the Earth's air. Without these small pushes, the station would sink lower and lower over the years, the air would become thicker around it, and in the end it would come down into the thick air below. So the reboost is a kind of routine maintenance, like checking the tyres of a vehicle or oiling a door. Nobody on the ground sees it, but it keeps everything working.

The reboost was planned by the teams on the ground, and the engines were controlled from there and by the station's computers. The crew's job was to prepare. Noor and Tomás went through the modules and checked that nothing loose was floating where it could be pushed by the small change in speed. Tools were put away. Cameras were fixed in place. An experiment that measures tiny movements was paused. Noor tightened the straps on a bag of clothes that she had been meaning to tidy for days.

When the engines fired, there was no roar. There was only a soft, steady push. Noor was holding a handrail near the window, and she felt a gentle tug through it as the station moved forward around her. A pen that someone had forgotten drifted slowly towards the back of the module. Tomás caught it without looking, as he catches most things. After several minutes, the engines stopped, and everything floated as before. The station was now a little higher and would stay up longer.

A few minutes later, a calm voice from the ground confirmed that the reboost had gone as planned. Tomás wrote the time into the log, and Noor and Ade went back through the modules, undoing the morning's preparations one by one. The paused experiment started again. The cameras were unfastened. By lunchtime, the station looked exactly as it had before, only a little higher above the clouds.

That evening, Noor told Sami about the reboost. He works with vehicles every night, and he understood at once. "So it is like the truck going uphill," he wrote. "If you stop pressing the pedal, you slow down." Then he asked something else. "Does the universe slow down too? If everything flew apart after the Big Bang, shouldn't it be slowing down by now?"

Noor smiled at the screen. For most of the twentieth century, astronomers asked exactly the same question, and they expected the same answer. Tonight's story is about what they found instead. It is one of the biggest surprises in the history of astronomy, and it is still a mystery.

Expecting the Slowdown

Think of throwing a ball straight up into the air. As it rises, the Earth's gravity pulls on it, and it slows down. If you throw it gently, it slows, stops and falls back into your hand. If you could throw it extremely hard, it might escape the Earth completely, but it would still be slowing down all the way, because gravity never stops pulling.

For many years, astronomers thought of the expanding universe in a similar way. After the hot beginning, the galaxies have been moving apart, because space has been stretching. But all the matter in the universe pulls on all the other matter through gravity. So it seemed natural to think that this pull would slowly brake the expansion. The only question seemed to be how strong the braking was. Would the expansion slow down so much that it one day stopped and turned around? Or would it slow down forever without quite stopping?

Albert Einstein's theory of gravity, which he completed in 1915, allowed several different futures for a universe full of matter. In 1922, Alexander Friedmann worked out some of these possibilities on paper. In one kind of universe, the expansion slows and then reverses, and everything falls back together. In another kind, the expansion slows but never stops. Which kind we live in would depend on how much matter the universe contains. For decades, measuring that amount and the slowing of the expansion was one of the central tasks of astronomy.

To answer that, astronomers needed to compare the expansion now with the expansion long ago. Remember that looking far away means looking back in time. If you can measure how fast very distant galaxies were moving away when their light left them, and compare that with nearby galaxies, you can see whether the expansion has changed. It is a little like checking the speed of a vehicle on an old recording and comparing it with its speed today.

The difficult part was measuring distances to galaxies that are extremely far away. The pulsing stars that Henrietta Leavitt studied are bright, but not bright enough to be seen at the greatest distances. Astronomers needed a much brighter kind of lamp, one whose true brightness they knew. They found it in a special kind of exploding star.

Exploding Stars as Measuring Lamps

Some exploding stars begin as white dwarfs, the small, dense remains of stars like the Sun that we met on an earlier night. When a white dwarf has a close partner star, it can slowly gather matter from that partner. When it becomes too heavy, it explodes in a very bright and fairly regular way. Explosions of this kind reach almost the same peak brightness each time, and astronomers learned how to correct for the small differences between them. This made them useful as measuring lamps that can be seen across billions of light-years.

