How old is the universe?

The universe is about 13.8 billion years old. This value comes from measurements of the cosmic microwave background and the expansion rate.

Where did the Big Bang happen?

The Big Bang did not happen at a place but everywhere at once. Space itself began with it, so there is no center of the expansion.

What proves the Big Bang?

The strongest evidence is the cosmic microwave background, the oldest light in the universe. The expansion of space and the abundance of hydrogen and helium also fit the model.

Was the Big Bang an explosion?

No, that is a common misconception. There was no explosion of matter into empty space. Instead, space itself began and has been expanding everywhere ever since.

How hot was the universe at the start?

In the first fractions of a second it was many billions of degrees hot. Only after about 380,000 years did it cool enough for the first atoms to form and for light to travel freely.

Is the universe expanding into something?

By current understanding, no. It is not matter flying through an existing space, but space itself growing between the galaxies. No outside is needed for that.

The Big Bang: How Our Universe Began

The Big Bang is the best explanation for how our universe began. About 13.8 billion years ago, everything we see today was packed into a tiny, incredibly hot point. Since then, space has been expanding and cooling.

13.8 bn years old

380,000 years to the first light

2.725 K background radiation today

What the Big Bang really was

Many people picture the Big Bang as an explosion in empty space. That image is misleading. Space and time themselves began with the Big Bang. There was no outside to explode into, and no single place where it happened.

Not a bang in one place, but everywhere

The Big Bang did not happen at a point but throughout all of space at once. Every location in today’s universe was part of the same hot, dense state. So there is no center from which everything moves away.

A helpful image is a rising dough with raisins. As it rises, all raisins move apart from one another, yet none is the center. Space between galaxies grows in just the same way.

Space expands, matter does not fly into it

Galaxies do not fly apart through space. Instead, the space between them grows. The more space lies between two galaxies, the faster the distance increases. This behavior also explains cosmic inflation right after the start.

The timeline: from the first second to today

The history of the universe falls into clear stages. Each lasted a different length of time, but together they add up to 13.8 billion years.

The first second

In the first fractions of a second, the universe was unimaginably hot. During this time the smallest building blocks of matter, such as quarks and electrons, appeared. Soon after, protons and neutrons formed.

The first atomic nuclei

After a few minutes it was cool enough for protons and neutrons to bind into simple nuclei. This produced mainly hydrogen and helium. To this day, about three quarters of ordinary matter is hydrogen.

The first light after 380,000 years

For a long time the universe was opaque, because light kept bouncing off free particles. Only after about 380,000 years did nuclei and electrons capture the first atoms. Suddenly light could travel freely. We see this light today as the cosmic microwave background.

The first stars and galaxies

About 200 million years later, gas clouds grew dense enough for the first stars to ignite. Their radiation ended the cosmic dark age. What lay before is covered in the spoke on what came before the Big Bang.

How we know the Big Bang happened

The Big Bang is not a belief but the best explanation for several independent observations. Three of them are especially strong.

The cosmic microwave background

The most important piece of evidence is the oldest light in the universe. It fills the entire sky and has a temperature today of 2.725 kelvin, almost absolute zero. The satellites COBE, WMAP and Planck have measured it ever more precisely.

The expansion of space

Edwin Hubble showed back in 1929 that distant galaxies move away from us. The farther away they are, the faster they recede. Run this motion backward and everything once sat close together.

The abundance of the elements

The measured amounts of hydrogen and helium match the predictions for the first minutes exactly. No other model explains this ratio so well. Three independent findings give the same picture.

What cosmic inflation explains

Right after the start, space expanded extremely fast for a tiny instant. This phase is called inflation. It explains why the universe looks almost the same in every direction.

Without inflation this would be hard to understand, because distant regions never had contact. The rapid expansion smoothed space out and seeded tiny density variations from which galaxies later grew.

Dark matter and dark energy

Visible matter makes up only a small part of the universe. About 95 percent is dark matter and dark energy. We know their effects but not their nature.

Dark matter holds galaxies together through its gravity. Dark energy drives the expansion of space and even accelerates it. Both are open puzzles, closely tied to the question of information as reality.

How fast is the universe expanding?

The expansion rate is called the Hubble constant. Different methods give slightly different values. Researchers call this gap the Hubble tension.

Measured via the background radiation, the rate comes out a little smaller than when measured via nearby galaxies. Perhaps it is only a measurement error. Perhaps it is a hint of new physics.

What came before the Big Bang?

This question sounds simple but is tricky. If time itself began with the Big Bang, perhaps there was no before at all. Our theories describe the universe only from a tiny fraction of the first second onward.

Some models still propose a before, such as eternal inflation or cyclic universes. None is proven. More on this in the spoke on what came before the Big Bang.

Common misconceptions

Three errors are especially stubborn. Knowing them makes the Big Bang clearer.

First, the Big Bang was not an explosion of matter into empty space. Second, there is no center of the expansion. Third, the universe is not necessarily finite just because it had a beginning in time.

What researchers want to settle next

New telescopes such as the James Webb Space Telescope keep delivering fresh data. They reveal galaxies that formed earlier than expected and sharpen our picture of the young cosmos.

The big open questions remain the nature of dark energy, the exact expansion rate and the very first moment. This is exactly where gravity and quantum mechanics meet, and where it will be decided how fully we understand the beginning.