In the 1920s the American astronomer Edwin Hubble made and astounding discovery about galaxies: they all moving away from the earth at speeds that depend on their distance – the father away they are, the faster they are receding. This is known as Hubble law and it occurs because the universe itself is expanding, which means that all the galaxies are rushing apart from one another.
It is quite easy to measure a galaxy’s speed, by looking at its light and seeing how the wavelength changes according to the Doppler effect. Hubble’s law tells astronomers how to calculate the distance from the speed. So if a distant galaxy is found to be receding at, say, 3,000,000 km/h, by multiplying Hubble’s constant (about 13, when calculating in kilometer per hour and light years) by the galaxy’s speed in kilometer per hour, they can work out that it is 40,000,000 light years away. In this way, astronomers have measured distances to galaxies that lie a staggering 12,000 million light-years away.
Picture Credit : Google