Three hypotheses on spacetime expansion are presented. First, velocity is independent of spacetime expansion. Second, spacetime expansion does not change energy. Third, spacetime expansion is a change in size of spacetime and energy conversion is a change in shape of spacetime, or equivalent to change in velocity. These are mathematical arguments hence mathematical proofs are presented in each hypothesis. The difference in the calculation of velocity between Hubble’s law and Belmonte theory is discussed. The so-called dark energy emerges as a consequence of velocity calculation in Hubble’s law but it does not appear in Belmonte theory. It is shown that the cosmological constant in the general theory of relativity arises from a violation of the conservation of energy. The cosmological constant does not appear in Belmonte theory as energy is automatically conserved in the theory. Following the Second Hypothesis, the Belmonte Mass is predicted. This is the total mass of dark matter and baryonic matter in the observable universe. It is derived theoretically from the Planck vacuum energy density.