Explain how redshift supports cosmic expansion
Connect the distant-galaxy redshift pattern with an expanding Universe and the Big Bang model.
Section P5.4 · The Universe
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recognise matching spectral redshift.
- Distinguish evidence from an exact historical photograph.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does the word evidence mean here?
What you will learn
- Connect the distant-galaxy redshift pattern with an expanding Universe and the Big Bang model.
- Separate evidence for expansion from a literal explosion centred on Earth.
Build the evidence chain
Distant-galaxy light appears redshifted relative to corresponding Earth references. Interpreting that population pattern shows distant galaxies receding and supports a Universe in which large-scale separations increase. It is evidence for expansion, rather than just a claim that galaxies have red-coloured surfaces.
Running an expansion picture backward suggests a past with matter closer together and a much hotter, denser state. This supports the Big Bang model. The spectrum is evidence about a model of cosmic history, not a direct photograph of the initial event.
Use the model’s boundaries
An expanding-spacing analogy can be viewed from different reference points. It need not give Earth a privileged central position. The Big Bang is not simply an ordinary explosion of stars into already empty space from the Earth’s location.
Cosmic expansion describes large-scale behaviour. Gravitationally bound systems such as the Solar System need not expand in the same way. A simple diagram cannot establish the Universe’s full geometry, precise age or every galaxy’s individual motion; other observations and models are needed.
Sources for this lesson
Pause and explain
What does the distant-galaxy redshift pattern support?
Worked example
Explain the chain from matching distant-galaxy wavelengths to a Big Bang interpretation.
Show the worked solution
- Identify increased observed wavelengths.
- Interpret the distant population as recession and expanding separations.
- Infer an earlier denser state supporting the Big Bang.
Answer Redshift → expansion evidence → support for an earlier hot, dense Universe.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Write three linked stages of the expansion argument.
Give me a hint
Begin with wavelength evidence before stating the model conclusion.
Compare my reasoning
- Observed wavelengths are increased.
- Distant galaxies are interpreted as receding.
- Expanding separations support a denser past and the Big Bang.
Redshift observations; expansion interpretation; support for a denser past.
Look for these in your work
- Start from observation.
- Use supports rather than a direct-origin photograph claim.
Correct “redshift proves Earth is the centre of an explosion”.
Give me a hint
Recession from one observer does not establish a unique centre.
Compare my reasoning
- Redshift supports large-scale expansion.
- Observers at other positions can also see increasing separations.
- Earth’s reference point does not become a physical central origin.
Expansion evidence does not make Earth a unique explosion centre.
Look for these in your work
- Distinguish reference point from centre.
- Retain the expansion evidence.
A model strip has positions 0, 2 and 5 units; later every separation doubles. Describe positions relative to the middle marker.
Give me a hint
Subtract the chosen reference position before and after scaling.
Compare my reasoning
- Initially relative positions are −2, 0, 3.
- Later positions 0, 4, 10 give relative values −4, 0, 6.
- Separations grow about the middle observer too.
Relative positions change from −2, 0, 3 to −4, 0, 6; expansion is also seen from the middle marker.
Look for these in your work
- Use the same reference marker.
- Avoid claiming a privileged observer.
Common mistakes
- Earth is not established as a unique centre.
- Expansion is not ordinary stellar explosion.
- Bound systems need not all expand.
What can you explain now?
What does the distant-galaxy redshift pattern support?
Compare with the explanation
Large-scale cosmic expansion
The recession pattern supports increasing large-scale separations and the Big Bang model.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Separate evidence for expansion from a literal explosion centred on Earth.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.