What is the cosmic microwave background in simple terms?

It is the faint leftover glow from the Big Bang, light released when the universe was about 380,000 years old. The expansion of space has since stretched it into microwaves that fill the entire sky.

Why is the CMB evidence for the Big Bang?

A hot, dense early universe naturally produces radiation with a near-perfect blackbody spectrum that cools as space expands. The CMB has exactly that spectrum, comes equally from all directions, and has the temperature that the Big Bang model predicts.

How old is the universe according to the CMB?

Planck's measurements give an age of about 13.8 billion years. The figure comes from fitting the pattern of CMB ripples with the standard cosmological model.

Can we see anything older than the CMB?

Not with light, because the universe was opaque before it. In principle, neutrinos and gravitational waves from earlier times could reveal more, and scientists are searching for faint gravitational-wave imprints in the CMB's polarisation.

What temperature is the cosmic microwave background?

About 2.725 K, or roughly −270.4°C, just under three degrees above absolute zero. When it was released it was about 3,000 K, as hot as the surface of a cool red star.

The oldest light in the universe, the Big Bang's afterglow

Cosmic Microwave Background

Afterglow of the Big Bang · ≈ 46 billion ly (today)

The cosmic microwave background, or CMB, is the oldest light we can see. It was released when the universe was about 380,000 years old and had cooled enough for atoms to form, letting light travel freely for the first time. Since then the expansion of space has stretched that light from a hot glow into faint microwaves, filling the whole sky at a temperature of just 2.725 K.

The CMB is the strongest evidence that the universe began in a hot, dense state, the Big Bang. Its tiny temperature variations, about one part in 100,000, are the seeds of every galaxy. By mapping them precisely, missions such as COBE, WMAP and Planck have measured the age of the universe, 13.8 billion years, and the amounts of ordinary matter, dark matter and dark energy.

Key facts

Temperature today: 2.725 K≈ −270.4°C
Released: ≈ 380,000 years after the Big Bang
Temperature when released: ≈ 3,000 KStretched about 1,100 times since
Temperature variations: ≈ 1 part in 100,000
Discovered: 1965Arno Penzias and Robert Wilson
Peak wavelength: ≈ 1 mmPeak frequency about 160 GHz
Age of the universe: 13.8 billion yearsFrom Planck data
Photon density: ≈ 400 per cm³Throughout space
Key missions: COBE, WMAP, Planck

Where the CMB fits in the Big Bang timeline

In the first tiny fraction of a second, many cosmologists think the universe underwent inflation, an extremely rapid expansion that smoothed it out and stretched microscopic quantum fluctuations to cosmic sizes. Within about a second, protons and neutrons had formed. In the first few minutes, some of them fused into helium and traces of other light elements, a process known as Big Bang nucleosynthesis.

For the next few hundred thousand years the universe was a glowing fog of hot plasma. Free electrons scattered light constantly, so it could not travel far. At around 380,000 years, the temperature fell to about 3,000 K, electrons joined nuclei to form neutral atoms, and the fog cleared. That released light is the CMB. A long 'dark age' followed until the first stars ignited, roughly 100 to 200 million years after the Big Bang.

The surface of last scattering

Looking far away means looking back in time. When we observe the CMB, we see light from the moment the cosmic fog cleared, coming from every direction at once. This shell is called the surface of last scattering. Beyond it, the universe was opaque, so no light from earlier times can reach us.

Discovery: a hiss that would not go away

In the late 1940s Ralph Alpher and Robert Herman, working with George Gamow's ideas, predicted that a hot early universe should have left behind a cold glow of radiation. The prediction was largely forgotten. In 1964 and 1965, Arno Penzias and Robert Wilson at Bell Telephone Laboratories in New Jersey were using a large horn-shaped radio antenna and found a faint, steady microwave noise coming from every direction.

