Beginner · 1:52

The Big Bang

How the universe grew from a hot, dense beginning into a web of galaxies, over 13.8 billion years

Each is simulated from real physics and measurements; anything no eye could see, like magnetic fields and radio waves, is drawn and labelled as drawn.

Below are the 9 chapters of this simulation in Cosmic Scale: each one's caption, then the fuller explanation the app shows under “Read more”. Colours, arrows and lines mentioned in the text belong to the 3D scene in the app.

Open Cosmic Scale then the Phenomena tab

1. The universe today

Galaxies strung along a web of filaments around empty voids: the universe 13.8 billion years after the Big Bang. Space itself is still stretching, carrying the galaxies apart.

The region shown is about 800 million light-years across. Its galaxies gather in walls and filaments around voids a hundred million light-years or more wide, the pattern that surveys mapping millions of galaxies have revealed. The web here is drawn from a simple model rather than mapped, so it looks like the real one without being it.

Distant galaxies move away from us faster the farther they are: about 67 km/s faster for every 3.26 million light-years. They are not flying through space; the space between them is growing. Run the clock backwards, and everything must once have been packed tightly together.

2. Running the clock back

Run time backwards and the galaxies crowd together, fade into smooth gas and disappear into ever hotter light. There is no centre: from anywhere, everything closes in the same way.

The Big Bang was not an explosion at a point in space. Space itself was smaller everywhere at once, and every part of today's universe took part in it. Pick any galaxy: running backwards, all the others close in on it, just as they close in on ours.

Everything we can see today, now 93 billion light-years across, fitted into a region about 18 light-years across one second after the Big Bang. What happened at the very first instant, or whether there was a before, no one knows yet: our physics can follow the universe back only to when it was already a tiny fraction of a second old.

3. The first second

A millionth of a second in, space is filled with quarks, gluons and light at trillions of degrees. After about a hundred-thousandth of a second the quarks gather into protons and neutrons.

Nothing today is this hot, not even the core of a star. The whole universe was a thick soup of the smallest particles, colliding far too often for anything to hold together. As space grew it cooled, and at about two trillion degrees the quarks clumped in threes: protons and neutrons.

At first there were as many neutrons as protons. Neutrons are slightly heavier, so as it cooled they became rarer, until a little over a second in, when the two stopped turning into each other: one neutron for every six protons. By then neutrinos had slipped away; they still fill space today.

4. The first minutes

Between about three and twenty minutes, protons and neutrons fuse into helium, until a quarter of all ordinary matter is helium. Then the universe is too cool to make more.

Protons and neutrons first pair up into deuterium, but for a few minutes the light is still hot enough to break those pairs apart. Once it has cooled below about a billion degrees, the pairs survive and quickly join into helium. Almost every neutron ends up inside a helium nucleus.

The result, about three quarters hydrogen and one quarter helium by mass, with traces of deuterium and lithium, is what astronomers find in the oldest gas and stars. It is among the strongest evidence that the Big Bang happened, and it happened in the universe's first twenty minutes.

5. A glowing fog

For the next 370,000 years the universe is a glowing fog of nuclei, electrons and light. Light cannot travel far before bouncing off an electron, so nothing can be seen through it.

It is like being inside a star, but thinning and cooling as space grows. Matter and light push and pull on each other, and sound waves ring through the fog. The ripples they leave are tiny, about one part in 100,000; here they are drawn thousands of times stronger.

Slowly the fog cools from hundreds of millions of degrees to a few thousand, and its glow changes colour as a cooling star's does: from blue-white to yellow and then orange. There are about 1.6 billion particles of light for every proton or neutron.

6. The fog clears

At about 3,000 degrees, some 370,000 years after the Big Bang, electrons settle onto nuclei and the fog turns clear. The light set free then still reaches us from every side: the cosmic microwave background.

Once electrons are bound into atoms of hydrogen and helium, light passes between them freely. That light has been travelling ever since. Stretched by the growing universe to wavelengths a thousand times longer, it arrives today as microwaves, 2.7 degrees above absolute zero.

Its tiny ripples, spots about a degree across on the sky, are the seeds of every galaxy. Satellites have mapped them in detail, from COBE to WMAP and Planck; the real map is among the photos. Their pattern tells us the universe's age and what it is made of.

7. The dark ages

As space keeps growing, the afterglow reddens and fades. After a few million years there is no visible light at all, only darkness, while gravity quietly gathers matter into the ripples.

For a few million years the sky still glowed a dull red, like embers. Then its light slipped into the infrared, and the universe went dark. There were no stars yet, only cooling gas of hydrogen and helium.

In the darkness, dark matter, which neither shines nor blocks light and outweighs ordinary matter five to one, pulled together where the ripples had left it slightly denser, and the gas followed. It is drawn here in blue; in reality it is invisible.

8. First light

A hundred to two hundred million years in, the first stars ignite in the densest knots of the web, and the first galaxies gather. Their light slowly clears the gas around them.

The first stars were probably huge, hot and short-lived, made only of hydrogen and helium; none has been found yet. The James Webb Space Telescope has seen galaxies already shining less than 300 million years after the Big Bang.

The ultraviolet light of young galaxies split the hydrogen atoms around them apart again, and by about a billion years in, the gas between galaxies was clear. Their stars also forged the first heavier elements, from carbon to iron, that later stars and planets were made from.

9. The web grows

Over billions of years galaxies grow and merge along the web. The Sun forms 9.2 billion years after the Big Bang, and for the last six billion years space has been growing ever faster.

Most stars were born when the universe was two to six billion years old; star birth has slowed about tenfold since. Our Sun and Earth formed from gas already enriched by generations of earlier stars, some 4.6 billion years ago.

In 1998 astronomers found that distant exploding stars were fainter than expected: the expansion is speeding up. Whatever drives it, called dark energy, makes up about 68% of the universe today. What it is remains one of the biggest open questions in physics.

Objects in this story

Real photos