The Cosmic Microwave Background: the oldest light in the universe

If you looked at an old tube television without a signal, a small part of the static was truly cosmic. It came from the early universe. We call this faint glow the cosmic microwave background.

Where this light comes from

In the first 380,000 years, the universe was so hot that light could not travel far. Charged particles kept scattering it. The cosmos was opaque, like thick fog.

Then the universe cooled. Atoms formed, and light could suddenly fly freely. This very light reaches us today, after 13.8 billion years. You can read more about the start of the universe in the knowledge section.

The accidental discovery of 1964

Two engineers, Arno Penzias and Robert Wilson, fought a stubborn noise in their antenna in 1964. They even cleaned out pigeon droppings, yet the hiss remained.

Only later did it become clear: they had caught the oldest light in the universe. For this discovery they received the Nobel Prize. It is one of the greatest accidental finds in science.

Why it looks the same everywhere

The background radiation is astonishingly uniform. Its temperature is 2.725 kelvin in every direction, almost the absolute zero point.

This evenness is itself a puzzle. Far-apart regions never had contact, yet they share the same temperature. Cosmic inflation offers the best explanation.

What the tiny fluctuations reveal

Look closely and the radiation is not perfectly smooth after all. There are fluctuations of a few millionths of a degree. These tiny differences are the blueprint of the cosmos.

Where it was a little denser, gravity pulled in more matter. From these seeds, galaxies and large structures later grew. The map of the fluctuations is therefore a baby photo of the universe.

How we measure it ever more precisely

Three satellites sharpened the picture step by step. COBE first detected the fluctuations in the 1990s. WMAP and later Planck mapped them in fine detail.

From these maps, researchers read off the age, composition and geometry of the universe. The background radiation has become a precision instrument of cosmology.

What it reveals about the whole universe

The measurements show that only about five percent of the universe is ordinary matter. The rest consists of dark matter and dark energy.

The age of 13.8 billion years also comes from this data. Hardly any other source reveals so much about the entire cosmos at once.

Why it is the best evidence for the Big Bang

The background radiation is the strongest proof of the Big Bang. No other model explains this uniform glow so convincingly.

It links the theory of a hot beginning with concrete measurements. Anyone who wants to understand the Big Bang cannot get past this faint glow.