Early in the twentieth century astronomers used the Doppler effect to
prove that galaxies are moving away from us at high speed. In other words the Universe is
expanding.
In 1925 the American astronomer Edwin Hubble proposed a simple
law that connected the speed of the galaxy with its distance from the Earth. It stated that the
further away a galaxy was the faster it was moving. The speed and the distance were
connected by a number called the Hubble constant. Hubble's law is:
So if a galaxy is 60 million light years (18.4 Mega
parsecs) from the Earth it will be moving away from us at 70x18.4 = 1288 km/s.
This
law is very important in our understanding of the size of the Universe. While it was possible
to find the distances of the stars in our galaxy (using parallax) and also the distance of some
of the nearest galaxies to us (using Cepheid variables) astronomers needed a way of
measuring the distances of distant galaxies. Hubble's law gave them that chance.
If
they could measure the speed at which a galaxy was moving away from the Earth they could
use the formula to calculate how far away that galaxy was.
When a vehicle such as a train moves away
from us with its hooter sounding the pitch of the note that we hear from the train gets lower.
(See Wave properties/Doppler effect foundation).
The same thing happens with light
– the frequency of the light that we receive is redder than the light emitted by the object. We
call this the Red Shift. The greater the speed the redder the galaxy looks and we can use this
reddening of the light to work out the speed of the galaxy.
The diagrams in Figure 1
show the reddening of the light coming from a number of distant galaxies. The two lines in
the galactic spectrum move towards the red as the speed of the different galaxies
increases.
