Chapter 7: The First Few Minutes

Night 70: The Water Machine

Day 70 was a maintenance day. On the schedule that the ground sent up each morning, Noor's main task had a plain name: replace a filter in the water recovery system. On a space station, water is too precious to waste. It is heavy to send up from the Earth, so the station collects almost every drop it can and cleans it again and again. The water vapour that the crew breathes out and the sweat that leaves their skin are taken from the air. Washing water is collected. Even urine is collected, cleaned and turned back into drinking water.

Noor had trained for this job on the ground many times, on a copy of the machine. On the station, it took most of the morning. She closed valves in the right order, checked the pressure readings, opened a panel and slid out the old filter. Every loose part was tied down or held in a bag. Ade floated nearby with the checklist and read each step aloud before she did it, and Noor repeated the step back to him. On the ground, this might seem slow. In orbit, it is how mistakes are avoided.

When the new filter was in place, the machine ran its tests. A row of small lights turned green, one after another. The teams on the ground watched the same readings on their screens and confirmed that everything looked normal. Noor wrote the time in the log and put the old filter in a bag, to be packed in a cargo ship and taken away later.

At lunch, she filled her drinking pouch from the station's water supply and held it up to the light. The water was perfectly clear. It tasted of nothing at all, which is exactly how good water should taste. Tomás raised his own pouch and said, as he did every time someone repaired the water machine, that yesterday's coffee had become today's coffee. Ade said that this was not a sentence he wanted to think about while eating.

That evening, Sami wrote that he had been unloading boxes of bottled water at a supermarket all night, and that he was glad nobody asked him to drink the water twice. Noor told him that, in a way, everyone on Earth drinks water that has been used before, by clouds, rivers, plants and animals, for billions of years. And then she thought further back, to where the water itself came from. A water molecule is made of two atoms of hydrogen and one atom of oxygen. The oxygen has its own story, which we will hear on a later night. But the hydrogen in every drop of water is much older. It comes from the first few minutes of the universe.

The Smallest Building Blocks

On the previous night, we met the oldest light, released when the universe was about 380,000 years old. Tonight we go much further back, to a time when the universe was only a few minutes old. To understand what happened then, we need to know a little about what atoms are made of. The words may be new, but the idea is simple, like learning the parts of a machine before you repair it.

Every atom has a tiny centre called the nucleus. Around the nucleus are even tinier particles called electrons. The nucleus itself is made of two kinds of particles. Protons carry a positive electric charge. Neutrons carry no charge at all, which is where their name comes from. Protons and neutrons are almost the same weight, and both are far heavier than an electron.

It is hard to picture how small a nucleus is. One comparison that teachers often use goes like this. If a whole atom were made as large as a big sports stadium, its nucleus would be about the size of a small pea in the middle of the field. The electrons would be somewhere far out in the stands. Almost all of the weight of the atom is in that tiny pea at the centre.

The number of protons in the nucleus decides what kind of atom it is. An atom with one proton is hydrogen. An atom with two protons is helium. An atom with six protons is carbon, and an atom with eight protons is oxygen. Each of these basic kinds of atom is called an element. The neutrons add weight and help hold the nucleus together, but they do not change which element it is.

The simplest nucleus of all is a single proton on its own. That is the nucleus of ordinary hydrogen. A helium nucleus is more complicated. It is made of two protons and two neutrons, packed tightly together. To build a helium nucleus, you need to bring protons and neutrons very close to each other, so close that a very strong force inside the nucleus can grip them. In ordinary conditions, this almost never happens, because protons push each other away with their electric charges. It takes enormous heat and pressure to force them together.

In the first moments of the universe, there were no atoms and no nuclei yet. There was only a hot mixture of free protons, free neutrons, electrons, light and other tiny particles, all moving very fast. It was far too hot for anything to stick together. Whenever a proton and a neutron joined, the heat knocked them apart again almost at once. The universe was like a kitchen so hot that no dough could ever rise, because everything burnt the moment it was mixed.

A Short Window for Building

As the universe expanded, it cooled. After a few minutes, it reached a temperature at which protons and neutrons could finally hold together. That temperature was still far hotter than the centre of the Sun is today, but it was no longer too hot. For a short time, the whole universe became a place where light nuclei could be built. Protons and neutrons joined in pairs, the pairs joined into helium nuclei, and a very small amount of a slightly heavier element, lithium, was made too.

