Red-Shift and the Expanding Universe Revision Notes | GCSE AQA Higher Physics

1. The Mystery of Moving Galaxies

When astronomers first looked at distant galaxies through powerful telescopes, they noticed something strange. The light from these galaxies wasn't quite the colour they expected.

Think of it like listening to an ambulance siren. As it moves towards you, the pitch sounds higher. As it moves away, the pitch drops lower. Light behaves similarly - but instead of pitch changing, it's the colour that shifts.

This discovery would revolutionise our understanding of the universe itself. The galaxies weren't just sitting still in space - they were racing away from us at incredible speeds.

Red-shift gets its name because visible light shifts towards the red end of the spectrum. Red light has a longer wavelength than blue light. When galaxies move away from us, their light gets 'stretched out' to longer wavelengths.

2. Red-Shift in Space

Red-shift happens because light from a galaxy moving away has a longer wavelength.

Picture yourself standing by a road as a car approaches, honking its horn. The sound waves get compressed together, making the pitch higher. As the car drives away, the sound waves get stretched out, making the pitch lower.

With galaxies, the same thing happens to light waves. As a galaxy moves away from Earth, its light waves get stretched to longer wavelengths. We observe this as red-shift.

The further away a galaxy is, the greater its red-shift.

3. Hubble's Revolutionary Discovery

In the 1920s, astronomer Edwin Hubble made measurements that changed everything. He didn't just find that galaxies were moving away - he discovered a pattern.

Hubble found that the further away a galaxy was, the faster it was moving away from us. This wasn't random - there was a clear relationship between distance and speed.

Think of it like dots on a balloon. As you inflate the balloon, every dot moves away from every other dot. The dots that are further apart move away from each other faster than the dots that are close together.

This pattern - more distant galaxies having greater red-shift - provided the first strong evidence that space itself is expanding. It's not that galaxies are flying through space away from us. Space itself is getting bigger.

4. Evidence for an Expanding Universe

The red-shift observations provide compelling evidence that the universe is expanding. But what does 'expanding universe' actually mean?

It doesn't mean galaxies are flying outwards through space like shrapnel from an explosion. Instead, space itself is stretching. Imagine the universe as a loaf of raisin bread rising in the oven - the raisins (galaxies) move apart as the dough (space) expands between them.

This expansion explains why we see red-shift from galaxies in all directions. If galaxies were just moving randomly through space, we'd expect to see some blue-shifted (approaching) and some red-shifted (receding). Instead, almost all distant galaxies show red-shift.

The further away a galaxy is, the faster it is receding.

5. Supporting the Big Bang Theory

If the universe is expanding now, what does that tell us about the past? If we could run time backwards, all the galaxies would get closer and closer together.

This leads to a remarkable conclusion: the universe must have been much smaller and denser in the past. Taken to its logical extreme, everything in the universe was once concentrated in an extremely hot, dense point.

Red-shift observations provide crucial support for the Big Bang theory. The fact that space is expanding uniformly in all directions suggests it began from a single point and has been growing ever since.

The Big Bang wasn't an explosion in space - it was an expansion of space itself. This explains why we don't see a 'centre' to the expansion. From any galaxy's perspective, all other galaxies appear to be moving away.

6. Exam Technique and Common Mistakes

Red-shift questions often test your understanding of the relationship between observation and theory. Examiners want to see that you can link the evidence to the conclusions.

When describing red-shift, focus on what we observe: the wavelength of light increases (gets longer). Avoid saying the light 'becomes red' - this suggests a colour change rather than a wavelength shift.

For expanding universe questions, emphasise that space itself expands. Many students incorrectly describe galaxies 'flying through space' like debris from an explosion. The key insight is that space stretches, carrying galaxies apart.

Remember that red-shift provides evidence for theories - it doesn't prove them absolutely. Science builds understanding by gathering supporting evidence from multiple sources. Red-shift is one key piece of evidence supporting our current model of an expanding universe that began with the Big Bang.

Worked example: Worked Method: Red-Shift and the Expanding Universe

Use the relevant formula, substitute values with units, calculate, then state the final answer clearly.

1. Write down the correct physics formula. — Write down the correct physics formula.
2. Substitute known values with units. — Substitute known values with units.
3. Rearrange if needed and calculate carefully. — Rearrange if needed and calculate carefully.
4. Give the final answer with correct unit and sensible precision. — Give the final answer with correct unit and sensible precision.

Answer: Final answer should include unit and appropriate significant figures.