Red-shift
- When a light source moves away from us, the light we see has a longer wavelength: it is shifted towards the red end of the spectrum. This is red-shift.
- Light from most distant galaxies shows red-shift, so they are moving away from us.
- The further away the galaxies are, the faster they are moving away and the bigger the red-shift.
The expanding universe
- Red-shift is evidence that the universe is expanding: all the distant galaxies are moving away from each other.
- It supports the Big Bang theory: the universe began from a very small region that was extremely hot and dense, and has been expanding ever since.
- Cosmic microwave background radiation, left over from the early universe, is further evidence for the Big Bang.
- (Edexcel) The Steady State theory said the universe has always existed, with new matter made as it expands. It cannot explain the cosmic microwave background radiation, so the Big Bang theory is the accepted one.
What we don't yet know
- Since 1998, observations of supernovae suggest that distant galaxies are moving away faster and faster: the expansion of the universe is accelerating.
- Scientists think this is caused by dark energy. Galaxies also seem to contain far more mass than we can see, called dark matter.
- There is still much about the universe that is not understood. Scientists develop new theories as new evidence is found.
Key terms
- Red-shift
- The increase in the wavelength of light from a galaxy moving away from us.
- Big Bang theory
- The theory that the universe began from a very small, extremely hot and dense region and has been expanding since.
- Expanding universe
- The idea that galaxies are moving further apart over time.
- Cosmic microwave background
- Microwave radiation coming from all directions, left over from the early universe.
- Dark energy
- An unknown cause of the accelerating expansion of the universe.
- Dark matter
- Mass that can't be seen but whose gravity affects galaxies.