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Entropy in a Big Crunch

  1. Jul 28, 2006 #1
    I know there are a lot of assumptions being made here but assuming that the big bang is scientific fact and that a big crunch will eventually occur I have a question about the application of the 2nd law of thermodynamics.

    The entropy of the universe at the time of a big crunch must be higher than the entropy at the time of the big bang preceeding the big crunch to satisfy the law, correct? But the sum of the energy (mass, temperature, etc) of the singularity at the big crunch must be the same as prior to the big bang, othewise energy is being created/destroyed. How can the entropy increase if the singularity at the big crunch is essentially the same as prior to the big bang?

    Does the law not hold in this situation? Or am I missing something?

  2. jcsd
  3. Jul 28, 2006 #2


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    I believe the current expected outcome for the fate of the universe is that it will just keep expanding and won't collapse back in on itself.

    However, ignoring that... If the compression of the universe as it tended towards the big cruch could take place reversibly (and I'm not saying it can), then we've no problems, because the net change in entropy would be zero, which is certainly permitted by the second law (in a closed system, entropy must either decrease or remain constant).

    Of course, trying to apply such concepts to the universe as a whole is a difficult task. For one thing, the universe is not in equilibrium, and we typically deal with entropy in equilibrium situations. Furthermore (I may be stepping outside of my realm of knowledge here), the extensivity of entropy seems to fail in systems where long-range forces (e.g. gravity) are important (e.g. the universe). And what if the universe were not a closed system? Then its entropy need not increase as it crunched, even if the crunch took place irreversibly...
    Last edited: Jul 28, 2006
  4. Jul 28, 2006 #3
    Most of the energy that is "lost" because of entropy isn't really lost (It can't be, because like you pointed out, that would break the Law of Conservation of Energy). It's just changed into a kind of energy that isn't really usable. For example, during almost all reactions energy is given off as heat. This energy is virtually impossible to re-obtain, and even when it is possible to do so, it always takes more energy to recapture the energy than the energy you actually get.

    I don't think that the Big Crunch could make that energy usable again. IIRC, nothing can reverse the trend toward Entropy, but I don't really know for certain.
  5. Jul 28, 2006 #4
    That was my next question. I was thinking its possible that it could escape into another system (making the ultimate system change in entropy increase but our universe entropy decrease), but that just opens up even more questions so I didn't go there.

    I understand that in any process entropy may increase or remain the same but the entropy of the universe already has increased since the big bang, has it not? Therefore my understanding is that the entropy of the universe may never be lower than it is today (unless it escapes into another system). But it is already higher than at the big bang, so any crunch would have to reduce the entropy to return to its original value. How is that possible?
  6. Jul 28, 2006 #5


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    About the "closed system" bit - I was being sloppy with definitions, I think. I should have said "isolated system".

    As for the universe's entropy increasing, yes, that would seem to be the case, but again... I think we have to tread carefully when applying the concept of entropy to the universe as a system (due to extensivity/non-equilibrium issues). But, Ateowa makes a good point - the total energy at the end must be the same as in the beginning (assuming an isolated universe), but it does not have to be the same kind of energy, and so the entropy need not be the same either. That is, perhaps the singularity at the beginning is not the same singularity as at the end?

    At any rate, seeing as we don't really understand the big bang very well, I'd say we understand a big crunch even less, and we also don't quite understand how entropy fits into the big picture of it all. Perhaps the Second Law needs to be modified somehow at some point? Perhaps the universe simply can't collapse into a big crunch so we need not worry about it? (Someone more knowledgable on these subjects may have something to say about these speculations).

    At any rate, they're fun questions to think about. =)
  7. Jul 28, 2006 #6
    Big Crunch

    well, here's my thinking on this particular subject.

    1. Big Crunch

    One of the ten top priorties of the gov. (within the next ten years) is to determine if the proton is stable. Some have estimated that a halflife of 10^38yrs is probable. But, only a guess.

    2. If the proton is stable or any other particle is stable. Then we have a condition where the decay of normal(baroynic)matter will not result in a total loss of all matter but only that portion of matter subject to decay. if this proves to be the case, then all radiation given up thru normal decay would eventually be reclaimed since the strong force will continue to increase in dominance over the conditions responsible for the current expansion.

    Just some of my thoughts. john
  8. Jul 28, 2006 #7
    The idea is simple. Entropy increases because less energy is avaliable to do work. Radiation contains energy and is unable to do work until it contacts a massive particle. The more time that radiation is in transit, the more entropy is. Consider getting from work at home to work at the job. When you are in your car stuck in traffic, between point A and point B, you can't do any real work (consider this). Only when you are at the place where you can "settle" will you do your work, even if your job is at the race track. So basically, since the Big Bang universe is expanding, this implies that entropy is increasing, in the same way that going a greater distance by car increases entropy.

    To fix this, point B must be brough closer such that power input on masses is greater the than power output of masses. In other words, point B must be opaque, and therefore must be really large with respect to its distance away from us. If radiation is like flies, then we need a flyswatter. How is this flyswatter (or photon swatter) going to form?

    If it already exists, then it must exist beyond 13.7 billion light years. It would be have to be capable of absorbing a vast majority of the radiation in the universe. It is likely that such an object would have a black body spectrum. Such an object would increase the amount of work available. Its size would have to overshadow that of galaxies and superclusters.

    If it does not exist and must be formed in the future, there would have to be an unknown mechanism that leads to a big crunch, decreasing the distance between point A and point B, allowing the amount of available work to increase.

    To return to a state of a big crunch requires attractive forces which increase with distance. Perhaps we're looking down the gradient of such forces such that they appear as though they were repulsive forces?

    The cause of entropy decrease must be absorbent (increasing proximity, hence energy able to do work), whereas the cause of entropy increase must be emissive (decreasing proximity, hence energy unable to do work). When a gluon is emitted is it always reabsorbed. Is our universe like a gluon, or is it like photon forever be unable to do work in accelerating universe?
    Last edited: Jul 28, 2006
  9. Aug 4, 2006 #8
    This all assumes that the second law of thermodynamics is ubiquitous throughout spacetime. What if planck's constant actually varies through spacetime. ie: a singularity, either a big bang or a big crunch, could be explained by planck's constant = 0. 'Entering' into a black holes event horizon would be entering into a part of spacetime where planck's constant may be negative.

    Maybe we can think of spacetime as a 3D topographic surface where the 'hills' represent big bang singularities and the 'basins' represent big crunch singularities. Planck's constant can be determined by taking the slope derivative at that point. This would give us our arrow of time dependant upon how we are moving across the 'topographic' surface

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