They ruled out equipment faults, city interference and even pigeon droppings inside the antenna. Meanwhile, a team at nearby Princeton led by Robert Dicke, including Jim Peebles, was preparing to search for exactly this radiation. The two groups published together in 1965, one reporting the signal and the other explaining it. Penzias and Wilson shared the 1978 Nobel Prize in Physics.

COBE, WMAP and Planck

NASA's COBE satellite, launched in 1989, showed that the CMB has an almost perfect blackbody spectrum, exactly what a hot early universe predicts. In 1992 it also detected the first faint temperature differences across the sky. John Mather and George Smoot received the 2006 Nobel Prize in Physics for this work.

NASA's WMAP, operating from 2001 to 2010, mapped these variations in much finer detail, and ESA's Planck, observing from 2009 to 2013, produced the sharpest all-sky maps yet. Planck's final results in 2018 gave an age of 13.8 billion years and a universe of about 5% ordinary matter, 27% dark matter and 68% dark energy. Ground telescopes such as the Atacama Cosmology Telescope and South Pole Telescope continue to refine these measurements.

Reading the ripples

The CMB is astonishingly uniform, but not perfectly so. Some patches are warmer or cooler by about one part in 100,000, a few hundred-thousandths of a degree. These anisotropies trace slightly denser and thinner regions in the early universe. Over billions of years, gravity pulled matter into the denser regions, building the galaxies and clusters we see today.

The sizes of the spots carry a wealth of information. Sound waves rippled through the early plasma, and the typical spot size, about one degree across, reveals that space is very nearly flat on the largest scales. The relative heights of other peaks in the pattern measure how much ordinary and dark matter there is. The light's polarisation adds further clues about when the first stars reionised the universe.

Mapping a sphere

Because the CMB surrounds us in every direction, its maps are drawn on a sphere, often flattened into an oval projection. The 3D view shows that sphere coloured by temperature, with warmer regions in red and cooler ones in blue.

The dipole

One side of the sky looks slightly warmer than the other by about one part in 1,000. This is not a feature of the early universe but a Doppler effect caused by our own motion, about 370 km/s, relative to the CMB. Astronomers remove it before studying the much smaller primordial variations.

Surprising facts about Cosmic Microwave Background

A small part of the static on an old analogue television tuned between channels came from the cosmic microwave background.
Every cubic centimetre of space contains around 400 CMB photons, many more than there are atoms.
The CMB was released when the universe was about 1,100 times smaller than today, and its light has been stretched by the same factor.
While hunting for the source of the noise, Penzias and Wilson removed nesting pigeons and cleaned their droppings out of the antenna, but the hiss remained.
Some features in the CMB, such as an unusually large 'cold spot', are still debated as either chance fluctuations or hints of new physics.

Frequently asked questions

What is the cosmic microwave background in simple terms?: It is the faint leftover glow from the Big Bang, light released when the universe was about 380,000 years old. The expansion of space has since stretched it into microwaves that fill the entire sky.
Why is the CMB evidence for the Big Bang?: A hot, dense early universe naturally produces radiation with a near-perfect blackbody spectrum that cools as space expands. The CMB has exactly that spectrum, comes equally from all directions, and has the temperature that the Big Bang model predicts.
How old is the universe according to the CMB?: Planck's measurements give an age of about 13.8 billion years. The figure comes from fitting the pattern of CMB ripples with the standard cosmological model.
Can we see anything older than the CMB?: Not with light, because the universe was opaque before it. In principle, neutrinos and gravitational waves from earlier times could reveal more, and scientists are searching for faint gravitational-wave imprints in the CMB's polarisation.
What temperature is the cosmic microwave background?: About 2.725 K, or roughly −270.4°C, just under three degrees above absolute zero. When it was released it was about 3,000 K, as hot as the surface of a cool red star.

Keep exploring

The 3D views in Ember’s Universe are scientific visualisations: physically based where possible, compressed or exaggerated where the real scales would hide everything.

Sources