Cosmic Microwave Background
The leftover glow of the early universe
Do Now: A pizza oven stays warm long after it is turned off. If the universe started out extremely hot, what might we still be able to detect today?
Leftover heat — a faint glow of radiation coming from every direction in the sky. It was predicted in the 1940s and discovered in 1965.
A. Where the Glow Came From
The early universe was so hot that atoms could not hold onto their electrons. Light kept bouncing off free electrons — the universe was like a thick fog.
About 380,000 years after the Big Bang, the universe cooled to about 3,000 K. Electrons joined nuclei to form atoms, and light could finally travel freely.
That light has been traveling through space ever since. It is the oldest light we can see.
B. Stretched Into Microwaves
It comes from every direction in the sky, almost exactly the same everywhere.
Its spectrum is a nearly perfect blackbody curve — the signature of something that was once hot and uniform. Review blackbody radiation
C. Discovered by Accident
1948: Ralph Alpher and Robert Herman predicted that the Big Bang should have left behind radiation at a temperature of a few kelvins.
1964–65: At Bell Labs in Holmdel, New Jersey, Arno Penzias and Robert Wilson were testing a large horn-shaped radio antenna. They kept picking up a faint hiss that came from every direction, day and night.
They checked everything — they even cleaned pigeon droppings out of the antenna. The hiss stayed. Nearby at Princeton, Robert Dicke’s team realized it was the leftover glow of the Big Bang.
1978: Penzias and Wilson won the Nobel Prize in Physics.
Satellites (COBE, WMAP, and Planck) mapped the whole sky. The glow is the same everywhere to about 1 part in 100,000. Those tiny differences show where matter was slightly denser — the seeds that grew into galaxies.
D. Why It Supports the Big Bang
A rival idea, the “steady state” model, said the universe never had a hot beginning. Why is the cosmic microwave background a problem for that idea?
A universe that was never hot and dense has no way to make a smooth glow coming from every direction. The Big Bang predicted this radiation before it was found, and the measured temperature and spectrum match the prediction.
Check Your Understanding
5 quick questions — pick an answer to see if you’re right.
1. The cosmic microwave background is best described as
2. Why is the radiation now in the microwave part of the spectrum?
3. The temperature of the cosmic microwave background today is about
4. From which direction does the cosmic microwave background come?
5. Who discovered the cosmic microwave background in 1965?