WHY ARE MOST GALAXIES MOVING AWAY FROM US?
The simple answer to this question is that the whole universe is expanding. Pick any two galaxies at random, and chances are they’re moving away from each other.
Edwin Hubble measured the distance to the Andromeda galaxy in 1924 by observing a certain type of pulsating star called a Cepheid variable, which exhibits a close relationship between intrinsic luminosity and pulsation frequency. So knowing the frequency tells you the luminosity, and comparing this with the observed brightness tells you the distance. When Hubble did this for Cepheids in the Andromeda galaxy, it became clear that it must lie outside the Milky Way.
As well as measuring the distances to other galaxies, Hubble also studied their ‘redshift’. This is a property of the light spectra emitted by astronomical objects that arises from a phenomenon called the Doppler shift. Think of a vehicle whizzing past you. As it’s approaching the sound is higher pitched, then as it recedes into the distance it’s lower pitched. What happens is that sound waves are bunched up when they’re moving towards you, and stretched out when they’re moving away. It’s the same with light waves, except we perceive the difference as a change in colour instead of pitch: if the source is moving away from us, we see it shifted towards the red end of the spectrum – hence the term ‘redshift’. Hubble’s great discovery in 1929 was that a galaxy’s redshift is proportional to its distance – a result now known as ‘Hubble’s law’. It gave astronomers a new way of measuring the distance to galaxies that were too far away for the Cepheid method, simply by measuring their redshift.