GUT Era

The GUT Era is when three of the four fundamental forces are combined, but gravity has become distinct.  There are a class of theories called Grand Unified Theories (GUTs) that attempt to describe all forces except gravity in a single framework.  The leading type are so-called string theories, and some are partially successful, but there are further details to be worked out.  Theorists would say that in the GUT Era the gravity force "froze out" of the universe.  The GUT Era lasted from 10-43 s to 10-38 s.  Near the end of this era, grand unified theories predict that the universe cooled to the point that the nuclear strong force began to freeze out, leaving three fundamental forces: gravity, the strong force, and the still combined electroweak force.  This "phase transition" released a huge amount of energy, causing space to undergo a rapid inflation.  In a mere 10-36 s, pieces of our universe the size of an atomic nucleus might have grown to the size of our solar system.  We will later discuss observations of the universe that seem to require such extreme inflation.  Note that this inflation is very very large compared to the speed of light, but again, space itself is what is expanding, so it does not have to obey the speed limit of the speed of light.