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B Does the 2LOT rule out an eternal universe?

  1. Jan 24, 2017 #1
    If the universe is past eternal, then why has it not reach heat death by now? If given infinite past time it should have already arrived at such a state and should exist in thermodynamic equilibrium. Some argue that the 2nd Law of Thermo rules out any possibility of a past eternal universe.
     
    Last edited: Jan 24, 2017
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  3. Jan 24, 2017 #2

    PeterDonis

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    There are at least two ways of responding to this that I'm aware of:

    (1) An eternal universe does not have to reach and then remain stuck in the same state. The classical version of this is that the universe we observe is a random fluctuation from an overall "heat death" thermal equilibrium state in a localized region. The quantum version of this is a theory like eternal inflation, in which the eternal "background" is a state that continually generates new observable universes, including ours, by means of phase transitions in an underlying quantum field.

    (2) An eternal universe might not have a state of maximum possible entropy, which is what "heat death" requires. For example, our best current model of our observable universe is that it is spatially infinite and will keep expanding forever. If this is correct, then the entropy of the universe has no maximum--there is no finite upper bound. So entropy can continue increasing forever without ever reaching a "heat death" state.

    However, both of these answers have the universe, or at least our observable universe, having a finite extent in the past -- a finite past time before which there was something quite different (or possibly nothing at all, depending on which speculative model you want to discuss). So they don't really say anything about why "an infinite past time" is or is not possible. That's because we know from actual observations that, at the very least, there was a finite past time at which what is now our observable universe was in a very different state, so any model which has something similar to our current universe existing for infinite past time is ruled out by the data.
     
  4. Jan 24, 2017 #3

    Chalnoth

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    Not necessarily. The second law of thermodynamics does not inevitably result in higher entropy at later times. Rather, it's a probabilistic law. Small, simple systems can definitely go down in entropy, and often do. Larger systems do as well, but far more rarely.

    Ultimately, the question of whether our universe is eternal in the past comes down to whatever caused the extremely low-entropy state near the Big Bang. Our universe is likely to be eternal into the future.
     
  5. Jan 24, 2017 #4
    So if the universe is (speculatively) past eternal , then that still doesn't mean heat death must have already happened b/c entropy can indefinitely grow without reaching a heat death limit, or in the alternative, new universes may grow from quantum field phase transitions?
     
    Last edited: Jan 24, 2017
  6. Jan 24, 2017 #5

    Chalnoth

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    Nope, because entropy doesn't have to always grow.

    We do know that there was something that produced an extremely low-entropy state some 13.7 billion years ago. Whether or not there was anything before that time is an open question, and the second law of thermodynamics doesn't help all that much. We would need to know more about what, precisely, caused that low-entropy state.

    One perspective to take on this is that it doesn't matter: the physics of that event 13.7 billion years ago wiped out any record of what happened before, so in some deep sense it can be considered irrelevant. Experimentally it may be impossible to determine.
     
  7. Jan 24, 2017 #6
    The universe eternal? Let's look at it closer from a more logical approach.
    For a first shot, very recent studies strongly indicate now that the universe is lining up for a rip.
    Secondly, for a material universe, there seems to be recorded another nature that perhaps needs some attention also. Namely with any material composition whether it be this cosmos or some multiverse (assuming they arise from hot inflation) will experience decoherence. Likely every composition. None of this sounds eternal or perdurable or even stable for the macro level sizes. Does it?

    At quantum level, it is very likely with many scientists, the energy level adjusts...downward. Hence, with that wondrous change, there will be a whole new universe not sharing the same properties our unstable, ephemeral existence does. In short, it goes from false vacuum to true vacuum and it may finally have what so many have postulated and wanted, something finally immutable.
     
  8. Jan 24, 2017 #7
    Entropy? The old reliance on entropy and arrow of time may finally be addressed. There need not be any and it is not essential to a universe. Entropy and the arrow of time are in fact secondary. It is amazing how one concept has occluded others. For the arrow of time to be born, something must drive it. OK? Something must give it its direction, forward. Entropy and time don't have to necessarily have a direction. But for forward, there must be something that takes arrow of time and entropy out of an inert state. It would have to be monotonic to keep one direction. The only energy that does that in the universe and is the source of taking entropy out of its inert state is.... dark energy.
     
  9. Jan 24, 2017 #8

    Chalnoth

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    This is false. Where did you get this impression?

    The current expectation of the far future of our universe is heat death, as described here:
    https://en.wikipedia.org/wiki/Future_of_an_expanding_universe

    The Big Rip is highly speculative and would require some pretty major violation of known laws of physics to occur.
     
  10. Jan 24, 2017 #9

    PeterDonis

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    Reference, please? AFAIK the "Big Rip" scenario is highly unlikely given our best current data.

    If by this you mean decoherence does not apply to macroscopic objects, of course it does. Why wouldn't it?

