The discovery of the expansion of the universe in the 1920s modified the sort of heat death predicted on the basis of the second law, but it didn’t alter the fundamental question. If the universe will expand forever, then it will never actually arrive at equilibrium. Because the volume of space is constantly growing, the matter and energy always have more room to spread out. Nevertheless, as the universe expands, its available energy is used up and it becomes increasingly cold, dark, dilute, and dead.
