The universe began with Big Bang and is expanding, and the expansion is accelerating at present.
Entropy of the universe becomes larger and larger; hence the Second Law of Thermodynamics is
valid in this process. The acceleration of the expansion of the universe is considered due to “dark
energy”. The amount (density) of dark energy decreases when some amount of dark energy changes
to matter. Energy is equivalent to matter: Einstein formula E=mc2, which is shown to be valid for the
conversion of matter to energy, and also for the conversion of energy to matter. When the large
amount of dark energy decreases by the change to matter, the decrease of dark energy induces the
change of the dynamics of the universe from expansion to shrinking, and the universe shrinks smaller
and smaller. According to the Second Law of Thermodynamics (Entropy Principle), the randomness
of the state of the universe becomes larger with time in this shrinking process. Finally, the universe
shrinks to a point with very large randomness in the state and vanishes (Big Crunch): the end of the
universe. Thus, the Second Law of Thermodynamics describes all processes of the evolution of the
universe from the beginning to the end. The validity of the Second Law of Thermodynamics (Entropy
Principle) in the universe has been established in this article, for the first time. The assertion of
Clausius in 1865 has been shown to be valid for the first time here, after a long time since his
Department of Systems Engineering, Faculty of Engineering, Shizuoka University, Hamamatsu City, 432-8561 Japan.