Topics Space and the universe

What happened in the Big Bang?

The Big Bang was not an explosion in space. It describes how the universe, all of space and everything in it, began expanding from an extremely hot, dense state about 13.8 billion years ago. Space itself stretched, and as it did, it cooled. There was no outside, and no center where it went off.

What makes it interesting is how much is well supported and how much is open. We have solid evidence for the hot early universe: the universe is still expanding, a faint glow of ancient light fills the sky, and the mix of light elements like hydrogen and helium matches what the theory predicts. But the very first instant, and whether anything came before it, is unknown. Physicists have ideas, not answers.

An episode would walk through the story in order: the first fractions of a second, the cooling into particles and atoms, the release of that first light, and the slow growth of stars and galaxies. It would also stay honest about the edges. bre's hosts are AI and can be wrong, so you can press Talk and ask what a word means or where the evidence comes from.

What a bre episode would cover

An outline of the episode bre would make for this question. Every episode is written fresh when you ask, so yours will differ.

  1. Not an explosion in spaceWhy the Big Bang is better pictured as space itself stretching, with no center and no edge it spread into.
  2. The first fractions of a secondThe universe starts extremely hot and dense. Many ideas exist about the earliest moments, including a rapid growth called inflation, which is still being tested.
  3. Cooling into particles and atomsAs things cooled, particles joined into protons and neutrons, then the first light nuclei, and much later whole atoms.
  4. The first light we can still seeRoughly 380,000 years in, light could finally travel freely. That glow, the cosmic microwave background, still reaches us today.
  5. From gas clouds to galaxiesGravity pulled slightly denser regions together over hundreds of millions of years, forming the first stars and galaxies.
  6. What we still don't knowWhat happened at the very start, and whether there was a before, remains an open question in physics.

How the episode might open

A sample exchange between two of bre’s AI hosts, bre and Tess. Both are AI; this is written by AI, as every bre episode is.

  1. breAI host

    Let's start with the picture most people have: a tiny dot goes bang, and stuff flies outward into empty space. That picture is wrong, and it's worth saying why.

  2. TessAI host

    Okay, real talk. I've definitely pictured the dot. So what's the right picture?

  3. breAI host

    Think of space itself stretching. There was no empty room outside for things to fly into. Every point in space moved away from every other point, and the whole thing was hot and dense.

  4. TessAI host

    So there's no center? No spot where it started?

  5. breAI host

    No center we can point to. It happened everywhere at once, because everywhere was part of it.

  6. TessAI host

    Fine. That's weird, but I can live with it. Okay, so what happened before it?

  7. breAI host

    I don't know. Nobody does. Our best physics breaks down at the earliest instant, and anyone who gives you a confident answer is going past the evidence.

  8. TessAI host

    Good. I'd rather hear that than a tidy story. So what do we actually know?

Questions people also ask

Was the Big Bang an explosion?
Not in the usual sense. An explosion happens at a place and sends material outward through existing space. The Big Bang describes space itself expanding everywhere, from a hot, dense early state. There was no outside it spread into and no single center.
How long ago did the Big Bang happen?
About 13.8 billion years ago, according to the standard model of cosmology. That figure comes from measuring the universe's expansion and the oldest light we can detect. Scientists refine it as measurements improve, but it has stayed close to that value for years.
What evidence supports the Big Bang?
Three main things: galaxies are moving apart, so the universe is expanding; a faint microwave glow fills the sky, left over from the early universe; and the amounts of hydrogen and helium match what the theory predicts.
What was there before the Big Bang?
No one knows. Current physics cannot describe the very first instant, and it is unclear whether the question even has a meaning. Researchers have proposed several ideas, but none is established or confirmed by observation.

Related topics

More: all 300 topics, space and the universe, or the longer reads on /learn.

bre’s hosts are AI, and every episode is generated, so they can be wrong: check anything that matters. This page outlines what an episode would cover. It is for interest and learning, not medical, financial or legal advice.