Phenomena · Space Codex
Cosmic microwave background
Light released about 380,000 years after the Big Bang, when the universe cooled enough to become transparent.
The oldest light in the universe
At a glance
Key points
Today it is a 2.7 K glow filling the whole sky.
- Released 380,000 years after the Big Bang
- Now just 2.7 K
- Fluctuations of 1 part in 100,000
A baby picture
Tiny temperature differences in the CMB are the seeds of every galaxy. Planck mapped them most precisely, fixing the universe's age and composition.
Seeing it
The CMB is microwave light; to the eye the sky would look black. All-sky maps use false colour.
An accidental discovery
In 1964 and 1965 Arno Penzias and Robert Wilson could not get rid of a faint, steady hiss in a horn antenna at Bell Labs in New Jersey, whichever way they pointed it. Nearby, Robert Dicke's group at Princeton was preparing to search for exactly this relic heat of the Big Bang, and the two teams published side by side in 1965. Penzias and Wilson shared the 1978 Nobel Prize in Physics.
Three generations of satellites
NASA's COBE showed in the early 1990s that the CMB has an almost perfect blackbody spectrum and detected its faint ripples, work recognised with the 2006 Nobel Prize for John Mather and George Smoot. WMAP sharpened the map from 2001, and ESA's Planck, observing from 2009 to 2013, measured the temperature pattern in finer detail than ever. Ground-based telescopes in Chile and at the South Pole now map its polarisation.
Reading the ripples
The hot and cold spots record sound waves that rang through the glowing plasma before it became transparent. Their typical size on the sky, about one degree, shows that space is very nearly flat, while the relative heights of the peaks in the pattern fix the amounts of ordinary and dark matter. The whole sky is also slightly warmer in one direction, because the Solar System moves through the CMB at about 370 km/s.
The Hubble tension
Using the standard cosmological model, Planck's map predicts today's expansion rate as 67.4 km/s/Mpc. Measurements that climb a distance ladder of Cepheid stars and supernovae give about 73, a gap of about 5σ. The Atacama Cosmology Telescope's final 2025 analysis also found a low value. Whether the mismatch points to new physics or to hidden errors is unresolved.
Missions
- 2009–2013
Planck
ESACompletedMapped the cosmic microwave background at the highest precision to date.
Related entries
- Cosmic timeline13.8 billion years in one view
- Dark matter and dark energyThe 95% we cannot see
Sources
- Dicke, Peebles, Roll & Wilkinson 1965, ApJ 142:414 (opens in a new tab)
- Nobel Prize in Physics 2006: press release (opens in a new tab)
- ACT DR6: power spectra and ΛCDM parameters, 2025 (arXiv) (opens in a new tab)
- ESA: Planck and the cosmic microwave background (opens in a new tab)
- Planck Collaboration 2018, A&A 641:A6 (arXiv) (opens in a new tab)
- Penzias & Wilson 1965, ApJ 142:419 (opens in a new tab)
- Planck 2018 results I (arXiv) (opens in a new tab)
- Riess et al. 2022, ApJL 934:L7 (arXiv) (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.