Redshift

OCR

GCSE

Redshift describes the observed increase in wavelength of electromagnetic radiation emitted by sources moving away from an observer, shifting spectral absorption lines toward the red end of the spectrum. This phenomenon provides critical evidence for the metric expansion of the universe, demonstrating that distant galaxies are receding at velocities proportional to their distance. When analyzed alongside Cosmic Microwave Background Radiation (CMBR), redshift underpins the Big Bang theory as the prevailing cosmological model.

Objectives

Exam Tips

Pitfalls

Key Terms

Mark Points

What You Need to Demonstrate

Key skills and knowledge for this topic

Award 1 mark for stating that light from distant galaxies shows an increase in wavelength (shift towards the red end of the spectrum)
Credit responses that link redshift to the galaxy moving away from the observer
Award 1 mark for stating the relationship: the more distant the galaxy, the greater the redshift and the faster it is moving away
Candidates must link this observation to the conclusion that the universe is expanding from a hot, dense initial point (Big Bang)

Example Examiner Feedback

Real feedback patterns examiners use when marking

"You correctly identified that the galaxy is moving away, but you must explicitly mention the 'increase in wavelength' to gain the technical mark"
"Good link to the Big Bang theory. To improve, explain *why* redshift supports this (i.e., the universe is expanding)"
"Avoid saying 'galaxies are red'. Instead, state that the 'light spectrum is shifted towards the red end'"
"You have described the motion well; for higher marks, discuss the relationship between the distance of the galaxy and its speed"

Marking Points

Key points examiners look for in your answers

Award 1 mark for stating that light from distant galaxies shows an increase in wavelength (shift towards the red end of the spectrum)
Credit responses that link redshift to the galaxy moving away from the observer
Award 1 mark for stating the relationship: the more distant the galaxy, the greater the redshift and the faster it is moving away
Candidates must link this observation to the conclusion that the universe is expanding from a hot, dense initial point (Big Bang)

Examiner Tips

Expert advice for maximising your marks

💡When explaining the Big Bang, always use the phrase 'expanded from a hot, dense singularity' or 'hot, dense point' to secure the AO1 mark
💡In 6-mark questions, structure your answer logically: Observation (Redshift) → Mechanism (Wavelength increase due to recession) → Conclusion (Universe is expanding)
💡Be careful with the word 'proves'; scientific theories are 'supported by evidence' rather than proven. Use 'provides evidence for' instead

Common Mistakes

Pitfalls to avoid in your exam answers

Stating that the galaxies themselves are expanding or turning red, rather than the space between them expanding and stretching the light waves
Confusing redshift with the heating of objects (thinking red means hot) rather than a Doppler shift in frequency/wavelength
Failing to distinguish between the observation (redshift) and the theory it supports (Big Bang), often treating them as the same thing

Study Guide Available

Comprehensive revision notes & examples

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Key Terminology

Essential terms to know

The Doppler Effect applied to light waves

Metric expansion of the universe

Evidence for the Big Bang Theory

Relationship between recession velocity and distance

Likely Command Words

How questions on this topic are typically asked

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Explain

Compare

Suggest

State

Practical Links

Related required practicals

•{"code":"Modelling Expansion","title":"Balloon Model","relevance":"Demonstrates how points on an expanding surface move away from each other, with further points moving faster"}

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