Chapter 14: Questions at the Edge

Night 164: The Quiet Hours

On mission day 164, Noor had an unusual shift. An experiment on the station needed a small adjustment in the middle of the crew's night, at a time when the teams on the ground wanted someone awake to watch it. Tomás and Ade would sleep as usual. Noor would go to bed early, wake up when her alarm rang, do the work and keep an eye on the experiment for a few hours before she slept again. The ground would be watching too, from their screens far below. She would not really be alone, but the station would feel very quiet.

Her alarm rang at the agreed time. She unzipped her sleeping bag, pushed gently out of her small cabin and drifted through the dim module. At night, the lights on the station are turned low. The fans hummed, as they always do, because in weightlessness air does not move by itself, and without the fans, the breath of a sleeping person could gather around their face. Small green and blue lights glowed on the control panels. Somewhere a pump clicked on and off.

The adjustment took only twenty minutes. Noor followed the procedure on her tablet, step by step, and the ground confirmed each step in a soft voice through her headset. Then there was nothing to do but watch the numbers on a screen and wait. She floated to the large window. The station was on the night side of the Earth. Below, the planet was a dark shape, with thin threads of light where people live and, here and there, the flicker of a distant thunderstorm. Above, there were more stars than she could count.

Along the edge of the dark planet, there was a thin, faint glow, a little greenish, lying above the Earth like a ribbon. It is called airglow. High in the atmosphere, atoms and molecules that were energized by sunlight during the day slowly give back that energy as a very weak light. From the ground, it is almost impossible to notice. From the station, on a dark night, it outlines the whole curve of the Earth. Noor had seen it many times by now, but it still made her stop and look. Then the station rushed on, and before long a bright line of dawn appeared at the edge of the planet, and the night was over until the next orbit.

In the quiet, Noor thought of Sami, who was probably driving on a dark road at that very moment. People who work at night know this feeling. The world is asleep, the work is calm, and the mind begins to wander into large questions. Earlier in the day, Sami had sent a message with exactly such a question. "If the universe began 13.8 billion years ago," he wrote, "where is its edge? And what was there before it began?"

These are some of the oldest questions people have asked, and some of the hardest. Tonight's story is about them. It is a story with more open doors than closed ones. We will see what astronomers can say, and we will be honest about the places where nobody knows.

How Far Can We See?

Let us begin with what we can see. Light travels at about 300,000 kilometres per second, and the universe is about 13.8 billion years old. So it is natural to think that we can see only 13.8 billion light-years in every direction, because light from farther away has not had time to reach us. This idea is close to the truth, but it misses one important detail: while the light was travelling, space was stretching.

Imagine a worker walking along a moving walkway at an airport, except that the walkway is growing longer as she walks. She walks at a steady speed, but by the time she reaches the end, the place where she started is much farther behind her than the distance she walked. Light from the most distant places behaves a little like this. It has been travelling towards us for nearly 13.8 billion years, but during that time, the space between us and its starting point has stretched enormously.

So the most distant places whose light has reached us are now much farther away than 13.8 billion light-years. According to current measurements, the edge of the region we can see is now about 46 billion light-years away in every direction. Astronomers call this region the observable universe: the part of the universe from which light has had time to reach us since the beginning. The oldest light, which we met on an earlier night, comes from very close to this edge.

Looking towards this edge is also looking back in time. The farther we look, the earlier the light left its source. The most distant galaxies that telescopes have found are seen as they were when the universe was only a few hundred million years old. Beyond them, there is the oldest light, released about 380,000 years after the beginning. And beyond that, ordinary light cannot take us, because before that time the universe was a glowing fog that light could not cross. So the observable universe is not only a space. It is also a long record of history, with the youngest pages closest to us and the oldest pages farthest away.

It is important to understand what this edge is. It is not a wall, and it is not the end of the universe. It is a horizon, like the horizon you see at sea. When you stand on a beach, you can see the water only up to a certain line, but the sea does not stop there. The line is only the limit of your view. In the same way, the edge of the observable universe is the limit of what we can see from where we are. A being in a galaxy far away would have its own observable universe, centred on itself.

