IGCSE Physics / Cosmology
Evidence for the Big Bang revision notes
Edexcel International GCSE Physics (4PH1), chapter 28: Cosmology, specification point 8.14P, 8.18P. Free lesson.
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Red-shift and cosmic microwave background radiation provide evidence that the universe began in a hot, dense state and has been expanding. Each observation must be linked to what it shows about the Big Bang.
How red-shift shows the universe is expanding
Light from almost all distant galaxies is red-shifted, meaning its wavelengths have become longer. This shows that those galaxies are moving away from us. The link between distance and speed is what shows expansion, not simply that galaxies are moving away. Galaxies that are further away have a bigger red-shift, so they are moving away faster. This shows that the universe is expanding, with every galaxy moving away from every other. Run the expansion backwards and everything comes together in one small region. This supports the Big Bang.
What the cosmic microwave background is
The cosmic microwave background (CMB) is microwave radiation reaching us from every direction in space. It is leftover radiation from the hot early universe. As the universe expanded and cooled, the radiation was stretched to microwave wavelengths. The Big Bang theory predicted the CMB before it was found.
How to explain the evidence in a six-mark answer
Describe each piece of evidence, then link it to what it shows about the Big Bang. For red-shift, include the change in speed with distance. For the CMB, state where it comes from and how it links to the hot early universe.
Keep the CMB separate from background radiation
The CMB is not the same as background radiation. Background radiation means ionising radiation around us on Earth, from rocks, the air and space. Use the full term cosmic microwave background so it is clear which radiation you mean.