The Big Bang: evidence, not just theory

The four lines of evidence

The accidental confirmation

In 1964 two radio astronomers at Bell Labs, Arno Penzias and Robert Wilson, were chasing persistent noise in an antenna. It came from every direction and wouldn't go away. They cleared out nesting pigeons, suspecting droppings.

Meanwhile a group at Princeton was preparing to search for exactly this radiation, predicted as a relic of a hot early universe. The two groups talked and realised the noise was the signal.

Penzias and Wilson got the 1978 Nobel Prize. It's one of the cleanest cases in science of a specific prediction being confirmed by people who weren't looking for it and first thought it was a fault. No expectation could have shaped the result.

The microwave background settles it. I'd stop entertaining 'just a theory' from anyone who hasn't read about it.

What it does not claim

It isn't a theory of the origin of the universe. It describes the evolution of the universe from an extremely hot dense state onward. What produced that state, or whether the question is well-posed, is outside the model.

It wasn't an explosion in space. It was an expansion of space, happening everywhere at once. There's no centre and no point you could travel to.

There's no edge. The observable universe has a boundary set by how far light has travelled, but that's a limit on our view instead of a physical edge. See how far can we see.

The name is a joke that stuck. It was coined dismissively by Fred Hoyle, a proponent of the rival steady-state model, and the model's supporters adopted it. It's a poor description, since it suggests both an explosion and a location.

Where it is incomplete

The earliest instants. Extrapolating back far enough reaches conditions where general relativity and quantum mechanics both apply and can't be reconciled. The model doesn't describe the first tiny fraction of a second.

Inflation. A brief period of extremely rapid expansion is invoked to explain why the universe is so uniform and so flat. It fits the data well and the specific mechanism isn't established.

Dark matter and dark energy are required for the model to fit observations, and neither is identified, see dark matter and dark energy.

The Hubble tension. Different methods of measuring the current expansion rate give values that disagree by more than their stated uncertainties. This is a live and unresolved problem and may indicate something missing.

None of these threatens the core picture, which is supported by four independent lines of evidence. They mark where work continues, and describing them plainly is more clear than presenting cosmology as settled.

What is the evidence for the Big Bang?

Four independent lines: the expansion of the universe, the cosmic microwave background radiation at 2.7 kelvin, the observed abundances of light elements matching predictions from the first few minutes, and the visible evolution of galaxies over cosmic time.

Was the Big Bang an explosion?

No. It was an expansion of space itself, occurring everywhere simultaneously rather than from a point. There's no centre and no location you could travel to. The name was coined dismissively by a critic and stuck.

How was the cosmic microwave background discovered?

By accident in 1964, when two radio astronomers troubleshooting persistent antenna noise found a signal from every direction they couldn't eliminate. A separate group was preparing to search for exactly that predicted radiation, and the two teams connected.

Does the Big Bang explain where the universe came from?

No. It describes the evolution of the universe from an extremely hot dense early state onward. What produced that state, or whether the question is even well-posed, lies outside the model.