This window did not stay open for long. The universe kept expanding and cooling. Within about the first twenty minutes or so, the temperature and the density had fallen so far that the building stopped. Particles were now too spread out and too slow to meet with enough force. Whatever had been made by then was what the universe had to work with. For many millions of years afterwards, until the first stars began to shine, no new elements were made at all.

You may wonder how anyone can know the temperature of the universe at an age of a few minutes. The reasoning is simpler than it sounds. Astronomers know how fast the universe is expanding today and how warm the oldest light is today. When the universe was smaller, the same light was squeezed into a smaller space, so it was hotter, in a simple and well understood way. By running the film backwards with these rules, physicists can work out how hot the universe was at each moment, all the way back to the first minutes.

There was another reason why time was short. A neutron on its own, outside a nucleus, does not last long. In about a quarter of an hour, on average, a free neutron changes into a proton. So the neutrons were slowly disappearing all the time. The only neutrons that could survive were the ones that had been built into helium nuclei. You could compare this to a bakery where the yeast slowly loses its strength. The baker has to use it quickly, or it will be gone.

When the building stopped, there were far fewer neutrons than protons. Roughly speaking, there was one neutron for every seven protons. That number decides the result. Imagine sixteen particles: two neutrons and fourteen protons. The two neutrons join with two of the protons to make one helium nucleus, which is four particles in total. The other twelve protons stay single, as twelve hydrogen nuclei. So, by weight, four parts out of sixteen are helium, which is one quarter. The other three-quarters are hydrogen.

That is the prediction of the first few minutes: by mass, about three-quarters hydrogen and about one quarter helium, with tiny traces of a heavier kind of hydrogen and of lithium. Hundreds of thousands of years later, when the universe cooled further and the electrons settled around these nuclei, the first atoms formed. Those were the atoms of the glowing fog, the fog that cleared and released the oldest light.

Two Stories That Met

There is a lovely meeting of two stories here. The first is the story of the prediction. In 1948, George Gamow, Ralph Alpher and Robert Herman, whom we met on the previous night, worked out how a hot, dense young universe could build light nuclei in its first minutes. Over the following years, other physicists improved the calculations, using better measurements of how protons and neutrons behave. The details of their work took many years to get right, but the main result stayed the same: the young universe should have made mostly hydrogen and about a quarter helium.

The second story began even earlier, with a young woman named Cecilia Payne. In the 1920s, she was a student working at an observatory, studying the light of stars. On the night of the moving galaxies, we saw that light can be spread into a spectrum, a band of colours with dark lines in it. Each element leaves its own pattern of lines. For a long time, astronomers had assumed that because the Sun's spectrum showed lines of iron and many other elements, the Sun must be made of roughly the same things as the Earth.

Payne used new ideas from physics to understand how the temperature of a star changes the pattern of its lines. When she took this into account, she came to a startling conclusion. Stars are made mostly of hydrogen and helium. The heavier elements are there, but only in small amounts. She finished this work in 1925. At the time, the idea seemed so unlikely that a respected senior astronomer persuaded her to write that her result was probably not real. A few years later, other astronomers, including that same senior astronomer, reached the same conclusion by other routes. Payne had been right.

It is worth thinking about Payne's position for a moment. She was young, she was a woman in a field where few women were allowed to do such work, and her result went against what almost everyone believed. Many people in her place would have dropped the idea. She did not drop the work. She was careful, she published it, and the evidence was there for others to check. Her result is now one of the foundations of astronomy.

Put the two stories together, and something remarkable appears. The physicists asked what the first few minutes should have made. The astronomers measured what stars and gas clouds are actually made of. Both answers point to the same thing: a universe made mostly of hydrogen, with about a quarter helium. When astronomers measure the oldest gas clouds and the oldest stars, which have had the least time to be changed by later stars, the amounts agree well with the prediction.

One of the Strongest Clues

On the first night, this book named three main clues that the universe had a hot, dense beginning. The first was that the galaxies are moving apart. The second was the oldest light. The third is tonight's story: the amounts of the lightest elements. This third clue is especially strong because it is a test with numbers. A theory that only says "things were hot once" could fit many results. A theory that says "about one quarter helium, not one half, not one tenth" can be checked, and it could have failed.

