The Expanding Universe and Hubble's Law

This lesson covers the discovery and evidence for the expanding universe and Hubble's law — as required by the Edexcel GCSE Physics specification (1PH0), Topic 7: Astronomy. This is a Paper 2 topic. You need to understand Hubble's observations, what they tell us about the universe, and the concept of the accelerating expansion.

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Hubble's Discovery

In the 1920s, the American astronomer Edwin Hubble made a groundbreaking observation. By studying the light from distant galaxies, he found:

1. Almost all galaxies are red-shifted — they are moving away from us.
2. The further away a galaxy is, the greater its red-shift — meaning the faster it is receding.

What This Means

• If almost all galaxies are moving away from us, the universe is expanding.
• The further away a galaxy is, the faster it is moving away — this is a systematic pattern, not random motion.
• This was the first observational evidence that the universe is not static — it is getting bigger over time.