Red-shift

There is an observed increase in the wavelength of light from most distant galaxies. The further away the galaxies, the faster they are moving and the bigger the observed increase in wavelength. This effect is called red-shift.

It is called red-shift because red is the longest wavelength of visible light, so an increase in wavelength shifts the light towards the red end of the spectrum.

You should be able to explain qualitatively the red-shift of light from galaxies that are receding. A galaxy moving away stretches the light waves reaching us, increasing their wavelength — and the faster it recedes, the greater the stretch and the larger the red-shift.

The observed red-shift provides evidence that space itself — the universe — is expanding, and supports the Big Bang theory.

You should be able to explain that the change of each galaxy's speed with distance is evidence of an expanding universe.

The argument turns on the pattern, not on any single galaxy. Almost every distant galaxy is red-shifted, so almost all of them are moving away from us. And the further away a galaxy is, the faster it is receding. That combination is not what you would see if galaxies were simply flying apart through space from an explosion at some particular place — it is what you see if space itself is expanding everywhere, carrying the galaxies with it. On that picture every observer, in every galaxy, would see the same thing, which is why the pattern does not put us at the centre of anything.

The Big Bang theory suggests that the universe began from a very small region that was extremely hot and dense.

You should be able to explain how red-shift provides evidence for the Big Bang model.

The reasoning runs backwards in time. Red-shift shows that the universe is expanding now, and that everything is getting further apart. Running that expansion backwards, everything must have been closer together in the past — and further back still, concentrated in a very small, extremely hot and dense region. That is what the Big Bang theory describes.

You should also be able to explain how scientists are able to use observations to arrive at theories such as the Big Bang theory.

This is the wider point about how science works, and it is examined directly. Scientists measure something they can observe — here, the wavelengths of light from many galaxies. They look for a pattern in the measurements, and they propose a theory that would explain it. A good theory does more than fit what has already been seen: it predicts things that have not yet been looked for, and those predictions can then be tested. If new observations agree, confidence in the theory grows; if they disagree, the theory must be changed or replaced.

Since 1998 onwards, observations of supernovae suggest that distant galaxies are receding ever faster.

That is not what was expected. Gravity should be pulling the galaxies together and slowing the expansion down, so an expansion that is speeding up needs an explanation nobody has yet confirmed.

You should be able to explain that there is still much about the universe that is not understood — for example dark mass and dark energy. Dark mass, or dark matter, is proposed to explain why galaxies hold together and why they bend light more than the matter we can see would account for; it emits no electromagnetic radiation, so it cannot be observed directly. Dark energy is the name given to whatever is causing the expansion to accelerate. Both are, at present, names for observations that current theories do not explain.