8.2.1a Red-shift and the expanding universe

Red-shift: light stretched to longer wavelengths

1. Light from a galaxy carries a pattern of wavelengths produced by the elements in its stars.
2. For most distant galaxies this pattern is shifted towards longer wavelengths than the same pattern measured in a laboratory.
3. Longer wavelengths lie towards the red end of the spectrum, so this increase in wavelength is called red-shift.
4. Red-shift is seen when a galaxy is receding, meaning it is moving away from the observer.
5. A bigger red-shift means a bigger increase in wavelength and so a greater recession speed.

The universe is expanding

1. Most distant galaxies show red-shift, so they are moving away from us.
2. The further away a galaxy is, the faster it is receding.
3. The faster a galaxy recedes, the bigger its observed red-shift.
4. This pattern is evidence that space itself is expanding, so the distances between distant galaxies keep increasing.
5. If galaxies were just moving at random, the most distant ones would not consistently be receding fastest, so the ordered pattern points to an expanding universe.

Red-shift supports the Big Bang theory

1. Light from most distant galaxies is red-shifted.
2. Red-shift shows these galaxies are receding.
3. More distant galaxies recede faster and show a bigger red-shift.
4. This relationship is evidence that the universe is expanding.
5. If it is expanding now, it must have been smaller in the past.
6. Following the expansion backwards supports the idea that the universe began from a very small, extremely hot and dense region, which is the Big Bang theory.
7. Red-shift provides evidence for the Big Bang theory rather than proving it beyond all doubt.

8.2.1b Supernovae and the unknown universe

Supernovae reveal that the expansion is accelerating

1. A supernova is the explosion of a star, giving a sudden, very large increase in brightness.
2. Since 1998, observations of supernovae in distant galaxies have been used to study how the universe is expanding.
3. The red-shift of a galaxy's light shows it is receding, and a bigger red-shift means a greater recession speed.
4. The observed brightness of certain supernovae lets scientists estimate how far away their galaxies are.
5. Comparing these distances with the galaxies' red-shifts gave a surprise: the distant galaxies are receding faster than expected.
6. This is evidence that the expansion of the universe is accelerating, or speeding up.

How scientists use observations to build theories

1. Scientists cannot directly observe the beginning of the universe, so they use present-day observations to build and test explanations.
2. They collect evidence, such as red-shift measurements and supernova observations.
3. They look for patterns in that evidence and propose a theory to explain them.
4. The theory is used to make predictions, which are checked against further observations.
5. The theory is then accepted, modified or rejected depending on how well it matches the evidence.
6. For example, red-shift observations support the Big Bang theory, and the later supernova evidence for accelerating expansion made scientists develop their explanations further.
7. A scientific theory is not just a guess; it is the best explanation that fits the evidence, and it can change when better evidence appears.

Dark mass and dark energy: what we do not yet know

1. There is still much about the universe that scientists do not understand, and two examples are dark mass and dark energy.
2. Dark mass is proposed because some gravitational effects cannot be explained by the matter we can see.
3. Dark energy is proposed because the supernova observations suggest the universe is expanding at an increasing rate.
4. Scientists do not yet know what dark mass and dark energy really are, so they keep gathering evidence and testing possible explanations.