Light from the early Universe

What is the cosmic microwave background?

The cosmic microwave background, or CMB, is a bath of microwave radiation that fills the entire Universe.

It reaches us from every direction in the sky, making it one of the most important clues we have about what the Universe was like long before stars, galaxies, and planets existed.

Step one

What does “microwave” mean here?

Visible light is only a small part of the full electromagnetic spectrum. There are many other kinds of light, including radio waves, infrared, ultraviolet, X-rays, and microwaves.

Microwave radiation is familiar from everyday technology, from microwave ovens to wireless communications. The CMB is light in this microwave part of the spectrum.

The electromagnetic spectrum, showing visible light as only a small slice of the full range of possible light.

Arno Penzias and Robert Wilson with the Holmdel Horn antenna used in the 1964 discovery of the CMB.

Step two

What exactly is the CMB?

The CMB is the faint microwave glow that fills the Universe and can be seen in every direction. It was discovered in 1964 by Arno Penzias and Robert Wilson while they were developing technology for satellite communications.

What they found turned out to be far more important than a source of noise in an antenna. It was a signal from the early Universe itself.

Step three

Where did it come from?

The early Universe began in a hot, dense state and then expanded and cooled. After about 380,000 years, it had cooled enough for atoms to form. Before that moment, light was constantly scattered by charged particles and could not travel freely.

Once atoms formed, that light was finally released and began streaming across space. The CMB is that ancient light, still traveling through the Universe almost 13.8 billion years later.

A timeline showing a brief history of the Universe — A simplified history of the Universe, including the moment when the CMB was released.

What we measure

Why does the CMB look almost the same everywhere?

The CMB is extremely uniform. Its dominant signal is the monopole: a nearly perfect all-sky temperature of about 2.72 K.

But that is not the whole story. Tiny fluctuations are imprinted on top of that near-uniform glow, and these anisotropies are only at the level of roughly 100 microkelvin. Those small patterns carry information about conditions in the very early Universe and the seeds of later cosmic structure.

One of the goals of the Simons Observatory is to produce even better measurements of these anisotropies and search for new signals hidden within them.

Illustration of the CMB monopole at 2.72 kelvin — The CMB monopole: the nearly uniform 2.72 K background temperature seen across the whole sky.

Map of the cosmic microwave background from the Planck satellite — The CMB as mapped by the Planck satellite, showing anisotropies at the level of about 100 microkelvin across the sky. Credit: ESA.