Cosmic microwave background — Science definition

Radiation observed across the sky left from an early stage of the universe.

Where it is taught

In use

Evidence includes galaxy redshift patterns, widespread cosmic microwave background radiation, and the abundances of light elements.

Spectra & Evidence for an Expanding Universe

Questions using cosmic microwave background

Problem 1

The cosmic microwave background is
  1. light from nearby stars
  2. faint radiation left from the early universe
  3. heat from Earth
  4. a measurement error
Answer and explanation

faint radiation left from the early universe

It is thermal radiation released when the universe first became transparent.

Problem 2

Why is the cosmic microwave background considered strong evidence rather than one observation among many?
Answer and explanation

It was predicted before it was found, its temperature matches what the model required, and its tiny variations match the structure needed to form galaxies. A prediction confirmed in detail by a later measurement is far stronger evidence than a result a theory was adjusted to fit, which is why it carries particular weight.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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