The Fifth Fundamental Force, the Equation of the Flat Universe, and the Classical Unification of the Fundamental Forces

<jats:p>Whether the universe is a flat or a closed system remains an open question. We derive the reason for the inflationary universe. Furthermore, we discuss why a surface-tension force is required. We present a model of the universe with an equation for its radius that suggests that the universe is infinite and flat. Further, we classically unify all the fundamental forces. Since the universe may have evolved from a point and a closed system to a flat system, our paper may help resolve the cosmological crisis. We provide the theoretical basis of all our findings.</jats:p>