The Planck epoch is an era in traditional (non-inflationary) Big Bang cosmology immediately after the event which began the known universe. During this epoch, the temperature and average energies within the universe were so high that subatomic particles could not form. The four fundamental forces that shape the universe— gravitation, electromagnetism, the weak nuclear force, and the strong nuclear force —comprised a single fundamental force. Little is understood about physics in this environment. Traditional big bang cosmology predicts a gravitational singularity — a condition in which spacetime breaks down —before this time, but the theory relies on the theory of general relativity, which is thought to break down for this epoch due to quantum effects.