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Entropy has been increasing since the Big Bang. But how? It seems relatively intuitive, but the universe is cooling down. Often when things cool down, entropy decreases. So why, in the case of the universe, does it increase? What are the other factors that are causing entropy to increase?

Qmechanic
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Lara
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  • Since entropy is disorder that might increase until no useful work could be obtained from the system involved, could you clarify (perhaps with an edit) why you feel it might decrease during a cooling process? – Edouard Dec 13 '22 at 04:57
  • Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. – Community Dec 13 '22 at 05:20
  • Related: https://physics.stackexchange.com/questions/14004/how-can-it-be-that-the-beginning-universe-had-a-high-temperature-and-a-low-entro/167535#167535 – hft Dec 14 '22 at 00:59

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When a cup of hot water for example cools down, its entropy decreases. (Note however that there is a heat transfer to its neighbourhood, and the entropy of water + neighbourhood increases).

But if this cup of water is placed at the outer space, far from the sun, it also loses heat by radiation. But at the same time it boils away, increasing its entropy.

So, it is not true that by cooling down, a system always decreases its entropy.

Claudio Saspinski
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