Cosmic microwave background radiation (CMBR) is detected all around us. Explain how the existence and properties of the CMBR provide evidence for the Big Bang theory.

Pearson Edexcel International GCSE Physics (4PH1) — 8.3 Cosmology · Explain · 4 marks · View as Markdown

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

Astronomers can point a telescope in any direction in the sky and detect low-energy microwave radiation. This radiation is present throughout the universe and is nearly uniform in all directions.

Model answer (4 marks)

The CMBR is the remnant of the high‑energy radiation released shortly after the Big Bang. As the universe expanded, this radiation was stretched to longer wavelengths, now observed as microwaves. It is detected uniformly in every direction, filling all of space. This isotropic background is exactly what the Big Bang model predicts for a universe that began from a single hot, dense point and has since expanded.

Examiner tips

Use the word ‘remnant’ or ‘afterglow’ for the radiation; mention ‘high‑energy’ and ‘short‑wavelength’ early on.
Explain the wavelength stretching as ‘red‑shifting’ due to expansion.
State that the CMBR is seen in all directions and that this uniformity supports a single origin.
Keep the answer concise – 4 points, 1–2 sentences each.

Common mistakes

Confusing the CMBR with starlight or other background radiation.
Failing to mention that the radiation was originally high‑energy and has been red‑shifted to microwaves.
Omitting the isotropy (uniformity) of the CMBR.

Mark scheme (4 marks)

Shortly after the Big Bang, a large amount of high-energy / short-wavelength radiation was released
As the universe expanded, this radiation was stretched to longer wavelengths / became microwaves
CMBR is detected in all directions / everywhere in the sky
This is consistent with the Big Bang, where the universe expanded from a single point, so the radiation fills all of space

Key terms in this question

cosmic microwave background radiation (CMBR) · Big Bang theory