Redshift and the Doppler effect

Stand by a road as an ambulance goes past and listen to the siren. The note is high as it comes towards you and drops the moment it passes. The siren itself has not changed. Sound is a train of waves, and a moving siren chases its own waves, so the ones in front are squeezed together and the ones behind are stretched apart. Squeezed waves sound higher and stretched waves sound lower. This is the Doppler effect, named after Christian Doppler, who described it in 1842.

Light is a wave too, and does the same. Light from something coming towards you is squeezed, which shifts it towards the blue end of the rainbow. Light from something going away is stretched towards the red. These are called blueshift and redshift. You could never see the change by eye, because it is far too small. But the light of a star is crossed by dark lines at exactly known places, and a shift moves every line the same way. Measure how far the lines have moved and you have the speed.

In 1912 an American astronomer called Vesto Slipher did this for the Andromeda galaxy, and found it rushing towards us at about 300 km every second. It was the fastest thing anyone had measured. He went on to other galaxies, and by 1917 he had 25. The surprise was that 22 of the 25 were moving away.

In 1929 Edwin Hubble added distances and found the rule: the further away a galaxy is, the redder its light. The reason is stranger than a siren. The galaxies are not flying through space like ambulances. Space itself is growing, and it stretches the light that is crossing it. The longer the light has been travelling, the more it has been stretched.

The stretching can be enormous. The galaxy JADES-GS-z14-0 is one of the most distant known. Its light left it about 300 million years after the Big Bang, and every wave of it has been stretched to 15 times its first length. Light that set out as ultraviolet arrives as infrared, which is why it took a telescope built for infrared to find it.

Back among the nearer stars, the same small shifts do a great deal of everyday work. They show how fast stars are moving and how fast galaxies spin. They show that some stars are really two stars circling each other. And they found the first planet round a star like the Sun, in 1995, from the tiny to-and-fro shift in the star’s light as the planet swung it round.