Red-shift and the Big Bang: the change in observed frequency from a moving source, red-shift of distant galaxies, and the Big Bang and Steady State theories with their evidence.

What this dot point is asking

Edexcel statements 7.8 to 7.15 (separate physics) want you to describe how a moving source changes the observed frequency and wavelength, the red-shift of light from distant galaxies and how it shows the Universe is expanding, the Big Bang and Steady State theories, and how red-shift and the cosmic microwave background (CMB) support the Big Bang as the accepted model.

A moving source changes observed frequency

This effect applies to all waves, including light and sound (you hear it as the change in pitch of a passing siren). For light, a receding source shifts the light towards the longer-wavelength, lower-frequency red end of the spectrum, which is what "red-shift" means. The faster the source recedes, the greater the shift.

Red-shift of galaxies

Astronomers see the same red-shift in galaxies in all directions, with the more distant ones receding fastest. This does not mean we are at the centre; rather, space itself is expanding, carrying the galaxies apart, so every observer would see the same thing. The red-shift is the key observational evidence that the Universe is getting bigger.

The Big Bang and Steady State theories

Both theories accept that the Universe is expanding (both explain the red-shift), so red-shift alone cannot decide between them. The difference is the origin: a single beginning (Big Bang) versus an eternal, unchanging-density Universe (Steady State). To choose between them, scientists needed further evidence.

The evidence: the CMB

The discovery of the CMB was decisive. The Big Bang theory had predicted that the early hot Universe would leave behind radiation that, after billions of years of expansion, would now be detected as microwaves; the Steady State theory made no such prediction. This is a clear example of how the model with more supporting evidence becomes the accepted one.

How Edexcel examines this

This is separate-physics only and examined on both tiers within that route, often as the highest-value astronomy question. The red-shift question rewards defining red-shift as an increase in observed wavelength from a receding source, stating that distant galaxies are red-shifted (moving away) and that more distant galaxies recede faster, and concluding that the Universe is expanding. The theory-comparison question rewards correctly describing both the Big Bang and Steady State theories and, crucially, citing the CMB as the evidence the Big Bang predicts but the Steady State theory cannot explain, which is why the Big Bang is accepted. Examiners frequently penalise the claim that red-shift alone settles the debate; emphasise that both theories explain red-shift and the CMB is the deciding evidence. You may also be asked how both theories account for red-shift (both have an expanding Universe), so be ready to give that nuance. Linking this to how scientific models change with evidence, as with the heliocentric model, strengthens an answer.

Try this

Q1. State what happens to the observed wavelength of light from a galaxy moving away from us. [1 mark]

• Cue. It increases (the light is red-shifted).

Q2. State the main evidence that makes the Big Bang theory the accepted model rather than the Steady State theory. [1 mark]

• Cue. The cosmic microwave background (CMB) radiation.