Light received from a distant galaxy contains identifiable spectral lines. A galaxy is redshifted when those lines are observed at longer wavelengths than the corresponding lines in a reference spectrum.
The reference may come from a nearby source or a laboratory measurement. If the whole pattern is displaced towards longer wavelength, the galaxy is receding.
Evidence for expansion
- Almost all distant galaxies are observed to be redshifted.
- The light reaching us has been stretched to longer wavelengths.
- More distant galaxies generally show larger redshifts.
- A greater redshift corresponds to a greater recessional speed.
Together, these observations indicate that the separation between galaxies is increasing and that the Universe is expanding.
A useful analogy is a set of marks on the surface of an inflating balloon. As the surface stretches, every mark becomes more widely separated from the others. Similarly, cosmic expansion increases the space between galaxies.
Exam Tip: For an expansion-evidence question, give the complete link: spectral lines have moved to longer wavelengths, so distant galaxies are receding; widespread recession is evidence that the Universe is expanding.