The Big Bang: what it was, what caused it and what came before

What is the Big Bang?

The Big Bang is the scientific explanation of how the universe developed from a very hot, very dense early state into the universe we see today. Since then, space has been expanding and cooling. As it cooled, particles formed, then atoms, then stars and galaxies. NASA puts the start of that timeline at around 13.8 billion years ago.

Was the Big Bang an explosion?

Not in the usual sense, despite the name. An explosion happens at one place and throws material outwards into empty space. The Big Bang happened everywhere at once, and it was space itself that stretched. There is no centre it all came from and no edge it is spreading into. Picture dots drawn on a balloon: as the balloon inflates, every dot moves away from every other, and no dot is the middle.

Try it: this question gives you a statement about the Big Bang and asks whether it is a myth or a fact.

What was before the Big Bang?

This is one of the biggest open questions in science. The honest answer is that we don't know, and our current physics can't yet answer it. Close to the very start, the universe was so hot and dense that general relativity (our theory of gravity) and quantum physics (our theory of the very small) would both matter at once, and we don't have a tested theory that combines them.

Scientists take several ideas seriously. None has been confirmed:

• There was no "before". If time began with the universe, asking what came before is like asking what is north of the North Pole. The question sounds sensible but has no answer.
• Inflation came first. Many cosmologists think a burst of extremely fast expansion, called inflation, came before the hot Big Bang. When it ended, its energy turned into matter and light. But, as NASA says, "scientists aren't sure what came before inflation or what powered it."
• A multiverse. In some versions of inflation, it never stops everywhere at once, and our universe is one bubble among many.
• A bounce. An earlier universe may have shrunk down and then rebounded into ours, possibly as part of a repeating cycle.
• A quantum beginning. The universe may have appeared through a quantum process that a future theory of quantum gravity could describe.

What would settle it? Each idea predicts slightly different patterns in the oldest light in the universe, or in gravitational waves left over from the earliest moments. Telescopes are searching for those signs now.

What caused the Big Bang?

This is closely tied to the last question, and the answer is the same: no one knows yet. The Big Bang theory starts a tiny fraction of a second after the beginning and explains what happened next. It isn't a theory of the cause. If inflation is right, the hot Big Bang was caused by inflation ending, which moves the question back a step: what started inflation? That remains unknown.

The evidence for the Big Bang

Although its beginning is a mystery, what happened after is backed by strong evidence. The theory predicted these observations before they were found, rather than being fitted to them afterwards.

1. Galaxies are moving apart

Light from distant galaxies is stretched to longer, redder wavelengths, called redshift. In 1929, Edwin Hubble showed that a galaxy's speed away from us rises in step with its distance, as described in OpenStax Astronomy. This is Hubble's law, and it is exactly what an expanding universe would look like from any galaxy in it.

2. The cosmic microwave background

For its first 380,000 years or so, the universe was a glowing fog of charged particles that light couldn't pass through. Then, ESA explains, it cooled to around 3000 K, electrons and protons joined into atoms, and the light was set free. That light still fills space. The expansion has stretched it into microwaves, and its temperature has fallen to just 2.7 K. Arno Penzias and Robert Wilson first detected it in 1964, by accident, with a radio antenna.

3. The amount of hydrogen and helium

In its first few minutes, the universe was hot enough for nuclear fusion, but only briefly. That made hydrogen, helium and a little lithium, and nothing heavier. OpenStax Astronomy estimates that about 10 times more helium was made in those first minutes than all generations of stars have made since. The amounts we measure match the theory's predictions.

How old is the universe?

If galaxies are moving apart, you can run the film backwards. Hubble's law says speed = H₀ × distance, where H₀ is the Hubble constant. If every galaxy has always moved at its current speed, they were all together a time of distance ÷ speed ago, which is:

H₀ is measured in kilometres per second per megaparsec, where a megaparsec is about 3.09 × 10¹⁹ km. With a round value of 70 km/s per megaparsec, 1/H₀ = 3.09 × 10¹⁹ ÷ 70 = 4.41 × 10¹⁷ seconds, which is about 14.0 billion years.

That is close to the accepted 13.8 billion years. The difference is because the expansion rate has changed over time: gravity slowed it for billions of years, and dark energy has been speeding it up since. Allowing for that history gives 13.8 billion.

Try one: this question asks you to rewind the expansion to estimate an age.

A timeline of the universe

The Big Bang course walks through each stage with interactive models, from the first minutes to dark energy. For the bigger questions of what the universe is and whether it goes on forever, try the Universe Ends course.

Common misconceptions

• "The Big Bang was an explosion at one point." It happened everywhere, as space itself expanded.
• "The Big Bang theory explains how the universe began." It explains how the universe developed from a very early moment, not its origin or cause.
• "Galaxies are flying through space away from a centre." Space between them is stretching, and there is no centre.
• "The universe started as a small dot in empty space." There was no outside space for it to sit in. The observable universe was once tiny, but the whole universe may always have been infinite.
• "It's just a theory, so it's a guess." In science, a theory is a well tested explanation. The evidence for the expansion after the Big Bang is very strong; what came before is the part that is unknown.

Read next

Physics · CoursesCosmology course: the Big Bang, redshift and dark energyAn interactive cosmology course in 24 short sections: expansion, redshift, Hubble's law, the age of the universe, inflation, the CMB and dark energy.