Space PhysicsAQA GCSE Physics (8463)

Red Shift

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)

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Worked examples3 Definitions4 Common mistakes3

Step-by-step worked examples

01.Defining red-shift
Foundation
Question
What is red-shift, and what does it tell us about a galaxy?
Solution
1. 1
  Wavelength is increased.
  $Light arrives with a longer wavelength than emitted.$
2. 2
  Source moving away.
  $Implies the galaxy is moving away from us.$
Answer
Red-shift is the increase in the wavelength of light from a galaxy moving away from us. The bigger the shift, the faster (and usually further) the galaxy.
02.Red-shift as evidence for the Big Bang
Higher
Question
Explain how the red-shift of distant galaxies supports the Big Bang theory (4 marks).
Solution
1. 1
  Red-shift means recession.
  $Galaxies are moving away from us.$
2. 2
  More distant ones move faster.
  $Distance \propto recession speed (Hubble)$
3. 3
  Implies expansion in every direction.
  $The universe is expanding.$
4. 4
  Run the film backwards.
  $Everything began compressed into a hot, dense point \to Big Bang$
Answer
Red-shift shows galaxies are moving away from us; more distant galaxies are red-shifted more, so they are moving away faster. This means the universe is expanding. Rewinding the expansion implies the universe started from a hot, dense point ~13.8 billion years ago — the Big Bang.
03.Dark matter and dark energy
Higher
Question
State what is meant by dark matter and dark energy, and why each is needed.
Solution
1. 1
  Dark matter explains rotation.
  $Galaxies rotate faster than visible matter alone could hold together.$
2. 2
  Dark energy explains acceleration.
  $Expansion of the universe is observed to be accelerating.$
Answer
Dark matter is invisible mass needed because galaxies rotate too fast for the visible matter alone. Dark energy is something driving the observed acceleration of the universe's expansion. Together they make up about 95% of the universe.

Practice with flashcards

Card 1 of 40 known · 0 to review

Key definitions and keywords

Red-shiftExaminer keyword: The increase in observed wavelength of light from a source moving away from the observer. Visible red-shift moves the spectrum toward the red end.
Big Bang theoryExaminer keyword: The theory that the universe began ~13.8 billion years ago from a single, very hot, very dense point and has been expanding ever since.
Dark matterExaminer keyword: Invisible mass needed to explain why galaxies rotate as fast as they do.
Dark energyExaminer keyword: Energy needed to explain the observed acceleration of the expansion of the universe.

Common mistakes and misconceptions

Mistake
'Red-shift proves the Big Bang.'
Why it happens
Scientific language is precise; theories are 'supported' not 'proved'.
How to avoid it
Use 'supports' / 'is evidence for'.
Mistake
Saying red-shift means a SHORTER wavelength.
Why it happens
Confusion with the Doppler 'higher pitch' direction.
How to avoid it
Red end = LONGER wavelength. Source moving AWAY stretches the wave.
Mistake
Imagining the Big Bang as an explosion in pre-existing space.
Why it happens
Everyday language.
How to avoid it
It's the expansion OF space itself, not an explosion within space.

Space PhysicsAQA GCSE Physics (8463)

Red Shift

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next. (2026)

PreviousNext

Master the topic

Worked examples3 Definitions4 Common mistakes3

Step-by-step worked examples

01.Defining red-shift
Foundation
Question
What is red-shift, and what does it tell us about a galaxy?
Solution
1. 1
  Wavelength is increased.
  $Light arrives with a longer wavelength than emitted.$
2. 2
  Source moving away.
  $Implies the galaxy is moving away from us.$
Answer
Red-shift is the increase in the wavelength of light from a galaxy moving away from us. The bigger the shift, the faster (and usually further) the galaxy.
02.Red-shift as evidence for the Big Bang
Higher
Question
Explain how the red-shift of distant galaxies supports the Big Bang theory (4 marks).
Solution
1. 1
  Red-shift means recession.
  $Galaxies are moving away from us.$
2. 2
  More distant ones move faster.
  $Distance \propto recession speed (Hubble)$
3. 3
  Implies expansion in every direction.
  $The universe is expanding.$
4. 4
  Run the film backwards.
  $Everything began compressed into a hot, dense point \to Big Bang$
Answer
Red-shift shows galaxies are moving away from us; more distant galaxies are red-shifted more, so they are moving away faster. This means the universe is expanding. Rewinding the expansion implies the universe started from a hot, dense point ~13.8 billion years ago — the Big Bang.
03.Dark matter and dark energy
Higher
Question
State what is meant by dark matter and dark energy, and why each is needed.
Solution
1. 1
  Dark matter explains rotation.
  $Galaxies rotate faster than visible matter alone could hold together.$
2. 2
  Dark energy explains acceleration.
  $Expansion of the universe is observed to be accelerating.$
Answer
Dark matter is invisible mass needed because galaxies rotate too fast for the visible matter alone. Dark energy is something driving the observed acceleration of the universe's expansion. Together they make up about 95% of the universe.

Practice with flashcards

Card 1 of 40 known · 0 to review

Key definitions and keywords

Red-shiftExaminer keyword: The increase in observed wavelength of light from a source moving away from the observer. Visible red-shift moves the spectrum toward the red end.
Big Bang theoryExaminer keyword: The theory that the universe began ~13.8 billion years ago from a single, very hot, very dense point and has been expanding ever since.
Dark matterExaminer keyword: Invisible mass needed to explain why galaxies rotate as fast as they do.
Dark energyExaminer keyword: Energy needed to explain the observed acceleration of the expansion of the universe.

Common mistakes and misconceptions

Mistake
'Red-shift proves the Big Bang.'
Why it happens
Scientific language is precise; theories are 'supported' not 'proved'.
How to avoid it
Use 'supports' / 'is evidence for'.
Mistake
Saying red-shift means a SHORTER wavelength.
Why it happens
Confusion with the Doppler 'higher pitch' direction.
How to avoid it
Red end = LONGER wavelength. Source moving AWAY stretches the wave.
Mistake
Imagining the Big Bang as an explosion in pre-existing space.
Why it happens
Everyday language.
How to avoid it
It's the expansion OF space itself, not an explosion within space.

Red Shift

Master the topic

Step-by-step worked examples

01.Defining red-shift

02.Red-shift as evidence for the Big Bang

03.Dark matter and dark energy

Practice with flashcards

Key definitions and keywords

Common mistakes and misconceptions

Red Shift

Master the topic

Step-by-step worked examples

01.Defining red-shift

02.Red-shift as evidence for the Big Bang

03.Dark matter and dark energy

Practice with flashcards

Key definitions and keywords

Common mistakes and misconceptions