Edexcel IGCSE Physics · Spec 8.3
Redshift & the Big Bang
Redshift and the evidence for the Big Bang theory.
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Redshift & the Big Bang
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Explained
Reading the light from distant galaxies
Split the light from a star into a spectrum and you see a pattern of dark lines. Those lines are produced by particular elements absorbing particular wavelengths, and the pattern is the same wherever the element is, because the physics of the atom is the same everywhere.
In light from distant galaxies, that whole pattern appears shifted towards the red end of the spectrum. The lines are still in the same relative positions, so it is recognisably the same pattern, but every line has moved. That shift is redshift.
What redshift means
A shift towards red means the wavelength has increased, and since the speed of light is fixed, a longer wavelength means a lower frequency.
Wavelength increases when the source is moving away from the observer. So the galaxies are receding, and almost every distant galaxy shows a redshift, which means almost all of them are moving away from us.
The further away a galaxy is, the greater its redshift, so the more distant galaxies are receding faster.
Why this is evidence for expansion
The natural reading is that we are at the centre of something, and that is the wrong reading.
Space itself is expanding, and the galaxies are being carried apart with it. On an expanding surface every point moves away from every other point, so an observer in any galaxy would see exactly what we see. There is no centre to be at.
An expanding balloon with dots inked on it is the standard analogy. Every dot moves away from every other, and the dots furthest apart separate fastest, which is the distance relationship observed.
The Big Bang
If the universe is expanding now, then running the expansion backwards means it was once smaller, hotter and denser. Extrapolating all the way back gives an extremely hot, dense beginning, which is the Big Bang.
The second piece of evidence is the cosmic microwave background: faint microwave radiation arriving from every direction, almost identically in all of them. It is radiation left over from the hot early universe, stretched into the microwave region by the expansion of space, and its uniformity is what makes a local source implausible.
What the examiners say about this topic
A principal examiner report for Edexcel International GCSE Physics discusses a question on a moving source, which uses the same reasoning as redshift. Most candidates knew the frequency would change, but the quality of the explanations varied a great deal.
The least commonly awarded marking point was the clear observation that the wavefronts were closer together, followed by recognising that the wave speed was constant. The report then notes that some candidates did not clearly distinguish between wavefronts, wavelength and waves, which led to confusion.
Those three words mean different things. A wavefront is a line joining points at the same stage of the wave. A wavelength is the distance between adjacent wavefronts. A wave is the whole disturbance. Using them loosely makes it impossible to tell what has actually changed.
The constant speed is the step that makes the whole argument work. If the wavelength increases and the speed does not change, the frequency must fall. Leaving that out gives a description of the observation without the reasoning behind it.
What redshift does not tell you
It gives a galaxy's speed along the line of sight, not its distance directly, and not its speed across the sky.
It also does not tell you what happened before the Big Bang or what caused it. The theory describes the universe from an extremely early moment onwards, and questions asking what came before are outside what the evidence can support.
Spec 8.3
What you need to know
- Explain redshift
- Describe the evidence for an expanding universe
- Outline the Big Bang theory
Active recall
Quick check
Answer each question before opening the answer.
What is redshift and what does it show?
Light from distant galaxies is shifted to longer (red) wavelengths, showing they are moving away and the Universe is expanding.
How does redshift support the Big Bang theory?
More distant galaxies show greater redshift, so they move away faster — consistent with expansion from a single point.
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