American astronomer Edwin Hubble made the observations of the expansion of the universe in 1925 and was the first to prove that the universe is expanding. He proved that there is a direct relationship between the speeds of distant galaxies and their distances from Earth. This is now known as Hubble's Law.

Hubble was the first to prove that the universe is expanding. He did this by measuring the redshift of a number of distant galaxies together with their relative distances by measuring the apparent brightness of Cepheid variables in each galaxy. When he plotted the redshift against relative distance, he found that the redshift of distant galaxies increased as a linear function of their distance. This is now called Hubble's law. The only explanation for this observation is that the universe was expanding.

The linear relationship between recession velocity, *v*, and the distance, *x*, of a local galaxy, can be expressed as:

Equation 47 - Hubble's Law

Where H_{0} is the Hubble constant.

A plot of the Hubble law shows that all distant galaxies seem to be moving away from us, which is not what we might initially expect since the gravitational attraction between the various bodies in the universe would act to pull galaxies together. Applying the cosmological principle, which is one of the guiding principles in modern cosmology, we may conclude that all distant galaxies are moving away from each other. This indicates that the Hubble law needs to have a cosmological origin - that is, one producing global behaviour.

Do all observers in the universe, independent of their position, measure the same phenomenon (the Hubble Law) or is our position special?

Although we observe detail in the local universe, on very large scales the universe seems to have a uniform appearance and we conclude that our location in the universe has no special significance. The universe changes its appearance with time but we believe that at any given time it looks the same - on average - everywhere.

Given the cosmological principle, we conclude that the universe is expanding, or more precisely, that space and spacetime are expanding.

The distances between objects are changing with time due to the expansion of the universe. When we observe objects at large distances, we are effectively looking back in time because of the light travel time. To compare distances and sizes from

different epochs (times) is not straightforward since we must remove the effects of the expansion. Cosmic Scale Factor can help with this and you can read more about it in the next article.