It did not fail. The helium is there in about the right amount, wherever astronomers look, in old stars and in gas between galaxies. The traces of heavy hydrogen also fit the prediction. And the numbers that come from these light elements agree with the numbers that come, completely separately, from the patterns in the oldest light. Two very different kinds of measurement, made with very different instruments, tell the same story about the young universe.

There is also a test that can be done on the Earth. The building of light nuclei depends on how easily protons and neutrons join at a given temperature. Those rates are not guessed. They are measured in laboratories, where machines fire protons and other particles at each other and count what comes out. The calculations of the first minutes use those measured numbers. In this sense, a small laboratory on the Earth and the whole young universe are following the same rules.

There is one honest puzzle to mention. The amount of lithium that astronomers find in some very old stars is lower than the calculations of the first minutes predict. Scientists are still studying why. Perhaps something inside those old stars has slowly destroyed part of their lithium. Perhaps something in the calculation needs to be improved. This puzzle is not yet fully understood. But it is a small puzzle inside a large agreement, and it does not change the main picture of the first few minutes.

It is also worth knowing what the first minutes did not make. They made almost nothing heavier than lithium. There was no carbon, no oxygen, no iron, no gold. The universe that came out of the first minutes was very simple: hydrogen, helium, a little of their close relatives and nothing else. Every other element, including the oxygen that you are breathing and the carbon in every living thing, had to be made later. That work was done by stars, and their story belongs to later nights.

So when you hold a glass of water, you are holding two very different histories. The oxygen in it was made inside stars, long after the beginning. But the hydrogen, the "two" in the two atoms of hydrogen and one of oxygen, was made in the first few minutes, about 13.8 billion years ago. That hydrogen has been in clouds, stars, comets, oceans and rain. Now it is in your glass.

Before Sleep

Before sleep, Noor filled one more pouch from the clean water supply and drank it slowly by the window. The Earth below was in daylight. She could see the blue of an ocean, scattered with white clouds, and the edge of a coast. All that water, she thought, and in every drop the same old hydrogen, waiting through billions of years.

She wrote to Sami: "The water you unloaded tonight is very old. The hydrogen in it was made in the first twenty minutes of the universe. Your boxes were heavier than you thought." Sami wrote back that his back had already told him that, thank you. Then he added that he would look at the next bottle a little differently.

Ade, who had seen her writing, asked what she was telling her brother tonight. When she explained, he was quiet for a moment and then said that he had read every page of the water machine's manual and never once thought of the water as old. Tomás said that this was the problem with experts: they know a thing so well that they forget to be surprised by it.

Tomorrow, or tonight, you may also fill a glass of water at the end of your working day. You do not need to think about the universe every time. But once, perhaps, you could hold the glass for a moment before you drink. In your hand is something made almost at the beginning of time, and it has travelled a very long way to reach you.

Common Misunderstandings

Many people think that all the elements were made in the Big Bang. It seems natural, since the Big Bang is the beginning of everything. But the first few minutes made only the lightest elements: hydrogen, helium and a tiny amount of lithium. Heavier elements such as carbon, oxygen, iron and gold were made much later, inside stars and in the violent events at the end of the lives of some stars. The universe began simple, and it became richer over time.

Many people also think that helium is a rare element, because on the Earth most people know it only from party balloons, and it is sometimes in short supply. In fact, helium is the second most common element in the universe, after hydrogen. It is rare on the Earth for a different reason. Helium is very light and does not join with other elements, so any helium in the air slowly escapes into space. Most of the helium that people collect on the Earth comes from deep underground, where it is slowly made by the breaking apart of some kinds of atoms in rocks. Out in the universe, helium is everywhere.

Finally, many people imagine that the first few minutes are pure guesswork, because nobody was there to see them. It is true that nobody watched, and it is true that some questions about the very earliest moments are still open, as we will see on the next night. But this part of the story is one of the best tested parts of all. It uses the same physics that is measured carefully in laboratories on the Earth, it makes clear predictions with numbers, and those predictions agree with what astronomers measure in the sky. On the next night, we will go back further still, to a time before even protons and neutrons existed, and there the ground under our feet will become much less firm.