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As entropy increases, the universe will evolve toward a thermodynamic equilibrium, and I understand that in a macroscopic sense, that is order since the temperature is the same everywhere. However, in a microscopic sense, it is chaotic, because as you increase temperature the movement of the atoms get faster and faster, now what I am unable to understand is, why is the heat death scenario considered a disorder? If it means the universe will be getting really cold, doesn't the mean that in the microscopic level the atoms will be moving very slowly? Isn't that more of an order than a disorder?

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    $\begingroup$ The comparison of entropy to disorder is generally a bad one. Everyone seems to do it though. $\endgroup$ – jjack Feb 23 '18 at 20:33
  • $\begingroup$ Heat death means that temperature will become uniform, not that the universe will become colder. $\endgroup$ – valerio Feb 23 '18 at 20:48
  • $\begingroup$ @valerio I have read in many resources that it meant the cooling of the universe, hence the confusion. $\endgroup$ – Dewton Feb 23 '18 at 21:05
  • $\begingroup$ @Dewton, there is little scientific evidence that entropy of the entire universe is increasing or that a thermodynamic equilibrium is being reached. It is a 19th century axiomatic concept that finds no robust support in modern theory - nor in the first law of thermodynamics! $\endgroup$ – Steve Feb 23 '18 at 22:11
  • $\begingroup$ @valerio I think that's just a semantic quibble. It's true that the "death" imagery comes from the full equilibration at a uniform temperature, but the currently accepted $\Lambda$-CDM cosmological model also implies that that temperature will get arbitrarily close to zero as the expansion of the universe dilutes the matter content ever lower. I think it's reasonable to incorporate that cooling into the "heat death" concept. $\endgroup$ – tparker Feb 23 '18 at 22:19
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Remember that the universe will have expanded to an extremely large size by then. The expansion results in both the cooldown (as the matter/energy density gets diluted) and the increase in entropy (as the energy's positional spread increases). The increase in the positional contribution to the entropy dominates the decrease in the momentum contribution.

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I doubt the "heat death" very seriously. First, our Universe is not enclosed system. We cannot ensure somehow that "thermodynamic closeness" could be verified. Even if we assume that, we do not know exact parameters of Universe to verify cosmology assumptions about the evolution of Universe, i.e. intensity of turning stars into black holes, gravitational evolution of Universe etc.

Also, we do not know if so called "global warming" exists ("does not" is not ruled out too), which is the problem of lesser scale, which could be solved simply by building science all over the world. Yet, there is infinitely small number of scientists to even rule out that one "small problem".

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