Recent comments in /f/askscience

fr293 t1_jcrn2xs wrote

Why do you think that the energy of the universe is constant? If you have an isolated system that is small enough that relativistic effects can be neglected, then energy is conserved. In systems as large as the universe, it’s not so simple. But if we are dealing with closed systems in which energy is conserved, then the flow of energy is determined in the most coarse sense by the second law of thermodynamics, which tells us that heat cannot move to from one body to a hotter body.

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