A student states: 'The Doppler effect explains why light from distant galaxies is red-shifted. The faster a galaxy moves away from Earth, the greater the increase in the wavelength of its light.' Explain what is meant by red-shift and describe what the pattern of red-shift observations from distant galaxies tells us about the universe.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Red‑shift is the observed increase in the wavelength of light emitted by a galaxy moving away from Earth – the light is shifted to longer, redder wavelengths.
The red‑shift of a galaxy increases with its distance: the farther a galaxy is, the greater its red‑shift, meaning it is moving away faster.
All distant galaxies are observed to be moving away from us, showing that the universe is expanding.
This pattern of red‑shifts provides evidence for the Big Bang theory, in which the universe started from a single point and has been expanding ever since.
The red‑shift of a galaxy increases with its distance: the farther a galaxy is, the greater its red‑shift, meaning it is moving away faster.
All distant galaxies are observed to be moving away from us, showing that the universe is expanding.
This pattern of red‑shifts provides evidence for the Big Bang theory, in which the universe started from a single point and has been expanding ever since.
Examiner tips
- Use the term "red‑shift" and explain it as a shift to longer wavelengths. Mention the distance–red‑shift relationship. State that all galaxies recede, implying expansion. Link the observation to the Big Bang theory.
Common mistakes
- Confusing red‑shift with blueshift or stating it is due to gravitational effects. Failing to mention that red‑shift increases with distance. Not connecting the observations to the Big Bang theory.
Mark scheme (4 marks)
- Red-shift is the observed increase in the wavelength of light emitted by a galaxy moving away from Earth / the wavelength of light from a distant galaxy is longer than expected
- The more distant the galaxy, the greater (larger) its red-shift / the faster it is moving away from Earth
- All distant galaxies are moving away from us / the universe is expanding
- This is consistent with / provides evidence for the Big Bang theory, in which the universe originated from a single point and has been expanding ever since
Key terms in this question
red-shift · Doppler effect · wavelength
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Cosmology questions
- Cosmic microwave background radiation (CMBR) is detected all around us. Explain …
- The Big Bang theory is the currently accepted model for the origin of the univer…
- A scientist observes two distant galaxies, Galaxy P and Galaxy Q. Galaxy P is tw…
- Explain how observations of red-shift from distant galaxies provide evidence for…