INSIGHT
We have said that the Big Crunch theoretically could not happen as it would violate the Second Law of Thermodynamics. But what does it mean?
First, we must know Thermodynamics is the branch of physics and chemistry that studies thermodynamic transformations, the relationship between heat, work, temperature, energy and radiation. These studies tend to be carried out on systems separated from the outside, and which therefore have no energy exchange, like the Universe as a whole, to comprehend how they reach equilibrium.
At the basis of this science, there are three laws (plus another implicit in the other three), true and proper axioms based on the experience that defines how thermodynamic transformations work. They are:
Zeroth Law: if A is in thermal equilibrium with B, they have the same temperature, and the B is in thermal equilibrium with C, then A and C are in equilibrium with each other. This principle explains that two bodies at different temperatures that exchange heat with each other achieve equilibrium when they are at the same temperature. The temperature is caused by kinetic energy, the movement of the molecules of a body. When two bodies are in contact, the tendency is to reach an average kinetic energy equal for all connected systems, and therefore the more energetic molecules pass energy to the less energetic ones, that is the heat passes from the hot body to the cold one
How heat transfer works
First Law: the variation of the internal energy of a closed thermodynamic system is equal to the difference between the heat supplied to the system and the work done by the system on the environment. In the eighteenth century, it was believed that heat was caused by a substance, the caloric, a fluid capable of moving through the mass, which, being in greater quantity in the hot body, passes to the cold one. The First Law, on the other hand, identifies heat as a form of energy that can be converted into mechanical work and stored, but which is not a material substance. Indeed, heat, measured in calories, and work or energy, measured in joules (one calorie = 4.186 joules), are equivalent. It is, therefore, a principle of conservation of energy, according to which no machine can produce work without consuming energy
Second Law: the Law that directly relates to the Big Crunch. It includes various statements, but it can be summarized as: it is impossible to develop a cyclic machine that transfers the heat from a cold body to a hot one without any other kind of outcome or it is not possible through a transformation to convert only in mechanical work an amount of energy took from a unique source. This Law, in a nutshell, tells us that entropy, the energy of a transformation wasted as heat, in a closed system can never decrease, but only remain unchanged or increase. It is therefore impossible for the Universe to return to a state of sheer energy as it was in its first instant
Third Law: considered a consequence of the Second Law, it reveals to us it is not possible to reach absolute zero with a finite number of transformations. Moreover, the entropy of a perfectly crystalline solid at 0 K is equal to 0. This means a perfectly crystalline solid at 0 K has no particles that move, and therefore there is no energy to transform into heat and entropy
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PIC CREDITS
Heat block conduction– Encyclopaedia Britannica, Inc.