If the universe always existed, it must have an infinite past. The universe cannot have an infinite past because the present cannot occur without a finite history, without a first cause which must be uncaused.
In philosophical terms, an uncaused cause is required to any event to prevent an infinite regress or the tracing of causes endlessly backward. If there is no such cause then the present event cannot happen. For example, an event 1 is caused by event 2 which is caused by event 3 which is caused by event 4 and so on forever. Will event 1 ever happen? The answer is no, because infinitival event never elapsed. There must, therefore, be an uncaused cause to a present event.
The universe cannot, therefore, exist now without coming into existence, without beginning in the finite past. That is, the universe must have begun to exist.
Laws of thermodynamics
The universe cannot always have existed because it should have already suffered a “heat death” by the laws of thermodynamics, and it has not.
The laws of thermodynamics apply to all physical and biological systems, all of which vary in their energy state and their ability to perform useful work on their environment. A system is an entity distinct from its environment with which it exchanges heat, work and other forms of energy. It could be anything such as gas inside a cylinder, or the universe.
A system’s condition is called its thermodynamic state, the characteristics of which are called state functions which, for gas inside a cylinder, are temperature and pressure.
A closed system is one which does not exchange heat, work or other energy with its environment. An open system is one which does.
A system is in thermodynamic equilibrium when there is no tendency for its state to change spontaneously. So the gas inside a cylinder will be in thermodynamic equilibrium if the temperature and pressure are uniform. Therefore, the system can only change by an external change in its state functions imposed by its environment.
First law of thermodynamics
The first law of thermodynamics says that the total energy of a system and its environment is conserved.
Since the universe is a closed system and does not, therefore, exchange heat, work or other energy with its environment, the total energy of the universe must, therefore, be constant whatever the processes taking place inside the universe.
Second law of thermodynamics
The second law of thermodynamics says that the entropy of closed systems increases with time until maximum entropy is reached at thermodynamic equilibrium.
Entropy is a measure of the uniformity of distribution of energy in a system. So for gas inside a cylinder with heat insulated walls [so that heat energy cannot be transferred into the cylinder from its environment], heat will continue to flow spontaneously from the hotter regions to the colder regions until temperature is uniformly distributed.
Since the universe is a closed system, its entropy will increase with time until it suffers a “heat death” at maximum entropy in thermodynamic equilibrium.