A problem with the Big Bang theory is the sheer evenness and uniformity of the background radiation.  If the universe did originate from an infinitesimally point, then microscopic fluctuations would of quickly become enourmous as teh universe expanded.  However the background radiation is the same no matter what direction one looks.

In 1980, the American physicist Alan Guth devised a way around this problem, the so-called Inflationary Universe. He theorized that shortly after the Big Bang (10^-35 seconds, or 100 billion trillion trillionths of a second, to be exact), the universe underwent a period of extraordinarily rapid expansion, which lasted until about 10^-33 seconds. During this time the universe grew exponentially, increasing in diamter a hundredfold or more, and in volume by 10³⁰ to 10⁶⁰ times.

Before this inflationary period, the universe's constituents would have been in contact with one another, so they would have reached the same temperature. And the rapid inflation would make the universe's expansion appear very flat and uniform. Inflation ended by 10^-33 or 10^-32 seconds after the Big Bang, and since then the universe has expanded just as it would have in the standard big-bang model.

Inflation was triggered when the temperature of the early universe fell to 10²⁸K. This may have been when there was a second symmetry break (the first occured at the end of the Plank Era) and the electro-weak and the strong nuclear forces that up until that time had been united become distinct.  The universe entered a state called a "false vacuum".  This requires a bit of explanation.

Inflation Theory relies on a proposal, originating in modern particle physics, that extraordinarily high densities can lead to a form of matter that would turn gravity on its head, causing it to become repulsive rather than attractive. This form of matter is called a false vacuum. Inflation is the proposal that the expansion of the universe that we see today is the result of the gravitational repulsion of a false vacuum that filled the universe during a small fraction of a second of its early history. During this time the energy of the vacuum drives inflation.  In the inflationary theory the extreme uniformity of the universe was established early, before inflation began.  After this uniformity was established, inflation took over to stretch the region of local space to great size (relative to it's prior size).  While this was happening matter and antimatter were being created out of the vacuum, keeping the density of the universe constant while it grew.  By 10^-32 seconds, the universe had expanded to 100 times its original diameter.  At this point Inflation stops. The universe undergoes a phase transition to a lower-energy state ("the true vacuum").  The inflation smooths out all irregularities, ensuring that the entire visible universe was in causal contact, and makes the geometry of the universe flat.   If it had not occured earlier, then this was may have been when the Grand Unified force separated into the strong nuclear and electroweak forces.  This constitutes the start of  the electro-weak era.
 
 

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