Penzias and Wilson Detect the Cosmic Microwave Background
A persistent hiss in a satellite antenna turns out to be the afterglow of the Big Bang itself
Quick facts
- Discoverers
- Arno Penzias and Robert Wilson, Bell Telephone Laboratories
- Instrument
- Holmdel horn antenna, New Jersey
- Measured temperature
- c. 3.5 Kelvin
- Recognition
- 1978 Nobel Prize in Physics
What happened
Arno Penzias and Robert Wilson, two radio astronomers at Bell Telephone Laboratories, were using a large horn antenna in Holmdel, New Jersey, originally built to test communications with NASA's Echo satellite, when they encountered a faint, persistent microwave signal they could not eliminate no matter where in the sky they pointed the instrument. The signal matched a temperature of about 3.5 degrees Kelvin and appeared uniformly across the entire sky regardless of time or season, leading the pair to conclude it originated from beyond the galaxy rather than from any local source of interference. Their result coincided with theoretical predictions made by physicists including George Gamow, Ralph Alpher, and Robert Herman back in the late 1940s, who had argued that if the universe began in a hot Big Bang, its residual heat should still be faintly detectable today; Penzias and Wilson's team and a group at Princeton University announced the discovery together in a pair of letters published in the Astrophysical Journal in July 1965.
Why it matters
The cosmic microwave background was the first direct observational evidence that the universe began in a hot, dense state, providing decisive support for the Big Bang theory over its chief rival, the steady state theory, which predicted no such uniform background radiation. Penzias and Wilson received the 1978 Nobel Prize in Physics for the discovery, and mapping the CMB's tiny temperature variations became one of the central projects of later cosmology.
How we know
Penzias and Wilson published their observations directly in the Astrophysical Journal in 1965, alongside a companion paper from the Princeton group explaining the theoretical significance of the signal, and the CMB has since been mapped in far greater detail by dedicated satellite missions that confirm the original 1965 measurement.
Sources
- NASA Goddard Space Flight Center, Cosmic Times. Murmur of a Bang · Reputable sourceimagine.gsfc.nasa.gov · The domain "imagine.gsfc.nasa.gov" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- European Space Agency. Cosmic Microwave Background (CMB) radiation · Reputable sourceesa.int · The domain "esa.int" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Related timelines
- Space Exploration → · The horn antenna Penzias and Wilson used was originally built to support NASA's Echo satellite program; see the Space Exploration timeline for the broader early satellite and space-race context of the era.