A Measurement of Excess Antenna Temperature at 4080 Mc/s
A faint 3.5 K hiss from every direction — the light left over from the Big Bang.
Two engineers cleaning up a stubborn hiss in their antenna had, without knowing it, picked up the faint afterglow of the Big Bang.
The big idea
If the universe began in a hot, dense state and has been expanding and cooling ever since, then it should still be glowing faintly everywhere — leftover heat from that first fireball, stretched by the expansion into faint microwaves. Theory put that glow at just a few degrees above absolute zero, coming equally from every direction.
In 1965 Penzias and Wilson found exactly that. Their sensitive radio antenna picked up a persistent noise — about 3.5 degrees above absolute zero — that they could not blame on the sky, the ground, the instrument, or any star or galaxy. It came from everywhere, never changed, and would not go away. It was the universe's own background warmth, the oldest light there is.
How it came about
The two worked at Bell Labs in New Jersey with a giant horn-shaped antenna, built for early satellite communications. To use it for radio astronomy they had to account for every scrap of unwanted noise. One faint hiss refused to be explained. They checked everything — even evicting a pair of pigeons nesting in the horn and scrubbing out their droppings. The hiss stayed.
Meanwhile, thirty miles away at Princeton, Robert Dicke's team had reasoned their way to the same prediction and were building an antenna to look for the glow. A phone call connected the two groups. The Princeton physicists realized at once what Penzias and Wilson had stumbled onto. The two papers ran side by side in the same journal in 1965 — one reporting the signal, one explaining it.
Why it mattered
At the time, scientists were split between a universe with a hot beginning and a 'steady state' universe that had always looked roughly the same. This faint, even glow was the deciding evidence: only a hot, dense early universe naturally leaves such an afterglow behind. It turned the Big Bang from one idea among several into the foundation of modern cosmology, and it handed astronomers a direct snapshot of the infant universe to study ever since.
A way to picture it
Open the oven door after baking and the air still pours out warmth, even though the flame is off — that warmth is the leftover heat of the fire. The cosmic microwave background is the leftover heat of the Big Bang, filling all of space. The fireball cooled long ago, but its glow is still here, just stretched and faint — about 2.7 degrees above absolute zero, the same hush in every direction you point.
Where it sits
The story runs from Planck's 1900 law of blackbody glow, through the 1920s discovery that the universe is expanding, to the 1940s prediction of relic radiation, and lands here. After Penzias and Wilson, satellites named COBE, WMAP and Planck measured this glow ever more precisely, reading from its tiny ripples the age and makeup of the cosmos. The radiation they happened upon is now the single most informative measurement in all of cosmology.
This excess temperature is, within the limits of our observations, isotropic, unpolarized, and free from seasonal variations (July, 1964–April, 1965). A possible explanation for the observed excess noise temperature is the one given by Dicke, Peebles, Roll, and Wilkinson (1965) in a companion letter in this issue.