Complex sentence: Inconceivably hot and dense, the state of the universe during the Planck epoch was unstable, tending to evolve, giving rise to the familiar manifestations of the fundamental forces through a process known as symmetry breaking.
Split into simpler sentences: Inconceivably hot and dense, the state of the universe during the Planck epoch was unstable. As it expanded and cooled, the familiar manifestations of the fundamental forces arose through a process known as symmetry breaking.