Since the beginning of man the question of the origin of the universe has been hotly contested. Specifically, was it created or has it always existed. It was the position of Judaeo-Christian religion that the universe was created from nothing or creatio ex nihilo. Ancient philosophers believed the universe was eternal in that it had existed forever. Physicists have been uncomfortable with the idea of a beginning since the work of Friedman which showed that the solutions of Einstein's equation showed that the universe had a beginning.
But if the universe is expanding then it must have a beginning. If you follow it backwards in time, then any object must come to a boundary of space time. You cannot continue that history indefinitely. This is still true even if a universe has periods of contraction. It still has to have a beginning if expansion over weights the contraction.
That the universe began has been proven a myriad of ways. Red shift shows that everything is moving away from everything else due to an expanding universe. An expansion that began when vast amounts of energy were released through matter anti matter annihilation during the creation of the universe. Cosmic background radiation shows the residue radiation left over from the matter and anti-matter mutual annihilation which occurred when the universe was filled with energy during the quantum tunneling event which is how the universe was created from nothing.
The problem with a universe that has existed forever (i.e. 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 and then it will stay in that state. A cyclical universe cannot avoid this problem. Since we do not see thermal equilibrium (good thing too because there would be no life) we know that the universe did have a beginning.
But if the universe is expanding then it must have a beginning. If you follow it backwards in time, then any object must come to a boundary of space time. You cannot continue that history indefinitely. This is still true even if a universe has periods of contraction. It still has to have a beginning if expansion over weights the contraction.
That the universe began has been proven a myriad of ways. Red shift shows that everything is moving away from everything else due to an expanding universe. An expansion that began when vast amounts of energy were released through matter anti matter annihilation during the creation of the universe. Cosmic background radiation shows the residue radiation left over from the matter and anti-matter mutual annihilation which occurred when the universe was filled with energy during the quantum tunneling event which is how the universe was created from nothing.
The problem with a universe that has existed forever (i.e. 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 and then it will stay in that state. A cyclical universe cannot avoid this problem. Since we do not see thermal equilibrium (good thing too because there would be no life) we know that the universe did have a beginning.
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