The Cosmic Microwave Background (CMB) radiation is observed uniformly across the entire sky. Explain what the CMB radiation tells us about the origin and early conditions of the universe.

WJEC GCSE Physics (Wales) — 2.1 Light, lenses and the universe · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Scientists detected a faint microwave radiation arriving from all directions in space with almost exactly the same intensity, now known as the Cosmic Microwave Background radiation.

Model answer (4 marks)

The CMB shows that the universe began with a Big Bang.
The early universe was extremely hot and dense.
The CMB is the remnant radiation from that early hot phase.
Since the Big Bang the universe has expanded and cooled, so the radiation is now observed as microwaves.

Examiner tips

Use the phrase "Big Bang" and "remnant radiation" to hit the key points.
Show the link between expansion, cooling and the shift to microwaves.
Keep each point short and to the point to fit the 4‑mark structure.

Common mistakes

Confusing the CMB with other background radiation, e.g. starlight.
Failing to mention that the radiation has been red‑shifted to microwaves by expansion.

Mark scheme (4 marks)

The CMB radiation is evidence for the Big Bang / universe began with a Big Bang
The early universe was very hot (and dense)
The CMB radiation is the remnant / leftover energy / radiation from the very early universe (after the Big Bang)
The universe has expanded and cooled since the Big Bang, which is why the radiation is now detected as microwaves (low energy / long wavelength)

Key terms in this question

Cosmic Microwave Background (CMB) · universe

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