Is the universe expanding?

Until early 20^th century, physicists thought the universe to be a static universe. But after the “General theory of Relativity” was first introduced in 1915, things started to change. General theory of relativity predicted the universe to be non-static. Though Einstein himself and most of the physicists tried to modify the theory to avoid the prediction of a non-static universe, some physicists like Alexander Friedmann, Lemaitre tried to explain it. In 1922, Friedmann developed a model of the universe and showed that the universe had to be non-static. He made two assumptions to make his calculations simpler –

i)  the universe looks identical in whichever direction you look

ii)  the first assumption is true if the universe is observed from anywhere else.

His work predicted that the universe was expanding and the farther a galaxy was from us, the faster it was moving away from us.

In 1924, American astronomer Edwin Hubble made some revolutionary works on finding new galaxies and their distances from us. His works led the physicists to take the prediction of an expanding universe seriously. Hubble found the same as Friedmann predicted.

Finding the distances to the distant galaxies was not easy as they appear fixed. In that case, he indirectly calculated the distances with the help of spectrum of light from those distant stars and galaxies. Spectrum carries most of the information about the star. We can determine the elements that are present in the atmosphere of that star. We know that every physical and chemical element absorbs a particular set of light frequencies. So, the light frequencies absorbed by those elements in the atmosphere of that star would be absent in its spectrum. Now, if we know exactly which frequencies are missing in the spectrum we can predict what elements there could be in the atmosphere of that star.  We check which frequencies are missing and then see which elements do absorb those frequencies. Every star has its own different spectra. Similarly we can determine the temperature of that star from the brightness and luminosity of its spectrum. Hubble used luminosity and apparent brightness of stars to find out the distances to the stars. This way he calculated the distances to the galaxies.

Now, after observing spectrums of different galaxies, Hubble found that spectrums of all the galaxies were red-shifted. This happens when something is moving away from us(Doppler effect). When the light source moves away from the observer, the observer experiences reduced frequency of that light i.e. the light is red-shifted for the observer. Hubble farther realized that the size of red-shift was directly proportional to the galaxy’s distance from us. It meant that the father the galaxy was from us, the faster it was moving away from us – exactly what Friedmann predicted. This implies that the universe is not static, instead it is expanding.