Afterword

Fifteen nights have passed. Noor has packed her notebook, said goodbye to the window and gone home to the heavy, familiar pull of the Earth. Sami has read his last message on his break, somewhere on a quiet road in the middle of the night. And you, I hope, have spent a few evenings with one of the largest stories there is.

It is a strange story when you think about it. It begins about 13.8 billion years ago, in a state so hot and dense that no atom could survive in it. It passes through a glowing fog, a long darkness and the first starlight. It travels through galaxies that grow and join together, stars that build new elements and scatter them, and a cloud that fell inward to become the Sun, the Earth and, in time, us. Every part of that story was pieced together by people who looked carefully, measured patiently and were willing to change their minds when the evidence asked them to.

Many of those people were working people. They had jobs, schedules and bosses. They measured photographic plates for long hours, kept old antennas running, taught classes and wrote calculations late at night. Some of them were not believed at first. Some of them were wrong for many years before they were right. Their work reminds us that understanding the universe is not only a matter of genius. It is also a matter of care, honesty and showing up again the next day.

The story is not finished. Nobody yet knows what dark matter is, what dark energy is, why there was more matter than antimatter, or what happened at the very first moment. Perhaps some of these questions will be answered within your lifetime. Perhaps some will take much longer. Either way, the sky above your street is part of the same story, and it is there every clear night after work, waiting for you to look up.

Reading This Book With Others

This book can be read alone, one chapter an evening, but it can also be shared. Some people read a chapter aloud to a partner or a grown-up child. Some talk about a night's story with a coworker during a break. Others read it in a small group and meet once a week to talk about two or three nights at a time.

If you read with others, you might begin each meeting by sharing one thing that surprised you and one thing that is still unclear. The unclear things are often the best part of the conversation. You do not need to find the answers together. It is enough to say the question clearly, and then to notice which parts of the book have already been answered by evidence and which are still open.

It can also be pleasant to step outside together after a meeting, even in a bright place, and look for the Moon or a few bright stars. Talking about the light you can see, and how long it has travelled, often makes the stories of the book feel close and real.

One Question for Each Night

Night 1: When you first heard the words "Big Bang", what picture did you have in your mind, and how has that picture changed?

Night 8: The light of the Sun left it about eight minutes before it reaches you. What else in your daily life arrives as a message from the past?

Night 19: For a long time, people thought the Milky Way was the whole universe. Why do you think it was so hard to imagine that there was more?

Night 31: If every distant galaxy is moving away from us, why does this not mean that we are at the centre?

Night 44: What is the difference between a good argument and good evidence, and why did the question of the beginning need evidence?

Night 57: Two engineers trying to remove a noise found the oldest light. Have you ever found something important while trying to solve a different problem?

Night 70: The hydrogen in a glass of water was made in the first minutes of the universe. How does that change the way you see an ordinary glass of water?

Night 84: Today's physics stops working at the very first moment. How do you feel about a question that nobody can answer yet?

Night 97: The first stars probably lived short, bright lives. What do you think the universe would be like without them?

Night 110: Most of the matter in the universe is dark matter, and nobody knows what it is. Why do scientists still believe it is there?

Night 124: Which atoms in your body would you most like to trace back to the star that made them, and why?

Night 137: The Sun is not one of the first stars. What does it mean to you that earlier stars had to live and end before the Earth could form?

Night 150: When the measurements showed that the expansion is speeding up, scientists accepted a result that nobody had expected. Why is that hard to do, in science and in ordinary life?

Night 164: The observable universe has a horizon, not a wall. How would you explain the difference to a friend?

Night 178: If the whole history of the universe were one working day, all of written history would fit into a tiny part of the last second. What does that make you feel about your own day?

Quiz: Looking Back at the Fifteen Nights

Here are ten questions for a quiet evening. The answers follow after the last question, so you can check yourself or quiz a friend.

Question 1: About how old is the universe, according to current measurements?

Question 2: Why do astronauts on the space station float?

Question 3: About how long does light from the Sun take to reach the Earth?

Question 4: What did Henrietta Leavitt discover about certain pulsing stars, and why did it matter?

Question 5: What does it mean when the light of a distant galaxy is redshifted?

Question 6: When was the oldest light released, and what is its temperature today?

Question 7: By mass, roughly how much of the ordinary matter made in the first minutes was hydrogen, and how much was helium?

Question 8: What is the difference between dark matter and dark energy?

Question 9: Where were the carbon, oxygen and iron in your body made?

Question 10: Is the edge of the observable universe a wall?

Answers

Answer 1: About 13.8 billion years. Answer 2: Not because there is no gravity, but because the station and everyone inside it are falling around the Earth together; at the station's height, gravity is still about nine-tenths as strong as on the ground. Answer 3: About eight minutes. Answer 4: She found that the time a Cepheid star takes to brighten and fade again tells how bright it really is, so astronomers could use these stars to measure great distances.

Answer 5: Its light arrives with longer, redder waves, because space has stretched while the light travelled; for distant galaxies, it shows that they are moving away from us. Answer 6: At about 380,000 years after the beginning, when atoms formed and light could travel freely; today it is about 2.7 degrees above absolute zero. Answer 7: About three-quarters hydrogen and about one quarter helium, with a tiny amount of lithium.

Answer 8: Dark matter is invisible matter that we know only by its pull; it helps hold galaxies together. Dark energy is the unknown cause of the speeding-up of the expansion of the universe. Nobody yet knows what either of them really is. Answer 9: Inside stars, and in the explosions and collisions at the end of their lives. Answer 10: No. It is a horizon, the limit of how far light has had time to travel to us, and the universe almost certainly continues beyond it.