Red Shift

Red shift — the observed increase in the wavelength of light from a source that is moving away from us. The light is shifted towards the red end of the spectrum.

Because v = f λ and the speed of light is fixed, a longer wavelength means a lower frequency.

If a source were moving towards us the wavelength would be shortened instead, giving a blue shift.

How it is measured. Every element absorbs light at a characteristic set of wavelengths, producing dark absorption lines in a star's spectrum. These lines form a fixed, recognisable pattern. In light from distant galaxies the whole pattern appears at longer wavelengths than it does in a laboratory on Earth — shifted, but unmistakably the same pattern.

This is what makes the measurement convincing. The lines have not changed, only their position, so the shift must be caused by relative motion rather than by the galaxy being made of something unusual.

The everyday analogy is the change in pitch of a siren as an ambulance passes. Approaching, the sound waves are bunched up and the pitch is higher; receding, they are stretched out and the pitch drops. Red shift is the same effect for light — though the analogy should be used as an illustration, not as the explanation itself.

What observation shows:

1. Light from almost every distant galaxy is red shifted, so almost every galaxy is moving away from us.
2. The more distant the galaxy, the greater its red shift — so the more distant it is, the faster it is receding.

The conclusion, and a common misreading. These two facts together mean the universe is expanding. They do not mean the Earth is at the centre of it. Space itself is expanding, so every observer in every galaxy sees the same thing: everything else moving away, with the most distant receding fastest.

The usual image is raisins in a rising loaf. As the dough expands, every raisin moves away from every other, and the ones further apart separate faster. No raisin is the centre.

Example: hydrogen absorbs strongly at 656 nm in a laboratory. Observed at 670 nm in a distant galaxy’s spectrum, the same line has been stretched by the galaxy’s recession — the further the galaxy, the larger that stretch.