The method is simple to describe, even though the work is very hard. If you know how bright a lamp truly is, you can tell how far away it is by how faint it looks. A street lamp at the end of a long road looks dim, not because it is weak, but because it is far away. In the same way, a faint exploding star of this kind must be very far away.

In the 1990s, two separate teams of astronomers set out to use these explosions to measure the slowing of the expansion. The work required patience. These explosions are rare in any one galaxy, so the teams had to watch large numbers of galaxies, catch the explosions while they were bright and then measure them carefully with large telescopes. Many people worked through long nights, compared notes and argued about each step.

In 1998, both teams reported the same result. The distant explosions were fainter than expected. That meant they were farther away than they should have been if the expansion had been slowing down. The best explanation was the opposite of what almost everyone had expected. The expansion of the universe was not slowing down. For the last several billion years, it has been speeding up.

What made the result so convincing was that it came from two separate teams. The teams worked independently, used their own telescopes and their own methods of checking, and they were, in a friendly way, competing to be first. If only one team had found the speeding-up, other scientists might have suspected a mistake in its work. When both teams found the same surprising answer, the result became much harder to dismiss. In science, as in many workplaces, a result that two careful groups reach separately is worth more than a result that only one group has seen.

Many scientists were at first unsure. Perhaps dust was dimming the light? Perhaps the explosions long ago were a little different? These questions were tested again and again in the years that followed. Other kinds of measurement, including detailed maps of the oldest light and of the way galaxies are spread through space, have since pointed in the same direction. The speeding-up of the expansion is now a widely accepted part of the picture.

The full picture that came out of this work has two parts. For the first several billion years after the hot beginning, the expansion really was slowing down, just as astronomers had expected, because matter was packed more closely and its gravity was strong. But as space stretched and matter thinned out, its braking became weaker. Several billion years ago, an unknown push, which we will name in a moment, became stronger than the braking of matter, and the expansion began to speed up. So the astronomers of the twentieth century were not wrong about gravity. They were only missing a second, unexpected part of the story.

Giving the Mystery a Name

If the expansion is speeding up, something must be causing it. Scientists do not know what that something is. They gave it a name anyway: dark energy. It is called "dark" because it does not shine and cannot be seen directly, and "energy" because it seems to act on the whole universe. But the name is a label on an empty box. It tells us that something is there; it does not tell us what it is.

According to current measurements, the universe's total content can be divided roughly like this. Ordinary matter, the kind that makes up stars, planets, gas and people, is only about 5 percent. Dark matter, which we met on an earlier night, is about 27 percent. Dark energy is about 68 percent. In other words, everything we have ever seen with our telescopes is only a small part of what the universe contains. The rest is known only by its effects.

It may help to picture this as a household budget. Imagine that you study your spending for a month and find that only five parts in every hundred go to things you can name: rent, food, travel. About twenty-seven parts go to something you can feel the effect of but cannot see on any receipt. And about sixty-eight parts go to something even stranger, which seems to make the whole budget grow larger every month. You would want very much to understand those two hidden items. That is the position astronomers are in today.

One possibility is that dark energy is a property of empty space itself. If even completely empty space has a small amount of energy in it, then as space stretches, there is more empty space, and more of this energy. This energy would push the expansion along. Interestingly, this idea brings back a term that Albert Einstein added to his equations in 1917, when he wanted to describe a universe that stood still. He later gave the term up when the expansion was discovered. Today, the same term returns in a new role, not to keep the universe still but to describe its speeding-up.

But this is not the only possibility. Some scientists think that dark energy could change over time. Others wonder whether our theory of gravity needs to be changed when we look at the largest distances. At present, there is no agreed answer. Large new surveys of the sky are measuring the positions and distances of huge numbers of galaxies, to see more exactly how the expansion has changed over time. These measurements may one day show which ideas are on the right track.

