Continuing with post 1483 on entropy…
The second law of thermodynamics states:
The second law may be formulated by the observation that the entropy of isolated systems left to spontaneous evolution cannot decrease, as they always arrive at a state of thermodynamic equilibrium where the entropy is highest at the given internal energy. An increase in the combined entropy of system and surroundings accounts for the irreversibility of natural processes, often referred to in the concept of the arrow of time.
The second law states that there exists a useful state variable called entropy S. The change in entropy delta S is equal to the heat transfer delta Q divided by the temperature T.
delta S = delta Q / T
The Second Law is concerned with Entropy (S) which is produced by all processes and associated with the loss of ability to do work. The Second Law states that the entropy of the universe increases.
For energy to be available there must be a region with high energy level and a region with low energy level. Useful work must be derived from the energy that flows from the high level to the low level.
100% energy can not be transformed to work
entropy can be produced but never destroyed
So, the second law of thermo means in a closed system, entropy will always be increasing and can never decrease. And in any closed system, given enough time, it will reach maximum entropy and no more further work is possible. If the universe is a closed system, then this would also apply to the universe. Given enough time, eventually the universe will have maximum entropy and would reach “heat death”.
The ‘heat-death’ of the universe is when the universe has reached a state of maximum entropy. This happens when all available energy (such as from a hot source) has moved to places of less energy (such as a colder source). Once this has happened, no more work can be extracted from the universe. Since heat ceases to flow, no more work can be acquired from heat transfer. This same kind of equilibrium state will also happen with all other forms of energy (mechanical, electrical, etc.). Since no more work can be extracted from the universe at that point, it is effectively dead, especially for the purposes of humankind.
The heat death of the universe (also known as the Big Chill or Big Freeze) is a hypothesis on the ultimate fate of the universe, which suggests the universe will evolve to a state of no thermodynamic free energy and will, therefore, be unable to sustain processes that increase entropy. Heat death does not imply any particular absolute temperature; it only requires that temperature differences or other processes may no longer be exploited to perform work. In the language of physics, this is when the universe reaches thermodynamic equilibrium.
Even if the universe were to somehow collapse and start another Big Bang, entropy would still have to increase. So, eventually, the cycle of Big Bangs would have to stop and end up in a final heat death.
On a smaller scale, we see this in the impossibility of creating a perpetual motion machine. The reason it’s impossible to create any closed system to do work forever is because of the second law of thermodynamics.
Before the establishment of the second law, many people who were interested in inventing a perpetual motion machine had tried to circumvent the restrictions of first law of thermodynamics by extracting the massive internal energy of the environment as the power of the machine. Such a machine is called a “perpetual motion machine of the second kind”. The second law declared the impossibility of such machines.
So, based on the second law of thermo, it would be impossible for the universe to have experienced infinite cycles in the past since we are not currently at heat death.