The Cosmic Microwave Background: Light From the Early Universe

The cosmic microwave background, or CMB, is the oldest light that astronomers can observe. It was released about 380,000 years after the Big Bang, when the universe cooled enough for atoms to form and light could travel freely. Today it reaches Earth as faint microwave radiation at a temperature of about 2.725 kelvin, arriving almost equally from every direction.

What the CMB is

Before recombination, the universe was an opaque plasma of protons, electrons, and photons. Photons scattered constantly, so light could not travel far before it was absorbed or redirected. When the temperature dropped to roughly 3,000 kelvin, electrons bound to nuclei. The plasma became neutral gas, and the photons that had been trapped in it were free to stream outward.

Those photons have been traveling ever since. The expansion of space has stretched their wavelengths by a factor of about 1,100, which moved their peak emission from visible light into the microwave band. The CMB is therefore a snapshot of the universe at a single moment in its history, not a signal from a particular object.

Discovery and early measurements

In 1964, two engineers at Bell Laboratories detected a persistent excess radio noise that they could not remove from their antenna. They found that it came equally from all directions, and they later connected it to the cosmic background predicted by hot Big Bang models. They received the 1978 Nobel Prize in Physics for the discovery.

Early measurements suggested the basic character of the signal. It has a blackbody spectrum, meaning its intensity at each wavelength matches the pattern expected from a hot body in thermal equilibrium. COBE's precise spectral measurements in 1990 confirmed that match, which was one of the strongest confirmations that the universe had once been hot and dense.

Why the sky is almost uniform

The CMB is remarkably uniform. Its temperature is the same to about one part in 100,000 across the sky. That uniformity is itself a clue. Regions that are now billions of light-years apart had to be in contact early on to reach the same temperature, and models of rapid early expansion, called inflation, explain how this could happen.

The tiny variations that remain are called anisotropies. They are the seeds of all later structure. Regions slightly denser than average gravitationally attracted more matter, eventually forming galaxies, clusters, and the large-scale web of the cosmos.

Missions that mapped the CMB

Three major space missions transformed the CMB from a faint signal into a precision map. Each one improved the angular resolution and sensitivity of the last.

Mission	Launch year	Angular resolution	Main contribution
COBE	1989	About 7 degrees	Confirmed blackbody spectrum and first anisotropies
WMAP	2001	About 0.2 degrees	Precise maps and early cosmological parameters
Planck	2009	About 5 arcminutes	Highest-precision temperature and polarization maps

The resolution figures are rounded from published mission specifications. Each step down in angular size revealed finer structure in the early universe.

COBE's work was recognized with the 2006 Nobel Prize in Physics, awarded to its lead scientists for the discovery of the blackbody form and the anisotropy of the CMB. WMAP and Planck built directly on that foundation, and Planck's final data release in 2018 remains the most precise full-sky temperature map of the CMB.

Acoustic peaks and the shape of the universe

The sky map of CMB temperature variations can be broken into patterns of different sizes. When these patterns are plotted as a power spectrum, the result shows a series of peaks. The first peak appears at an angular scale of roughly one degree, and its position reveals the geometry of space. The measured position is consistent with a flat universe.

The heights of the later peaks reveal the composition. The ratio of odd to even peaks depends on the amount of ordinary matter, and the overall pattern depends on the amount of dark matter. Fitting the peaks with a single model gives the energy budget listed in the table below, using rounded values from the Planck analysis.

Component	Approximate share
Ordinary matter	About 5%
Dark matter	About 27%
Dark energy	About 68%

These numbers are consistent with the galaxy and lensing measurements described elsewhere in cosmology. The microwave background is the one measurement in this list that probes the early universe directly.

Polarization and the search for gravitational waves

Light from the CMB is slightly polarized. The polarization pattern splits into two types, which are called E modes and B modes. E modes come from density variations and have been measured in detail. B modes can come from gravitational waves produced during inflation, if inflation occurred at high energy.

Searches for primordial B-mode polarization have so far set upper limits rather than a confirmed detection. The limits already rule out some inflation models, and future experiments aim to reach lower levels. A detection would provide direct evidence for the early expansion phase and for the quantum origin of the first density variations.

The Hubble tension in the CMB

The CMB also provides one of the two main values of the Hubble constant. Fitting the microwave background data with the standard model gives an expansion rate of about 67 kilometers per second per megaparsec. Local measurements that use nearby distances give a higher value, about 73. The difference, roughly 8 percent, is called the Hubble tension.

This disagreement is not resolved. It may reflect unrecognized systematic effects in one method, or it may point to physics beyond the standard model. The CMB remains central to the question because it offers the cleanest view of the early universe.

Removing foreground emission

Measuring the CMB is harder than it sounds, because the Milky Way and nearby sources emit microwave light too. Dust grains in our galaxy glow in microwaves, and fast electrons in the galaxy emit synchrotron radiation. Both add signals that can look like cosmic structure if they are not removed.

Mission teams handle this by observing at several frequencies. The CMB has a known blackbody spectrum, while galactic foregrounds have different spectra. Comparing maps taken at different frequencies lets analysts separate the two components. Planck used nine frequency channels, which made this separation much more reliable than in earlier missions.

Because foreground removal is delicate, cosmologists check their results with independent methods and with simulated skies. Agreement between separate pipelines is a key part of trusting the final maps.

Key takeaways

The cosmic microwave background is light released about 380,000 years after the Big Bang. It has cooled to about 2.725 kelvin and arrives almost uniformly from every direction.

Its tiny temperature variations, about one part in 100,000, are the seeds of galaxies and clusters. COBE, WMAP, and Planck progressively measured these variations with higher precision.

The CMB supports a flat universe with about 5 percent ordinary matter, 27 percent dark matter, and 68 percent dark energy. It also sits at the center of the Hubble tension and the search for primordial gravitational waves, which remain open questions.