    If by this you mean that our current universe is in a "false vacuum", again, reference, please? AFAIK this is also considered very unlikely given our best current data.
     
  11. Jan 24, 2017 #10
    It depends on why the entropy was low at the big bang. in order to answer that we need a good theory of what happens at the very earliest moments of the big bang. We dont have that . There are many different proposal including but not limited to string gas cosmology, loop quantum cosmology, CCC, ekprrotic universe etc
    Many of these different proposals have different ideas about entropy. CCC is a cyclic model proposed by Sir Roger Penrose , he claims the entropy of the previous "aeon" is locked up in black holes which then evaporate setting the slate clean as it were. Loop quantum cosmology replaces the big bang with a big bounce, an expanding universe and a contracting universe are joined by a "a quantum bridge" the entropy of the previous state does not go through the bridge. Other models like the Aguirre Gratton model and Julian Barbours "Janus universe" also imply a contracting and then expanding universe but with the arrow of time reverse at the middle point.
    There are many different ideas out there and no one knows which ( if any) are right at the moment. Maybe there will be bits of ideas from the these different models that turn out to be right. The most mainstream idea for early universe cosmology is inflation which seems to predict a multiverse therefore there is no maximum entropy and it can increase forever. So the universe may have existed forever into the past , we dont know. It is not impossible that a signal from before the big bang could be detected, but dont hold your breath, see here for example:
    http://iopscience.iop.org/article/10.1088/0264-9381/31/5/053001/meta
     
  12. Jan 24, 2017 #11
    So if the universe is (very, very speculatively) past eternal, then that still doesn't mean heat death must have already happened b/c entropy does not have to grow?

    In other words, the 2LOT does not automatically rule out a past eternal universe? That doesn't mean the universe has an eternal past, but on the other hand, it also doesn't automatically mean it there universe doesn't have an eternal past. Maybe it does, maybe it doesn't. We have no data of events prior to the Big Bang, and if we did, it wouldn't be incompatible with the 2LOT.
     
    Last edited: Jan 24, 2017
  13. Jan 24, 2017 #12

    PeterDonis

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    No, because as far as we can tell, there is no maximum entropy for the universe, so it can keep growing forever without reaching thermal equilibrium.
     
  14. Jan 24, 2017 #13
    Okay, I'm getting two contradicting responses:

    1. Entropy can grow indefinitely without reaching heat death (thermal equilibrium), per PeterDonis.

    2. Entropy doesn't have to grow, per Chalnoth.

    But both points seem to agree that heat death doesn't rule out a universa with an eternal past (we just lack empirical data of a universe with an eternal past) b/c there won't be heat death period.
     
  15. Jan 24, 2017 #14

    PeterDonis

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    No, they're not contradictory. Entropy can grow indefinitely, but it does not have to. Both of those statements are perfectly consistent with each other.

    Yes.
     
  16. Jan 24, 2017 #15
    Thank you very much. You guys were very, very informative.
     
  17. Jan 24, 2017 #16
    Currently, science vacillates on many issues. One big central issue on the cosmos established by most is critical energy. And results say we are at a fine edge of continuity or being torn apart (rip) Or rest in peace? Those are given but will they result is the point of vacillation. Many observations and studies still are to be done.

    The recent entry of "little rip", not Big, is found in present time of early November 2016. This has been study of Robert Scherrer of Vanderbilt U., Bouhmadi-Lopez, Imanol Albarran of Technical University of Lisbon, Portugal and Joao Morais. Can be found online, perhaps as "Little Rip" and in magazine, "New Scientist" of the week of Nov. 9, 2016. Perhaps other sources have entertained this at that time and subject matter. Pleased to be of service and updating.
     
  18. Jan 24, 2017 #17
    It was meant that with material compositions, decoherence is likewise inevitable for such natures. Nature can change and we speculative humans won't detect it. Ignorance is bliss?
    For true vacuum, other than my own essays, I very recently looked it up in Wikipedia and still found it more extensively treated therein by false vacuum. Are we that afraid of true vacuum? Many write on black holes, supernovas, gamma rays, SFS. So why not true vacuum?
     
  19. Jan 24, 2017 #18

    PeterDonis

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    I asked for references, not personal opinions.

    Then please find it and post a link.

    Neither of which are valid sources for PF discussion. Please review the PF rules on acceptable sources and personal theories.
     
  20. Jan 25, 2017 #19
    [Moderator's note: irrelevant off topic content deleted.]

    A source for the paper on my treatment here of Entropy is Physical Review E. 93052125, 21 May 2015, plus references therein which are more than 2.

    For a paper that currently addresses a future rip and addresses the critical energy of the cosmos: arxiv.org, 1611.00392.
     
    Last edited by a moderator: Jan 25, 2017
  21. Jan 25, 2017 #20

    berkeman

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    Thread closed temporarily for Moderation...
     
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