Knowledge Organiser: Red Shift and Big Bang
This topic summary covers Knowledge Organiser: Red Shift and Big Bang within Red Shift & Big Bang for GCSE Physics. Revise Red Shift & Big Bang in Space Physics for GCSE Physics with 13 exam-style questions and 12 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 15 of 15 in this topic. Use this topic summary to connect the idea to the wider topic before moving on to questions and flashcards.
Topic position
Section 15 of 15
Practice
13 questions
Recall
12 flashcards
Knowledge Organiser: Red Shift and Big Bang
Key Terms
- Red shift — wavelength increases, galaxy moving away
- Doppler effect — wave change due to relative motion
- CMBR — 2.7 K microwave radiation from all directions
- Big Bang — origin of universe 13.8 billion years ago
- Hubble's Law — velocity proportional to distance
- Absorption spectrum — dark lines used to measure red shift
Evidence for Big Bang
- Red shift: galaxies moving away, universe expanding
- More distant = greater red shift = faster recession
- CMBR: predicted then discovered, from all directions
- CMBR temperature: about 2.7 K
- Correct H/He abundance predicted by Big Bang nucleosynthesis
Key Distinctions
- Red shift: longer wavelength, galaxy receding
- Blue shift: shorter wavelength, galaxy approaching
- Andromeda: blue-shifted (coming towards us)
- Space expands — galaxies carried with it, not through it
- Big Bang: created space itself, not explosion in space
CMBR Key Facts
- Temperature: approximately 2.7 K
- Detected from all directions equally
- Cooled-down afterglow of hot early universe
- Predicted before it was discovered (1965)
- Originally high-energy photons, now stretched to microwaves
Key Equations
- Δλ/λ ≈ v/c (red shift — change in wavelength proportional to recessional speed)
- Greater red shift → greater recessional speed → greater distance
- Hubble's law: v = H₀ × d (recession speed proportional to distance)
- Age of universe ≈ 1/H₀ ≈ 13.8 billion years
Common Mistakes
- Saying galaxies move through space away from us: Space itself is expanding — galaxies are carried apart by the expansion of space, not moving through it like objects in a room
- Confusing red shift and blue shift: Red shift means a galaxy is moving away (wavelength stretched, frequency reduced); blue shift means moving towards us — all distant galaxies show red shift
- Saying the Big Bang happened at a single point in space: The Big Bang was an expansion of space itself from an extremely hot, dense state — it did not happen "somewhere" in pre-existing space
- Thinking CMBR is visible light: The cosmic microwave background radiation is in the microwave part of the EM spectrum — it was originally high-energy radiation stretched by the expansion of the universe
- Saying red shift only proves the universe is expanding: Red shift AND CMBR together provide the strongest evidence for the Big Bang — red shift alone could have other explanations; CMBR is the "smoking gun"
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Practice Questions for Red Shift & Big Bang
What is the orbital period of a geostationary satellite?
Explain why a geostationary satellite stays above the same point on Earth's surface.
Quick Recall Flashcards
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