Early in the 20^th century, astronomers noticed that features in spectra from distant galaxies were shifted towards the red end of the spectrum.  The degree of redshift is related to the velocity of recession, or how fast away from us the galaxy is moving.  In 1929 astronomer Edwin Hubble produced a paper that plotted the relationship between the velocity of recession and the distance. 

Figure 4.1: Recreation of Hubble's Original Data

The gradient of the plot can be expressed as shown below:

The Hubble Parameter has the effective unit of 1/Time, which means that determining the value of the Hubble Parameter will give the age of the Universe.  The current value is:

Hubble's observations suggested that the farther away the galaxy the faster the recession.  This phenomenon is described as the Doppler redshift.  It is similar to the effect that makes a car sound lower-pitched as it travels away from you. A similar effect applies to light as well, so that if an astronomical object is travelling away from the Earth, its light will be shifted to longer, red wavelengths.

It is this Doppler red shift in the spectra of distant galaxies that leads scientists to conclude that the Universe is expanding.

	Olbers Paradox
	Big Bang