The Big Bang

The story

A story that begins with “everything”

When you look at the night sky you see stars, the Moon and a few planets. But everything that exists, from a grain of sand to your own body, has a story that starts much further back: with the Big Bang. Scientists estimate that it happened about 13.8 billion years ago. That is such a huge number that it is hard to imagine: if every year were one second, it would take more than 400 years to reach today.

Be careful with the name, though. “Big Bang” sounds like an explosion, but it was nothing like a firework going off, and it did not happen at one point in space. There was no empty space to explode into: space itself began to stretch, everywhere at the same moment. That is why you cannot point to the “place” of the Big Bang. The place is the whole Universe, which was then very, very much smaller.

Very hot and very dense

At the beginning the Universe was extremely hot and extremely dense, a soup of energy and tiny particles. As it expanded, it cooled down. In the first seconds quarks appeared and joined together to make protons and neutrons; in the first minutes protons and neutrons stuck together to form the nuclei of the first elements. What came out was mostly hydrogen, about three quarters of ordinary matter by mass, and helium, about a quarter, with a tiny bit of lithium. All the other elements, such as carbon and oxygen, would arrive much later, inside stars.

For hundreds of thousands of years the Universe was like a glowing fog: light could not travel, because it kept bumping into charged particles. About 380,000 years after the beginning the temperature dropped enough for nuclei to capture electrons and form atoms. At that point the fog cleared, and light could at last run free through space.

The oldest light

That light still exists and reaches us from every direction. It is called the cosmic microwave background. Over 13.8 billion years space has stretched so much that this light has been “pulled out” and can no longer be seen; it is now made of microwaves, with a temperature of about 2.7 degrees above absolute zero, that is, about -270 °C. It was discovered almost by accident in 1964 by two American radio astronomers, Arno Penzias and Robert Wilson, who were trying to find out where an annoying hiss in their big antenna was coming from. They received the Nobel Prize in Physics for it in 1978.

From the background radiation we can read the history of the Universe like an old photograph. The satellites COBE, WMAP and above all Planck, from the European Space Agency, have measured its tiny differences in temperature. From those little patches we can work out the age of the Universe: Planck found 13.8 billion years, with only a small margin of error. The patches are also the seeds from which galaxies later grew, pulled together by gravity.

How people arrived at this idea

The idea came from mathematics and from observations. In 1929 the American astronomer Edwin Hubble discovered that distant galaxies are moving away from us, and the further away they are, the faster they go. A few years earlier, in 1927, the Belgian priest and physicist Georges Lemaître, using the equations of Einstein's relativity, had calculated that the Universe was expanding; in 1931 he then suggested that it had started from a very small, dense state, his “primeval atom”. If everything is moving apart today, he reasoned, then in the past everything must have been closer together.

The name “Big Bang” came later, in 1949, in a BBC radio broadcast, from the English astronomer Fred Hoyle, who supported a different theory. He said it almost as a joke, but the name caught on and stayed.

What we do not know

Science has not yet answered every question. The Big Bang explains how the Universe developed from a very early moment, but it does not say what there was “before”, and it is not even certain that the word “before” makes sense, because time as we know it was born together with the Universe. Scientists are also studying dark matter and dark energy, which together seem to make up most of the Universe but which nobody has yet seen directly. Saying “we do not know yet” is part of a scientist's job.

The Big Bang is today the strongest picture we have, because many independent observations point in the same direction: the galaxies moving away, the background radiation, and the amounts of hydrogen and helium. It is the starting point of our timeline and of everything you will find in the next stops.