All revision notes topics

CosmologyEdexcel GCSE Physics: Revision notes

Section 1

What is red-shift?

If a wave source moves relative to an observer, the observed frequency and wavelength change. This is called the Doppler effect.

  • If the source moves away from the observer, the wavelength increases and the frequency decreases — for light, this shifts towards the red end of the spectrum, called red-shift.
  • If the source moves towards the observer, the wavelength decreases and frequency increases — blue-shift.
  • Light from distant galaxies shows red-shift, meaning those galaxies are moving away from us.
Key termsred-shiftDoppler effect
Think of it like this

Like a police siren sounding higher-pitched as it approaches and lower-pitched as it moves away — the same stretching/compressing happens to light waves.

Section 2

What does red-shift tell us about distant galaxies?

  • Light from almost all distant galaxies is red-shifted, showing they are moving away from Earth.
  • The further away a galaxy is, the greater its red-shift — meaning more distant galaxies are moving away faster.
  • This pattern is true in every direction we look, suggesting the whole Universe is expanding, not that Earth is at a special centre.
Key termsexpanding Universe
Exam tip

Exam answers should explicitly link greater distance to greater red-shift to greater recession speed — examiners credit each step separately.

Section 3

Big Bang theory vs Steady State theory

Big Bang theorySteady State theory
Origin of UniverseBegan from a single extremely hot, dense point and has been expanding and cooling ever sinceHas always existed and always looked the same, with matter continuously created as it expands
Explains red-shift?Yes — expansion of space stretches light from receding galaxiesYes — also predicts galaxies moving apart as the Universe expands
Explains CMB radiation?Yes — predicted as leftover radiation from the hot early UniverseNo convincing explanation

Both theories can account for red-shift, because both involve an expanding Universe. Red-shift alone cannot distinguish between them.

Key termsBig Bang theorySteady State theory
Common mistake

Don't say red-shift alone proves the Big Bang — both theories predict it. It is the CMB radiation that decides between them.

Section 4

Why is the Big Bang theory now accepted?

  • The cosmic microwave background (CMB) radiation is a faint radiation detected uniformly from all directions of space.
  • The Big Bang theory predicted this radiation as the leftover, cooled radiation from the hot, dense early Universe — the Steady State theory had no convincing explanation for it.
  • When CMB radiation was discovered, it strongly supported the Big Bang theory and led to it becoming the currently accepted model for the origin of the Universe, since there is now more evidence supporting it than the Steady State theory.
Key termscosmic microwave background (CMB) radiation
Example

Discovery of the CMB is often compared to finding the 'afterglow' of the Big Bang still filling space today.

Must Know

  • Red-shift: light from a source moving away has its wavelength increased and frequency decreased.
  • More distant galaxies show greater red-shift — evidence the Universe is expanding.
  • Both the Big Bang and Steady State theories can explain red-shift.
  • Only the Big Bang theory predicted the CMB radiation.
  • Discovery of CMB radiation is the key evidence that made the Big Bang the currently accepted model.
  • The Big Bang theory states the Universe began from a hot, dense point and has been expanding and cooling since.

That's the notes covered.

Carry on to the next subtopic.