Abstract: If the universe is a coexistence system of the thermal equilibrium state of photons and of the gravitational bound state, and the equilibrium state of photons is predominant in the system, the universe will inevitably expand, and if the thermal equilibrium is weakened or destroyed and the gravitional is in dominant position, the universe will change from expansion into contraction. The concept of the early universe nucleon and that of the late universe nucleon are introduced, so the mutual correlation exists between the evolution of the universe and that of nucleons. The dark matter, namely, the supersymmetry companion Ue,B0 of υe,F is given, which has big mass scale, and if it makes the total mass of the universe too much and beyond the limit, the universe will collapse. The condition which makes the universe change from expansion to contraction is that when the mass of the late nucleon decreases to the upper limit of the mass of a present photon, the thermal equilibrium of photons will be weaken and the gravitational bound state will preponderate over the universe. Before it change to contraction, the universe will expand at deceleration.