THE BIG BANG MODEL

The discovery of the expansion of the universe suggests another model of the universe, namely that the universe started in one gigantic explosion, and that the expansion we now see is the result of the pieces from that explosion flying apart.
To see why you are led to the idea of an explosion, imagine that you take a motion picture of the expanding universe and then run the motion picture backwards. If the expansion is uniform, then in the reversed motion picture we see a uniform contraction. The particles in this picture are the galaxies which are getting closer and closer together. There is a time, call it t = 0, when all the galaxies come together at a point. Now run the motion picture forward and the galaxies all move out as if there were an explosion at that point.
The explosion of the universe was first proposed by the Belgian priest and mathematician Georges Lemaître in the late 1920s. It was, in fact, Lemaître who explained Hubble’s red shift versus distance data as evidence for the expansion of the universe. In the late 1920s not much was known about nuclear physics, even the neutron had not yet been discovered. But in the 1940s after the development of the atomic fission bomb and during the design of the hydrogen fusion bomb, physicists gained considerable experience with nuclear reactions in hot, dense media, and some, George Gamov in particular, began to explore the consequences of the idea that the universe started in an initial gigantic explosion.
A rough picture of the early universe in the explosion model can be constructed using the concepts of the Doppler effect and thermal equilibrium. Let us see how this works.
We have seen that the red shift of the spectral lines of light from distant galaxies can be interpreted as being caused by the stretching of the wavelengths of the light due to the expansion of the universe. In a reverse motion picture of the universe, distant galaxies would be coming toward us and the wavelengths of the spectral lines would be blue shifted. We would say that the universe was contracting, shrinking the wavelength of the spectral lines. The amount of contraction would depend upon how far back toward the t = 0 origin we went. If we went back to when the universe was 1/10 as big as it is now, wavelengths of light would contract to 1/10 their original size. In the Einstein photoelectric effect formula, E_photon = hf = hc/λ, the shorter the photon wavelength, the more energetic the photons become. This suggests that as we compress the universe in the time reversal moving picture, photon energies increase. If there is no limit to the compression, then there is no limit to how much the photon energies increase.
Now introduce the idea of thermal equilibrium. If we go back to a very small universe, we have very energetic photons. If these photons are in thermal equilibrium with other forms of matter, as they are inside of stars, then all of the matter has enormous thermal energy, and the temperature is very high. Going back to a zero sized universe means going back to a universe that started out at an infinite temperature. Fred Hoyle thought that this picture was so ridiculous that he gave the explosion model of the universe the derisive name the “Big Bang” model. The name has stuck.