The Big Bang: How the Universe Began, History Textbook

A point of light

Watch the screen: a single point flashes and starts to grow. This is how scientists picture the start of our Universe about 13.8 billion years ago. It was not an explosion inside empty space. Space itself, together with all matter and energy, began to expand from an extremely hot and dense state. We call this the Big Bang model, and it is a scientific theory that has survived a century of tests. Long before science, every people told its own creation story, from sky gods to cosmic eggs. Those myths ask the question we still ask: where did everything come from?

The Universe is growing

Now the dots fly apart, and the farther a dot is, the faster it goes. In 1929 Edwin Hubble showed that distant galaxies are moving away from us, faster the farther they are. He read this from redshift: light from a receding galaxy is stretched toward red, the way a siren sounds lower as it drives away. The galaxies are not flying through space. The space between them is stretching. Run the film backwards and everything meets in one hot beginning. A Belgian priest and physicist, Georges Lemaître, had suggested such an expanding Universe two years earlier.

A rival idea

For a while the Big Bang had a rival. In 1948 Fred Hoyle, Hermann Bondi and Thomas Gold proposed the Steady State theory: the Universe never changes in the large, and new matter keeps appearing as it expands. Hoyle himself gave the Big Bang its name in a BBC radio talk in 1949. Scientists did not choose by taste. They looked for a test, because the two theories disagreed about the distant past. Steady State said the far past looks like today. The Big Bang said it was hotter and denser, and that its glow might still be around us.

The oldest light

The sky begins to glow in faint speckles. For its first 380,000 years the Universe was a hot fog through which light could not travel far. Then it cooled enough for atoms to form, and the light flew free. That light is still travelling. Stretched into weak microwaves, it reaches us from every direction, only about 2.7 kelvins above absolute zero. In 1965 two radio engineers, Arno Penzias and Robert Wilson, picked up this hiss with an antenna in New Jersey and could not get rid of it. It was the cosmic microwave background, and it settled the argument in favour of the Big Bang. Its tiny speckles are the seeds of galaxies.

Let the stars switch on

After the glow faded, the Universe went dark. Gravity then pulled gas into clumps, and about 100 to 200 million years after the beginning the first stars ignited. Scientists think they were giants made almost only of hydrogen and helium, and that they lived short, violent lives. Their explosions scattered heavier elements. The carbon in your body, the oxygen you breathe and the iron in your blood were all made inside stars. Stars gathered into the first galaxies. The James Webb Space Telescope now sees galaxies from less than 400 million years after the Big Bang, and astronomers are still refining this picture.

The Sun’s family is born

A cloud of gas and dust collapses. A young Sun lights up at its centre, and a flat disc spins around it. Grains stick together into pebbles, pebbles into rocks, rocks into planets. This happened about 4.6 billion years ago. We know the date from meteorites, leftover pieces from that time. Their radioactive atoms work like a clock and give an age of about 4.57 billion years. Rocky planets formed close to the hot Sun, and giants of gas and ice formed farther out. Eight planets grew in all, and the third one from the Sun is ours.

A giant collision

Early Earth was a ball of molten rock. Then, scientists think, a body about the size of Mars, nicknamed Theia, struck it. The debris formed a ring and gathered into the Moon. This is the giant-impact theory. It fits the Moon rocks that Apollo astronauts brought back, which are chemically very close to Earth’s own rock. Over millions of years Earth’s surface cooled and hardened. Water vapour in the air turned to rain, and oceans formed. Tiny zircon crystals from Western Australia show that liquid water existed about 4.4 billion years ago. A world with a Moon, tides and seas was ready for something new: life.

The Beginning of Everything