Illustration of a 'bubble' universe and superstrings. Bubble universes may have formed in the early universe, where conditions known as a false vacuum created a repulsive force that caused an incredibly rapid expansion of the universe. Areas of inflation formed bubbles completely isolated from each other. String theory tries to unify the four fundamental forces (gravity, electromagnetism, and strong and weak nuclear forces) using superstrings. These are one-dimensional physical objects where the vibrational state determines the particle type and resulting force.