The problem with a cyclical universe is with the 2nd Law of Thermodynamics. For every matter to energy or energy to matter exchange there is a loss of usable energy. So while the total energy of the universe does not decrease, the usable energy of the universe does decrease. If it is a periodic or cyclical universe then the entropy will increase with each cycle. The 2nd Law of Thermodynamics is a fundamental law of nature which tells us that entropy can only increase or stay the same. Entropy can never decrease. Which means that in a finite amount of time, a finite system will reach a maximum state of disorder which is called thermal equilibrium
Usable energy is actually called KINETIC energy, and kinetic energy is the energy of MOTION. In order for the universe to use up all of its kinetic energy, ALL motion must stop. But that is a violation of the Third Law of Thermodynamics, which says there is no temperature at all motion stops, so therefore thermal equilibrium is impossible.
The kinetic energy in the universe comes from transformations of mass.
E = mc2, equation in German-born physicist
Albert Einstein’s theory of
special relativity that expresses the fact that mass and
energy are the same physical entity and can be changed into each other. In the equation, the increased relativistic
mass (
m) of a body times the
speed of light squared (
c2) is equal to the
kinetic energy (
E) of that body.
In physical theories prior to that of special relativity, mass and energy were viewed as distinct entities. Furthermore, the energy of a body at rest could be assigned an arbitrary value. In special relativity, however, the energy of a body at rest is determined to be
mc2. Thus, each body of rest mass
m possesses
mc2 of “rest energy,” which potentially is available for conversion to other forms of energy. The mass-energy relation, moreover, implies that, if energy is released from the body as a result of such a conversion, then the rest mass of the body will decrease. Such a conversion of rest energy to other forms of energy occurs in ordinary
chemical reactions, but much larger conversions occur in
nuclear reactions. This is particularly true in the case of
nuclear fusion reactions that transform
hydrogen to
helium, in which 0.7 percent of the original rest energy of the hydrogen is converted to other forms of energy.
Stars like the
Sun shine from the energy released from the rest energy of hydrogen atoms that are fused to form helium.