Law of Thermodynamics

Modern scientists know that a beginning and an ending are demanded by one of the most validated laws in all nature - the Second Law of Thermodynamics.

Thermodynamics is the study of matter and energy, and the Second Law states, among other things, that the universe is running out of usable energy. With each passing moment, the amount of usable energy in the universe grows smaller, leading scientists to the obvious conclusion that one day all the energy will be gone and the universe will die. Like a running car, the universe will ultimately run out of gas.

You say, "So what? How does that prove that the universe had a beginning?" Well, look at it this way: the First Law of Thermodynamics states that the total amount of energy in the universe is constant. In other words, the universe has only a finite amount of energy (much as your car has only a finite amount of gas (the First Law), and whenever it's running it continually consumes gas (the Second Law), would your car be running right now if you had started it up an infinitely long time ago? No, of course not. It would be out of gas by now. In the same way, the universe would be out of energy by now if it had been running from all eternity.

The Second Law is also known as the Law of Entropy, which is a fancy way of saying that nature tends to bring things to disorder. That is, with time, things naturally fall apart. Your car falls apart; your house falls apart; your body falls apart. But if the universe is becoming less ordered, then where did the original order come from? Astronomer Robert Jastrow likens the universe to a wound-up clock. If a wind-up clock is running down, then someone must have wound it up.
The aspect of the Second Law tells us that the universe had a beginning.