The first law of thermodynamics says that the actual amount or energy in the universe remains constant—it doesn't change. The second law of thermodynamics says that the amount of usable energy in any closed system (which the whole universe is) is decreasing. Everything is tending toward disorder and the universe is running down. Now if the overall amount of energy stays the same, but we are running out of usable energy, then what we started with was not an infinite amount: You can't run out of an infinite amount. This means that the universe is and always has been finite. It could not have existed forever in the past and will not exist forever into the future. So it must have had a beginning.
