When we look at the spectra from distant galaxies, we find that the light from them is shifted towards the red end of the spectrum. The wavelength of the light we receive is longer than the wavelength of light due to the same atomic transitions on Earth. The reason for this is that those same distant galaxies are moving away from us, so the light under goes a Doppler shift. We can work out the speed with which they are receding by finding the Doppler shift – called the redshift in the case of light from distant galaxies – and we find that the speed with which galaxies are receding from us is directly proportional to their distance from us. This is shown on the graph below.
We can express Hubble's Law mathematically as
where
is the constant of proportionality, called Hubble's constant.is the gradient of the graph above:
is often expressed in SI units
We can easily find a simple estimate for the age of the universe by taking the reciprocal ofsince
) to give
equivalent to 15.3 billion years. This ignores gravitational effects which slow the expansion, or in fact any other complicating factors.