Big bang

A theory which states that the universe came into being in an instantaneous event between 15 and 20 billion years ago. Everything contained within the universe was created in that initial event and as time has gone by, the universe has expanded and the contents have evolved into the stars and galaxies of today. The theory uses the known laws of physics to describe everything that occurred in the universe following the Planck time, which ended when the universe was 10-43 seconds old. Following this space and time became distinct from matter and energy. Matter and energy continued to be completely interchangeable until the universe was fifteen seconds old. At this time, the elementary particles which make up atoms became stable. Atoms could still not form, however, because collisions between photonsand sub-atomic particles prevented them binding together. The forces of nature evolved into gravity, electromagnetism, the weak nuclear force and the strong nuclear force. As the temperature fell, because the universe expanded, atoms formed. Now that the sub-atomic particles were in orderly structures, the obstructions they had caused to the photons were removed. This allowed the photons to travel large distances through space without interacting with other particles. In effect, space became transparent to them and caused the releases of energy which are observed today as the cosmic background radiation. This also removed the radiation pressure from the matter and allowed it to be influenced by the force of gravity. This, in turn, caused galaxies to form as matter congregated together.