In 1929 Edwin Hubble published his observations of the redshift and distances of nearby galaxies. Hubble observed in the light from most of those galaxies that the spectral lines were shifted towards the red end of the spectrum as compared to a local laboratory source of the same atomic gas species. From this he interpreted that it was a Doppler effect (ie. due to the motion of the source), where the galaxies were receding from us, the observer. Thus the idea of the expanding universe was founded.
But one other important idea came from those same observations. He observed roughly the same redshift in light from the galaxies as a function of distance in every direction he looked. This became known as the Hubble law, which is the basis for the standard cosmology today–the big bang model. But the fact that this was in every direction and that the proportionality between the redshift and distance was the same in every direction meant that it looked to him like we, that is, our galaxy, was at the centre of the Universe. This is because the galaxies were moving away in a spherically symmetric way, putting us at the centre. This view of the Universe then would look something like the image in the figure on the right. Continue reading