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Before inflationary universe theory there was the 'Big Bang Theory'. Bog Bang theory is a fundamental theory on the beginning of the universe, the theory tells us that the current universe exploded out of very dense matter, and that the expansion of the universe was continuing today. However, while there was a lot of evidence to support the theory, including the way that the different planets and galaxies are moving away from each other, as well as background noise from that initial explosion which can be picked up by radio telescopes, there were still some problems with the big bang theory. The theory, which was relativity simple, appeared to explain most of the current characteristics of the universe, but it did not explain everything. The problems with the big bang theory related to the mathematics of the initial expansion, which did not explain why the temperature of the universe appeared to remain fairly consistent, and why the universe appeared to be almost flat.

After research Guth's developed a theory that would explain the way that temperatures had remained almost constant and why the universe appears almost flat. The idea was that when the big bang took lace there was a massive expansion with particles being pushed outwards very quickly; this was the same as big bang theory. However, instead of this explosion remaining constant, Guth believed that there was a 'super cooling' period, almost immediately after the big bang at less than 100 billion trillion trillionths of once second, which slowed down the expansion followed a very rapid expansion (inflation). This explained the temperature problem and the flat universe problem. The fast explosion period is called the inflationary period, and may be seen as similar, but faster, to the blowing up of a balloon. As all the particles were touching each other before the big bang, they were all the same temperature, so when the expulsion separates them and they are cooled rapidly, they will all still be at similar temperatures, wherever they ended up. It was also argued that the very rapid inflation (expansion) would explain how and why the universe appeared flat.

This is a theory which is now widely accepted and in 2006 the Wilkinson Microwave Anisotropy Probe sent back results which appear to support the theory. It may not explain everything about the beginning of the universe, but helps to explain many of the gaps in the big bang theory.