Discovery of cosmic microwave background radiation

In 1964, US physicist Arno Allan Penzias and radio-astronomer Robert Woodrow Wilson discovered the cosmic microwave background (CMB), estimating its temperature as 3.5 K, as they experimented with the Holmdel Horn Antenna.

There had been a prior measurement of the cosmic background radiation (CMB) by Andrew McKellar in 1941 at an effective temperature of 2.3 K using CN stellar absorption lines observed by W. S.

[5] Over two decades later, working at a Bell Telephone Laboratories facility atop Crawford Hill in Holmdel, New Jersey, in 1964, Arno Penzias and Robert Wilson were experimenting with a supersensitive, 6 meter (20 ft) horn antenna originally built to detect radio waves bounced off Echo balloon satellites.

They removed the effects of radar and radio broadcasting, and suppressed interference from the heat in the receiver itself by cooling it with liquid helium to −269 °C, only 4 K above absolute zero.

At that same time, Robert H. Dicke, Jim Peebles, and David Wilkinson, astrophysicists at Princeton University just 60 km (37 mi) away, were preparing to search for microwave radiation in this region of the spectrum.

Dicke and his colleagues reasoned that the Big Bang must have scattered not only the matter that condensed into galaxies, but also must have released a tremendous blast of radiation.

Penzias read the paper and called Dicke again and invited him to Bell Labs to look at the horn antenna and listen to the background noise.