Beyond the Horizon

What lies beyond our horizon? Almost certainly, more universe. As far as we can tell, the universe looks much the same in every direction and at every place on the largest scales: galaxies, clusters of galaxies and the great threads and empty regions of the cosmic web. There is no sign that the universe changes or comes to an end just beyond what we can see. So most astronomers expect that beyond our horizon, there are more galaxies, much like the ones we know.

How do astronomers know that the universe looks the same in every direction? They count and map galaxies across the whole sky and find that, on the largest scales, they are spread out in a similar way everywhere. The oldest light has almost exactly the same temperature in every direction, with only tiny differences. If our part of the universe were special, for example close to some centre or edge, we would expect to see a difference between one side of the sky and the other. No such large difference has been found.

But how much more? Here we reach a real unknown. The universe may be extremely large but still finite. Or it may go on without end. Both possibilities fit what we can measure today. Measurements show that, on the largest scales we can see, space is very nearly "flat", which is a way of saying that it does not seem to curve back on itself the way the surface of a ball does. If it does curve, the curve is so gentle that we cannot yet detect it within our horizon. So we do not know whether the whole universe is finite or infinite, and we may never be able to know for certain.

This can be a strange thought to hold at three in the morning. But there is also something calm in it. For most of human history, people did not know what lay beyond the next mountain range or the far side of the ocean. They went on working, building and wondering. We are in a similar position, only on a much larger scale. Our horizon is wider than any horizon people have ever known, and still there is more beyond it.

It is also worth remembering that the observable universe is not small. It contains hundreds of billions of galaxies, perhaps more, and each galaxy may contain hundreds of billions of stars. There is more inside our horizon than anyone will ever be able to study in detail. The unknown beyond it does not make the known any less wonderful.

An Idea Called Inflation

Now let us go back to the beginning, to the very first moments. On earlier nights, we followed the story from the first fractions of a second through the first minutes to the oldest light. But there are some puzzles about the very start that the basic picture of the hot beginning does not explain by itself.

One puzzle is that the oldest light has almost exactly the same temperature in every direction. Yet light from opposite sides of the sky comes from regions that, in the basic picture, never had time to share heat with each other. How did they come to be so alike? It is a little like walking into two separate rooms at the opposite ends of a large building and finding that the two cups of tea on the tables are exactly the same temperature, even though nobody could have carried heat from one room to the other. Another puzzle is why space is so nearly flat.

Around 1980, scientists proposed an idea to answer these puzzles. It is called inflation. According to this idea, in a tiny first moment, far less than a second, space grew extremely fast, much faster than it has grown since. A very small region, which had already become even in temperature, was stretched to a huge size. This would explain why distant parts of the sky look so alike, and it would also smooth out any curve, making space look flat, the way a small part of a huge balloon looks flat.

Inflation also suggests where the tiny ripples in the oldest light came from. In the idea of inflation, extremely small random variations, smaller than atoms, were stretched to enormous sizes during the fast growth. Those stretched variations became the slightly thicker and thinner regions that later grew into galaxies. The pattern of ripples that satellites have measured in the oldest light fits the predictions of inflation quite well.

So is inflation true? The honest answer is that we do not know yet. Some observations fit it, and many scientists consider it the best idea we have. But it has not been proven. There are different versions of the idea, and some scientists have suggested other explanations. Astronomers are looking for certain faint patterns in the oldest light that inflation might have left behind. If they find them, the idea will become much stronger. If they do not, the question will stay open. For now, inflation is an idea with support, not an established fact.

Before the Beginning?

And what about the second part of Sami's question: what was there before the beginning? This is where honest science must say very clearly: we do not know. As we saw on an earlier night, our present laws of physics stop working when we try to describe the very first moment. We have no tested theory that can tell us what happened at that point, or whether there was anything earlier.

