Red Shift

Visible light is part of the electromagnetic spectrum. Across the visible range:

Violet → blue → green → yellow → orange → red (shorter → longer wavelengths)

When the source of a light wave moves away from the observer, the observed wavelength is stretched — it becomes longer. Visible light shifted to longer wavelength looks redder. We say the light is red-shifted.

If the source moves towards us instead, the wavelength is squashed (shorter, bluer) — blue-shift. You don't need blue-shift for the spec, but it's the same effect in the opposite direction.

The Doppler effect for sound. You hear the same thing every day with sound: a police car driving towards you sounds high-pitched (waves squashed = shorter wavelength = higher frequency); as it speeds away the pitch drops (waves stretched = longer wavelength = lower frequency). Red-shift is the light equivalent.

How is it measured? Atoms emit and absorb light at specific, known wavelengths (spectral lines). When astronomers look at a distant galaxy they see the same patterns of spectral lines from the same elements (mostly hydrogen and helium) — but the whole pattern is shifted toward longer wavelengths. The size of the shift tells us how fast the galaxy is moving away.

Edwin Hubble (1929) observed that the further away a galaxy is, the larger its red-shift. Larger red-shift means the galaxy is moving away from us faster. The pattern is approximately linear (Hubble's Law):

recession speed ∝ distance

This tells us:

1. Almost every galaxy outside our own local group is moving away from us.
2. Galaxies that are further away are moving faster.
3. This is true in every direction we look — there is no "centre" we are moving away from.

The simplest explanation: the whole universe is expanding. Galaxies are not flying outwards from a single point in space — rather, space itself is stretching between them. A common analogy is dots drawn on the surface of a balloon — as the balloon is inflated, every dot moves away from every other dot, and dots further apart move apart faster.

Don't think of red-shift as a normal Doppler shift. Strictly, the red-shift of distant galaxies is caused by space stretching the light as it travels — not by the galaxy moving through space. For GCSE, AQA accepts the simpler description "galaxies moving away from us".

If galaxies are moving apart now, then in the past they must have been closer together. Rewind the expansion all the way and everything in the observable universe came from a single, very hot, very dense point about 13.8 billion years ago. That moment is called the Big Bang.

Red-shift = evidence the universe is expanding = supports the Big Bang theory.

Other evidence for the Big Bang (helpful background, not required but useful):

• Cosmic microwave background (CMB) — faint microwave radiation left over from the very early universe, found everywhere we look. Predicted by Big Bang theory, discovered in 1965.
• Abundance of light elements — the proportion of hydrogen, helium and lithium matches Big Bang predictions.

A common GCSE confusion. "Big Bang" doesn't mean an explosion in space — it means an extremely hot, dense beginning of space itself.

Astronomers find that:

• Galaxies rotate too fast for the gravity of their visible matter alone to hold them together. There must be additional, invisible mass we cannot see. This is called dark matter.
• The expansion of the universe is accelerating rather than slowing down. The energy driving this is called dark energy.

Together, dark matter and dark energy are thought to make up about 95% of the universe — ordinary atoms (you, the Earth, the Sun, every star) make up only about 5%.

"We are still learning a lot about both — neither has been directly detected. They are placeholder names for things that must be there to explain what we see."

AQA only require you to recognise that dark matter and dark energy exist as cosmological ideas and that current models depend on them. You don't need numerical detail.