Expansion of the Universe
In 1929, Edwin Hubble measured the speed at which galaxies are moving away from us. He looked at the light from lots of galaxies and measured the amount of redshift in the light. Hubble used his results to make a discovery. The speed of the galaxies was faster, the further away they were. This relationship between speed and distance is now called Hubble's Law.
The rate at which the Universe is expanding is called the Hubble constant. It is very hard to get an accurate and precise number for the Hubble constant. Scientists have used different way to work it out, but not all the answers agree. Recent answers suggest the Universe is expanding at a faster rate than scientists expected it to. Astronomers do not yet know why this would be happening. One idea is there could be some kind of force which is pulling galaxies apart. Astronomers have named this force, dark energy.