String Theory and The Big Bang
String theory will hopefully help physicists understand more precisely what happened in those early moments of the universe, so understanding the big bang theory is a key component of string theory's cosmological work.
In the history of cosmology, it soon became evident that an expanding universe was once very much smaller — so small, in fact, that it was compressed down to a single point (or, at least, a very small area). The theory that the universe started from such a primordial point and has expanded ever since is known as the big bang theory.
The theory was first proposed in 1927, but was controversial until 1965, when an accidental discovery supported the theory. Today, the most advanced astronomical observations show that the big bang theory is likely true.
The man originally responsible for the big bang theory was a Belgian priest and physicist, Georges Lemaître, who independently worked on theories similar to Friedmann’s. Like Friedmann, Lemaître realized that the universe defined by general relativity would either expand or contract.
In 1927, Lemaître learned of Hubble’s finding about distant galaxies moving away from Earth. He realized that this meant space was expanding, and he published a theory that came to be called the big bang theory.
The name big bang was given to the theory by Fred Hoyle, one of the theory’s greatest critics. In a 1949 series of BBC radio broadcasts, Hoyle was speaking dismissively of the idea that everything in the universe was created in one sudden big bang in the distant past.
The name stuck, much to big bang theorists’ dismay. Strictly speaking, the big bang theory doesn’t include a bang. Rather, the theory states that a tiny primordial particle began to expand, creating the universe. There is neither big nor bang in this theory.
Because you know that space is expanding, you can run the video of the universe backward in time in your head (rewind it, so to speak). When you do this, you realize that the universe had to be much smaller than it is now.
As the matter in the universe gets compressed into a smaller and smaller amount of space, the laws of thermodynamics (which govern the flow of heat) tell you that the matter had to be incredibly hot and dense.
The big bang theory reveals that the universe came from a state of dense, hot matter, but it tells nothing about how the matter got there, or whether anything else existed before the big bang (or even if the word before has any meaning when you’re talking about the beginning of time).