The Expantion of the Universe and the Big Bang

In the 20th century, great strides were made in the field of astronomy. First, the Russian physicist Alexandre Friedmann discovered in 1922 that the universe did not have a static structure. Starting out from Einstein's theory of relativity, Friedmann calculated that even a tiny impulse might cause the universe to expand or contract. Georges Lemaître, one of the most famous astronomers of Belgium, was the first to recognise the importance of this calculation. These calculations led him to conclude that the universe had a beginning and that it was continuously expanding right from the outset. There was another very important point Lemaître raised: according to him, there should be a radiation surplus left over from the big bang and this could be traced. Lemaître was confident that his explanations were true although they initially did not find much support in the scientific community. Meanwhile, further evidence that the universe was expanding began to pile up. At that time, observing a number of stars through his huge telescope, the American astronomer Edwin Hubble discovered that the stars emitted a red shifted light depending on their distances. With this discovery, which he made at the California Mount Wilson Observatory, Hubble challenged all scientists who put forward and defended the steady state theory, and shook the very basis of the model of the universe held until then.
Hubble's findings depended on the physical rule that the spectra of light beams travelling towards the point of observation tend towards violet while the spectra of light beams moving away from the point of observation tend towards red. This showed that the celestial bodies observed from the Californian Mount Wilson Observatory were moving away from the earth. Further observation revealed that the stars and galaxies weren't just racing away from us; they were racing away from each other as well. This movement of celestial bodies proved once more that the universe is expanding. In Stephen Hawking's Universe, David Filkin relates an interesting point about these developments:
…Within two years, Lemaître heard the news he had scarcely dared hope for. Hubble had observed that the light from galaxies was red shifted, and, according to Doppler effect, this had to mean the universe was expanding. Now it was only a matter of time. Einstein was interested in Hubble's work anyway and resolved to visit him at the Mount Wilson Observatory. Lemaître arranged to give a lecture at the California Institute of Technology at the same time, and managed to corner Einstein and Hubble together. He argued his "primeval atom" theory carefully, step by step, suggesting that the whole universe had been created "on a day which had no yesterday." Painstakingly he worked through all the mathematics. When he had finished he could not believe his ears. Einstein stood up and announced that what he had just heard was "the most beautiful and satisfying interpretation I have listened to" and went on to confess that creating the "cosmological constant" was "the biggest blunder" of his life.1
The truth that made Einstein, who is considered one of the most important scientists in history, jump to his feet was the fact that the universe has a beginning.
Further observations on the expansion of the universe gave way to new arguments. Starting from this point, scientists ended up with a model of a universe that became smaller as one went back in time, eventually contracting and converging at a single point, as Lemaître had argued. The conclusion to be derived from this model is that at some point in time, all matter in the universe was crushed together in a single point-mass that had "zero volume" because of its immense gravitational force. Our universe came into being as the result of the explosion of this point-mass that had zero volume and this explosion has come to be called the "Big Bang".
The Big Bang pointed to another matter. To say that something has zero volume is tantamount to saying that it is "nothing". The whole universe is created from this "nothing". Furthermore, this universe has a beginning, contrary to the view of materialism, which holds that "the universe has existed from eternity".