The Expanding Universe

This lesson covers the key evidence for the expansion of the universe — red shift, Hubble's observations and cosmic microwave background radiation (CMBR) — and the Big Bang theory, as required by the Edexcel GCSE Combined Science specification (1SC0).

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Red Shift and the Doppler Effect

The Doppler Effect

The Doppler effect is the change in the observed frequency (or wavelength) of a wave when the source and observer are moving relative to each other.

Situation	Effect on Frequency	Effect on Wavelength
Source moving towards observer	Frequency increases	Wavelength decreases (blue shift)
Source moving away from observer	Frequency decreases	Wavelength increases (red shift)

You can hear the Doppler effect when an ambulance siren sounds higher-pitched as it approaches and lower-pitched as it moves away.

Red Shift in Light from Galaxies

When scientists examine the light from distant galaxies, they find that the absorption lines in the spectrum are shifted towards the red end (longer wavelength). This is called red shift.