Many scientists hope that a new theory will one day help with this. Today, physics has two very successful theories that work in different areas. One is Einstein's theory of gravity, which describes stars, galaxies and the expansion of space. The other describes the world of the very small, of atoms and the particles inside them. At the very first moment, the whole universe was both extremely heavy and extremely small, so both theories would be needed at once. But nobody has yet found a complete and tested way to join them. Until someone does, the very first moment remains beyond the reach of our knowledge.

It may even be that the question itself has no meaning. Think about the question "What is north of the most northern point on Earth?" On the surface of the Earth, north is a direction, but if you stand at the very top of the planet, every direction is south, and there is no farther north to go. Some scientists think that time itself may have begun with the universe. If so, asking what happened "before" the beginning would be like asking what is north of the most northern point. This is not a new thought. More than a thousand years ago, some thinkers already suggested that time might have begun together with the world, and was not something that existed on its own before it.

Other scientists have explored the idea that our universe came out of something earlier, perhaps an earlier phase or a larger whole. Some versions of inflation and of other theories suggest that there could be many universes, each with its own beginning. These ideas are interesting, and some are carefully worked out in mathematics. But so far, there is no observation that shows that any of them is true. They are possibilities to explore, not discoveries.

Many people find this uncomfortable at first. We like stories that start with "once upon a time" and know what came before. But there is a kind of honesty in saying "we do not know yet" that is worth respecting. Every chapter of this book began with things that were once unknown: the distance of galaxies, the growth of space, the oldest light. Each of those questions was answered by patient work. Perhaps some of today's questions will be answered too, by people who are now children or by people who are not yet born.

A Message Written at Night

The numbers on the screen stayed calm. The experiment was behaving well. Shortly before her watch ended, the ground told Noor that she could go back to sleep. She thanked them, floated to the window one last time and looked at the stars above the dark Earth.

Then she wrote to Sami. "I am awake at the same time as you tonight," she began. "Your question has no full answer. We can see about 46 billion light-years in every direction, because space stretched while the light travelled. Beyond that, there is probably more universe, but nobody knows how much. And nobody knows what was before the beginning, or whether there was a before. Drive safely. We are both working in the dark tonight."

Sami read it a few minutes later, parked at the side of an empty road. He looked up through the windscreen. "Then we are both looking at the same unknown," he wrote back. "Good night, sister. Or good morning. I am never sure up there."

Noor zipped herself back into her sleeping bag when the watch was over. The station was still quiet. Tomás was asleep in his cabin, and Ade was asleep in his. In a few hours, the lights would brighten, the alarms of the others would ring, and the working day would begin again. For now, she closed her eyes with the questions still open, and she found that she did not mind.

If you are reading this late, after a long shift or a long day, you may also be awake while most people sleep. Night can make big questions feel heavy. But they can also feel like company. Under the same sky, people have been asking where the edge is and what came before for thousands of years. You are one of them tonight.

Common Misunderstandings

Many people think that the universe has an edge, like a wall at the end of a room, and that beyond the wall there is empty space or nothing at all. But no observation has ever shown such an edge. The edge of the observable universe is a horizon, the limit of our view, not the end of the universe. There is no known place where the galaxies stop and a wall begins. As far as we can tell, the universe has no edge of that kind.

Many people also think that, because the universe is about 13.8 billion years old, we can see only 13.8 billion light-years away. This seems logical, and it would be true if space had not been stretching. But because space has been stretching while the light travelled, the places where the most distant light started are now much farther away. That is why the edge of the observable universe is now about 46 billion light-years away in every direction, even though no light has travelled for more than about 13.8 billion years.

Finally, some people think that science has shown that there are many universes. You may have seen this idea in films or articles, sometimes presented as if it were a known fact. In reality, ideas about many universes are possibilities that come out of some theories. They have not been observed, and at present, there is no evidence that shows that other universes exist. They remain speculation, interesting but unproven. The next night is the last night of Noor's stay, and we will look back over the whole story, from the beginning to the evening sky above your own home.