When the Universe is 10 -43 seconds old, the gravitational force splits off from the other three forces, which are still combined into one force called the GUT force , where GUT stands for Grand Unified Theory . We actually have a partially-successful theory to unify the electromagnetic, the strong and the weak forces. The universe at this time is still unbelievably hot. When symmetry breaking occurs, a huge amount of energy is released. This expansion is called inflation .
Inflation is a relatively new addition to cosmological theory.
The flatness problem. As we discussed last Friday, the Universe may have some curvature it may have positive curvature, zero curvature, or negative curvature. The thing we didn t discuss is how the curvature is determined that I will discuss now. The Universe is expanding with energy left over from the Big Bang, and this expansion makes it get larger. At the same time, gravity from all of the matter in the Universe acts as an attractive force, pulling at the Universe, and trying to slow the expansion or even reverse it, causing the Universe to eventually contract.