Red-shift (AQA GCSE Physics): Revision Notes
Red-shift
What is red-shift?
Red-shift happens when light from distant galaxies changes as it travels to Earth. This change in light gives us important clues about how the universe works.
When a light source moves away from us, the light stretches out. This makes the wavelength longer and the frequency lower. We call this red-shift because the light shifts towards the red end of the spectrum.
When a light source moves towards us, the light squashes together. This makes the wavelength shorter and the frequency higher. We call this blue-shift because the light shifts towards the blue end.
Key Concept: Red-shift means moving away, blue-shift means moving towards.
How we discovered red-shift
In 1929, astronomer Edwin Hubble made an amazing discovery. He looked at light from distant galaxies and found that:
- Most galaxies show red-shifted light
- This means most galaxies are moving away from us
- The furthest galaxies are moving away the fastest
Hubble studied spectral lines - these are like fingerprints in starlight. He compared these lines from different galaxies:
- Nearby galaxies had lines shifted slightly towards red
- Distant galaxies had lines shifted much more towards red
Spectral lines act like unique fingerprints for different elements in stars. By analysing how these lines shift in colour, astronomers can determine whether a star or galaxy is moving towards or away from us.
This was the first evidence that our universe is expanding.
Red-shift and the Big Bang theory
Red-shift observations support the Big Bang theory. This theory suggests:
- The universe started from a very small, incredibly hot and dense region
- The universe is about 13.8 billion years old
- The universe has been expanding ever since it began
Since 1998, astronomers discovered something surprising. Distant galaxies are moving away faster than expected. This means the expansion is speeding up, not slowing down.
Dark matter and dark energy
Red-shift observations revealed two mysterious things in our universe:
Dark matter
Dark Matter
Scientists noticed that stars in galaxies move differently than expected. There must be invisible matter we cannot see or detect. This dark matter doesn't give off light or other electromagnetic radiation, yet it makes up most of the mass in galaxies.
Dark energy
Dark Energy
This is the mysterious force making the universe's expansion speed up. Scientists don't know what dark energy actually is - it cannot be explained by gravity alone, and it makes up most of the energy in the universe.
Example of red-shift evidence
Observational Evidence: Galaxy Red-shift Patterns
When we look at light from galaxies:
- Light from most galaxies is red-shifted
- This means most galaxies are moving away from us
- The further away a galaxy is, the greater its red-shift
- This shows the universe is expanding in all directions
This pattern provides direct observational proof that the universe is expanding uniformly in all directions.
Remember!
Key Points to Remember:
- Red-shift occurs when light sources move away from us - the light wavelength increases
- Edwin Hubble discovered that distant galaxies are red-shifted, proving the universe is expanding
- Red-shift supports the Big Bang theory - the universe started small and has been expanding for 13.8 billion years
- Dark matter and dark energy are invisible but make up most of the universe's mass and energy
- The further away a galaxy is, the faster it moves away from us - this shows the universe is expanding everywhere