Pigeon Droppings and the Oldest Light

In 1964 Arno Penzias and Robert Wilson were trying to get a radio antenna in New Jersey to behave. There was a persistent hiss, coming from every direction, at all times of day, in every season.

They checked everything. They eliminated the antenna's own electronics, urban interference, and nuclear test fallout. They found pigeons nesting in the horn, removed them and cleaned out what the paper delicately called a white dielectric material. The hiss remained.

Forty kilometres away, a group at Princeton was calculating what a hot early universe would have left behind. The two groups were put in touch, and the noise turned out to be the oldest light in existence.

Here is what it is. For its first 380,000 years the universe was too hot for electrons to stay bound to nuclei, so matter was an opaque plasma and light could not travel far before scattering. As expansion cooled it, electrons and nuclei combined into neutral atoms, the fog cleared, and the light released at that moment has been travelling ever since: stretched by expansion from the visible spectrum down into microwaves.

It arrives from every direction because it was released everywhere at once. It is the surface of last scattering, and it is the earliest thing anyone can see.

Which is the strongest single piece of evidence for a hot early universe, and it is worth being precise about what the resulting theory claims.

The Big Bang model describes the expansion and cooling of the universe from a hot dense state. It does not describe an origin, and it was not an explosion at a point in space. Space itself expands, everywhere simultaneously, with no centre and no edge. The name was coined derisively by Fred Hoyle, who backed a rival steady-state model, and it stuck, which is unfortunate, because it has been misleading people about explosions ever since.

There is a second, independent line of evidence that is less famous and arguably more impressive.

In the first three minutes, the universe was hot enough for nuclear fusion, and then it cooled below the threshold. The theory predicts what the resulting mix should be: about 75% hydrogen and 25% helium by mass, with traces of deuterium and lithium. The predicted proportions depend sensitively on the density of ordinary matter.

The measured abundances match. And because the match constrains the density of ordinary matter, it independently tells us there is not enough of it to account for the gravitational mass we observe, which is one reason the dark matter cannot simply be normal matter that has escaped notice.

Two of the most common questions about all this deserve straight answers.

What is the universe expanding into? Nothing. Expansion means distances between things increase everywhere; there is no outside. The standard analogy is dots on the surface of an inflating balloon (every dot sees every other receding, none is at the centre) and the analogy works only if you remember that in it, the surface is the whole universe, and the air is not part of the picture.

What came before? The model does not say. It describes the evolution from an early hot dense state and breaks down at the earliest moments, where both general relativity and quantum mechanics apply and no accepted theory combines them. Proposals exist (eternal inflation, a bounce, various multiverses) and none is established.

A question can be perfectly well-formed and unanswered, and saying so is more useful than either confident metaphysics or dismissing the question as meaningless.

Finally, the piece that was added because the model had a specific problem.

Look at opposite sides of the sky. The background temperature is the same to about one part in a hundred thousand. But those regions have never been in causal contact (light has not had time to travel between them since the universe began) so nothing could have equalised them. Why should they match?

Cosmic inflation proposes that in an unimaginably brief early moment, the universe expanded by an enormous factor. Regions now on opposite sides of the sky were in contact before inflation pushed them apart, which is why they agree.

It is a large claim invented to solve a specific problem, and what gives it standing is that it also predicted things it was not designed for: the universe's flatness, and the detailed statistical spectrum of the tiny temperature fluctuations in the background. Those fluctuations have since been measured with great precision, and they match. Which is roughly the best a speculative idea can do.