GCSE-readyRed-shift, the Big Bang and the Steady State theory
Two theories tried to explain the Universe: the Steady State theory and the Big Bang theory. Red-shift and the cosmic microwave background decided between them.
The Doppler effect and red-shift
Light from distant galaxies is shifted towards the red end of the spectrum: red-shift. So the galaxies are moving away from us.
The further away a galaxy is, the greater its red-shift, so the faster it is moving away. This means the Universe is expanding.
Two theories
Steady State theory: the Universe has always existed and always looks the same; as it expands, new matter is continuously created to keep the density constant.
Both theories can explain red-shift, because both describe an expanding Universe.
The cosmic microwave background
The Big Bang theory explains it as radiation left over from the hot early Universe, stretched to microwave wavelengths as space expanded.
The Steady State theory cannot explain the CMB. So the Big Bang is now the accepted model.
Galaxy A is twice as far away as galaxy B. How would their red-shifts compare, and what does this tell us?
- Further galaxies have greater red-shift.
- So A has a greater red-shift than B.
- A is moving away faster, which fits an expanding Universe.
Answer: A has a greater red-shift, so it is moving away faster; the Universe is expanding.
Red-shift means galaxies are moving away; further galaxies have more red-shift, so the Universe is expanding. Both the Big Bang and Steady State theories explain red-shift, but only the Big Bang explains the cosmic microwave background, so it is the accepted theory.
- Red-shift
- The increase in wavelength of light from a galaxy that is moving away from us. More distant galaxies show more red-shift.
- Big Bang theory
- The model that the universe began from a very small, very hot, dense region and has been expanding ever since.
- Wavelength
- The distance from one point on a wave to the same point on the next wave, for example crest to crest.
- Frequency
- The number of waves passing a point each second. Unit hertz (Hz). f = 1 ÷ T.
- Density
- Mass per unit volume: ρ = m ÷ V. Unit kg/m^{3}.
- Cosmic microwave background (CMB)
- Microwave radiation reaching us from every direction in space, left over from the early universe. Evidence for the Big Bang.
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A source of waves moves towards an observer. What does the observer detect?
- Shorter wavelength and higher frequency
- A higher wave speed
- Longer wavelength and lower frequency
- No change
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Answer: Shorter wavelength and higher frequency
The waves bunch up ahead of the moving source.
Light from nearly all distant galaxies is red-shifted. What does this suggest?
- The universe is expanding
- The universe is shrinking
- The galaxies are made of red stars
- The galaxies are getting hotter
Show the answer
Answer: The universe is expanding
Galaxies moving away in all directions suggests expansion.
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- Pearson Edexcel GCSE Physics (1PH0) specification
- HyperPhysics (Georgia State University): Discovery of Cosmic Background Radiation
Checked by the GCSE-ready editorial team, 16 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.
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