Its energy is not being released to its surroundings.
Even if the universe expands it would also suggest that the closed system is expanding.
Even if the universe expands it would also suggest that the closed system is expanding.
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The universe is the ultimate boundary for any system. Even though the universe constantly expands until it exhausts all of its usable energy, any transfer of energy must take place within the universe system. Basically, there is nothing beyond the universe (as far as thermodynamics is concerned); no parallel universe is supplying heat, so the universe is indeed a closed system.