Discovery of the Cosmic Microwave Background
Arno Penzias & Robert Wilson · 1965
"A faint, uniform microwave hiss detectable in every direction of the sky is the cooled-down afterglow of the universe's hot early state — direct physical evidence, not just a mathematical inference, that the Big Bang happened."
Two Bell Labs engineers trying to eliminate radio noise from a satellite antenna in 1965 couldn't get rid of a faint hiss no matter what they cleaned. It turned out to be leftover heat from the birth of the universe.
Penzias and Wilson ruled out every mundane source of interference — including, famously, pigeon droppings in the antenna — before realizing the signal was coming uniformly from all directions in space, not from any specific source. Unknown to them, Princeton physicists nearby (Dicke, Peebles) had predicted exactly this kind of relic radiation should exist if the Big Bang model were correct: as the early universe cooled from an extremely hot, dense state, the radiation filling it would stretch to microwave wavelengths and persist as a near-uniform background today. The two groups connected, and the discovery instantly converted the Big Bang from one competing cosmological theory (against the rival 'Steady State' model) into the scientific consensus, since the Steady State model had no mechanism to produce this background at all.
Why did the discovery of the cosmic microwave background settle the Big Bang vs. Steady State debate?
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The explanation above is written with AI assistance. These are the originals — go to them to check it.
- A Measurement of Excess Antenna Temperature at 4080 Mc/sThe Astrophysical Journal, 1965
Hubble's Law and the Expanding Universe
"Distant galaxies are receding from us at speeds proportional to their distance — the first hard observational evidence that the universe itself is expanding, which run backward in time implies it began from a single point."