Explain how scientists use the observed redshift of light from distant galaxies as evidence that the universe is expanding.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When astronomers study the light from distant galaxies, they observe that the spectral lines in that light are shifted towards the red end of the spectrum compared to the same lines measured in a laboratory on Earth.
Model answer (5 marks)
Redshift is the increase in wavelength of light received from a galaxy compared with laboratory values.
It indicates that the galaxy is moving away from Earth – the source is receding.
The more distant a galaxy, the greater its redshift, meaning it is moving away faster.
Because all galaxies, in every direction, show this effect, it implies that space itself is expanding.
This observation supports the view that the universe began from a single point – the Big Bang.
It indicates that the galaxy is moving away from Earth – the source is receding.
The more distant a galaxy, the greater its redshift, meaning it is moving away faster.
Because all galaxies, in every direction, show this effect, it implies that space itself is expanding.
This observation supports the view that the universe began from a single point – the Big Bang.
Examiner tips
- Use the term "redshift" and explain it as a wavelength increase. Show the link between redshift and recession velocity. Mention the distance–redshift relationship. Conclude that expansion of space explains the observation.
- common_mistakes
- :
- Confusing redshift with blueshift. Saying only that galaxies are moving without linking to space expansion. Forgetting to mention the distance–redshift correlation.
Mark scheme (5 marks)
- Redshift means the wavelength of light received from a galaxy is longer (shifted towards the red end) than expected
- Redshift indicates that the galaxy is moving away from Earth / the source is receding
- More distant galaxies show greater redshift, meaning they are moving away faster
- Galaxies moving away in all directions suggests space itself is expanding
- This supports the idea that the universe began from a single point / links to the Big Bang
Key terms in this question
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