Two Measurements That Do Not Agree

There is another open question, related to the expansion, that astronomers are working on today. It is about how fast the universe is expanding right now. There are different ways to measure this. One way is to measure the distances and speeds of nearby galaxies, using pulsing stars and exploding stars, as in the work we have described. Another way is to start from the detailed map of the oldest light and calculate, using our best theory of the universe, how fast the expansion should be today.

Both methods are careful, and both have been checked many times. Yet they give slightly different answers. The difference is not huge, but it is larger than the expected errors of the measurements. It is a little like two careful workers measuring the same room with two different tools and getting results that differ by a small but stubborn amount. Each worker checks their own tool, and each tool seems to be fine.

Nobody yet knows why the answers differ. Perhaps there is a small error hidden in one of the methods that nobody has found. Perhaps our theory of the universe is missing something, for example something about dark energy or about the early universe. Scientists are testing both possibilities with new telescopes and new methods. It is an honest example of how science works: when two good measurements disagree, scientists do not choose the one they like. They keep measuring until they understand.

New telescopes are helping with this question. The James Webb Space Telescope, which sees infrared light, can look through dust and pick out individual pulsing stars in galaxies more clearly than earlier telescopes could. Astronomers are using it, and other telescopes, to check the measurements of nearby distances step by step. Meanwhile, other teams are improving the calculations based on the oldest light. Each new result is followed closely, because a clear answer, whichever way it goes, would teach us something important.

For a working person, there is something familiar in this. Many of us have had a problem at work that would not go away, a number that did not add up or a machine that sometimes stopped for no clear reason. The solution often comes slowly, from patient checking. The universe, too, is being understood one patient check at a time.

Before Sleep

Late in the evening, Noor floated to the window. The station had risen a little higher that day, but from inside, nothing looked different. The Earth turned below, and the stars shone above the edge of the air. Somewhere beyond those stars, galaxies are moving apart a little faster each year, pushed by something that no one has ever seen.

She wrote to Sami. "You were right to ask. Astronomers expected the universe to be slowing down, like your truck going uphill without the pedal. But when they measured, they found the opposite. The expansion is speeding up. They call the cause dark energy, but nobody knows what it is. So tonight, your question is one of the biggest questions in science."

Before she slept, she thought about how strange it was to work on a station that needs pushing to stay where it is, inside a universe that seems to be pushed outwards by something no one understands. The station's slow sinking and the universe's speeding-up were both noticed in the same way, by people who looked carefully at numbers and saw that something did not fit.

At three in the morning, Sami read it in the cab of his truck. "So the universe has its foot on the pedal," he wrote back, "and nobody knows whose foot it is." Noor read his answer when she woke up, and she thought it was not a bad description at all.

It may be a comfort to know that even the people who study the universe for their whole working lives do not have every answer. If something in your own life is still unclear tonight, you are in very good company. Some questions are answered quickly. Others need patience, careful work and time.

Common Misunderstandings

Many people think that dark energy and dark matter are the same thing, because both have "dark" in their names. They are, in fact, very different. Dark matter pulls things together. Its gravity holds galaxies and clusters of galaxies together and helped them form. Dark energy seems to do the opposite: it pushes the expansion of the universe to go faster. Both are called "dark" because we cannot see them directly, and both are still not understood. But they are two separate mysteries.

Many people also think that dark energy is a kind of energy that we could collect and use, like the energy in a battery or in sunlight. It sounds like that because of its name. But dark energy, whatever it is, is spread extremely thinly through space. In the space of a room, or even of the whole Earth, there would be almost nothing of it. Its effect only becomes important over the enormous distances between groups of galaxies. It is not a source of power for machines, and nobody knows how it could be used, even in principle.

Finally, some people think that scientists already know how the universe will end, for example that it will tear itself apart, freeze or fall back together. You may have read such ideas presented as facts. The truth is that the far future depends on the nature of dark energy, and we do not know its nature yet. If it stays as it seems to be now, the expansion will go on speeding up, and distant galaxies will slowly move out of sight. But if dark energy changes, the future could be different. On the next night, during a quiet watch, we will go to the very edges of what we know and ask what lies beyond them.