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Glossary · Galaxies & Cosmology
Cosmic microwave background
The oldest light there is, released when the universe first became transparent.
For 380,000 years the universe was an opaque plasma. When it cooled enough for electrons to bind to nuclei, light was suddenly free to travel, and it has been travelling ever since — stretched by expansion from visible light to microwaves. It fills the sky at 2.7 K and is smooth to one part in a hundred thousand. Those tiny fluctuations are the seeds every galaxy grew from, and measuring them is how the age, composition and geometry of the universe are known. It is also the reason the universe has a photograph of its own childhood and nothing earlier. Before that moment the plasma scattered light so effectively that no image could form, so the microwave sky is a wall: everything before 380,000 years has to be inferred from its consequences rather than looked at, and gravitational waves are the only thing anybody expects to see through it.
Where it has come up lately
Related terms
Fast radio burst
A millisecond flash of radio energy from another galaxy, cause still unsettled.
Redshift
The stretching of light from distant objects, and the universe's odometer.
Hubble tension
Two good measurements of the universe's expansion rate that stubbornly disagree.
Dark energy
The name for whatever is making the expansion of the universe speed up.
Questions
What is Cosmic microwave background?
The oldest light there is, released when the universe first became transparent.
Why does it matter?
It is also the reason the universe has a photograph of its own childhood and nothing earlier. Before that moment the plasma scattered light so effectively that no image could form, so the microwave sky is a wall: everything before 380,000 years has to be inferred from its consequences rather than looked at, and gravitational waves are the only thing anybody expects to see through it.