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I First law of thermodynamics, mass/energy in the Universe

  1. May 30, 2018 #1
    Ok, so just a quick question, first law of thermodynamics basically states that matter cannot be created nor destroyed aka it can only change states and turn into energy or vice versa, the second law states that, IIRC, for perfect isolated systems entropy remains constant but for our universe entropy can only increase not decrease, so given this information how can we reconcile these two laws with our current best theory about the origins of the universe which is the "Big Bang" theory?

    Also given the fact that physical laws and our means of "seeing" in the past like EM radiation itself arose only after or with the big bang, will we ever be able with scientific experimental certainty tell how these laws go together with how the universe began, taking also into account the fact that we cannot determine the total size of the universe due to it's accelerated expansion?

    I have heard that typically we don't apply the first law to the universe, but if so does it help us much? because we know for a fact that the universe aka space-time comes together with a certain amount of mass/energy that we can observe within the universe so with or without the first law applied to cosmological scale do we have or will we ever have any certain knowledge of exactly where all this mass/energy came from?
  2. jcsd
  3. May 30, 2018 #2


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    The law of conservation of energy does not apply to the universe as a whole. It is a local conservation law. With no conservation law to use, the question of "where did it come from" does not arise. It does not have to come from anywhere -- it's not conserved.

    If one restricts one's attention to a local patch of space-time, one can ask "where did did the energy in that patch it come from". But the answer is simple -- it was either already there or it crossed the boundary into our patch. Our model of the universe does not extend to a time before the universe existed. It does not cover a transition from nothing to something. So again, there is no problem.
  4. May 30, 2018 #3


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    The entropy of an isolated system can increase. Suppose we have a cubical isolated container with volume of ##1m^3##, and containing an ideal gas that is for some reason all located in one half of the cube at the initial state. In a quite short time after the initial moment, the gas will be distributed to fill the whole cube evenly, while its temperature is the same as initially because expanding to vacuum does not consume energy and the temperature of an ideal gas depends only on the internal energy and number of moles. Now, as the gas is occupying a larger volume, its entropy has to definitely be larger than initially, because there's no temperature drop that would counteract the effect of the volume change on entropy.
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