Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Law of entropy vs. Big Bang Theory?

  1. Oct 19, 2008 #1
    Sorry if this isn't the right place for this post. Please do help me move this topic if necessary.

    I'm just a bit curious that the law of entropy states that the disorderness of a closed system will continually increase. However, the big bang theory says that the universe will expand until at some point, it will start contracting and thus big bang will occur again. Well doesn't this violate the law of entropy? If the universe can itself cluster all the galaxies together in some finite volume, then disorderness is decreasing.

    What's your thought on this?
  2. jcsd
  3. Oct 20, 2008 #2
    Perhaps we can think of the pre-BB condition as a state of total order, and the common history of the universe as a path from total order to total disorder, then maybe back again.

    I don't really understand your last statement. Are you attributing conscious motives to the universe? Where were the galaxies, would you say, before the universe 'clustered' them? Could you define 'universe?'
  4. Oct 20, 2008 #3


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Where do you get this, please? As I understand the standard picture in cosmology, it does not say this. It says to expect indefinite expansion, never starting to contract.
    What is your source? Please give a link so we can easily check it out and see if there is some confusion.

    It does seem that your question falls more into the Cosmology area, maybe the mentors will move it (as you suggested.)

    Another thing: where did you get this?
    Be careful not to be misled by unreliable sources, like popular books and religious websites! In big bang theory, the cosmologists never said that the universe must necessarily start out with all the matter in a finite volume!

    And if the numbers were different in a way that destined our universe to eventually collapse, then nobody has said that it would collapse to a finite volume. You seem to imagine that there is some professional consensus that in the case of a big crunch (which is not predicted, but what if) all the galaxies would be collapsed to a finite volume. That is not true. The question has not been decided.

    Some given finite chunk would of course collapse down to a finite volume---finite must collapse down to finite----but that does not necessarily mean ALL the galaxies.

    You may need some up-to-date reliable info about mainstream consensus cosmology. I have two good links in my sig.
    Lineweaver's cosmo Scientific American article, and the Einstein-Online website. Please see if you can get the links to work. Let me know if they don't work. Einstein-Online has more and is more recent. It has different sections, click on Cosmology to get stuff specifically about the largescale universe. It is non-mathy and intended for wide audience, but not as full of mistakes as ordinary un-vetted sites.
    Last edited: Oct 20, 2008
  5. Oct 20, 2008 #4
    As I understand, the idea that space will eventually collapse back on itself is called the "big crunch" theory, and in the last ten years it has been proven not to be true in our universe because of the discovery of dark energy. The expansion of space accelerates, so space will never collapse back on itself.

    (It may still be worth asking the question: If the big crunch theory were true in our universe, what would happen to the universe's entropy as it collapsed to a singularity?)
  6. Oct 21, 2008 #5


    User Avatar
    Science Advisor

    Understand that we could still have a big crunch in principle. There are various beyond the LCDM models that have us living in a DeSitter patch that is undergoing the end of inflation atm or somesuch, such that the sign of the CC could change at some future epoch.

    Unfortunately the constraints on these scenarios aren't very tight, so theres still a lot of room for speculation. We really need to get better probes on the gravity wave modes (LIGO and future experiments) to get some more detail.
  7. Oct 21, 2008 #6
    can u explain why the big crunch is not possible in our universe
  8. Oct 21, 2008 #7
    Yes, sorry that some of the words were misused and my knowledge our universe is only from highschool. And I only vaguely remember it. But I do remember being taught how we've found the distances between galaxies are continually increasing (red shift)? And then I think I remember being told that a theory is that at some point, blue shift will occur and the distances between galaxies will begin contracting?
  9. Oct 22, 2008 #8


    User Avatar
    Science Advisor

    That was the theory, untill about ten years ago. Around then, some guys decided to measure the rate at which the expansion of the universe was slowing. Geuss what they found out; it isn't! The expansion of the universe is not slwoign down, the way we would expect if it were coasting after a Big Bang, it's actually speeding up. Things are flying apart faster today than they were a billion years ago, and this acceleration has been going on for as far back as we can see. Now, we don't know what is causing the acceleration. It has been called "dark energy," but that just means, "whatever is causing the accelerating expansion."

    But, none of that addresses your original question. Untill recently, the "Big Crunch" was the prevailing theory, and so the question of how entropy works in a contracting universe must have been addressed.
  10. Dec 10, 2008 #9
    "If the universe can itself cluster all the galaxies together in some finite volume, then disorderness is decreasing."

    Entropy is at a maximium for a gas in a container, when the particles are spread out randomly. This is because repulsive forces make the particles bounce off each other.

    For very large collections of particles the attractive gravitational force comes into play, and the most likely, high entropy, configuration is all the particles stuck together.

    The universe at the instant of the big bang was also all collected in one place, but note that, somewhat counterintuitively, this configuration was not high entropy, but extremely low.
  11. Mar 21, 2009 #10
    I am not 100% on the laws of entropy and i do find them confusing. But i have read that the big crunch is possible with the laws of entropy because as the galaxys move towards each other they lose gravitational potienial energy which (supposedly) causes an increase in entropy.
  12. Mar 21, 2009 #11
    I might get a big bashing for saying this, but the law of entropy is only true until proven wrong, as are all the other fundamental theorems in physics. Conservation of energy and conservation of momentum are ideas, more than actual laws. We cannot prove that entopy exists, we can only say that in our current understanding it appears to be so that nature tends to bla bla bla... If you cannot sleep at night because you fear that one of the fundamental concepts of physics might be ruined by some "experiment" or event then you should have became a mathematician and not a physicist. :)
  13. Mar 21, 2009 #12


    User Avatar

    Staff: Mentor

    Was it, though? Or was it a flaw in the big crunch theory that hadn't yet been figured out and is now irrelevant?
  14. Mar 21, 2009 #13


    User Avatar

    Staff: Mentor

    That's all true, but more than a little miselading since these principles are put into practice on a coninuous basis. Every revolution of a car engine and electron moving through an electrical wire is an experimental confirmation of these principles. By now, the completely countless experiments that support them lead us to believe that in these particular cases, we have found the fundamental law on which the universe was based.
  15. Mar 21, 2009 #14
    what is it that causes entropy to increase,disorderness would keep increasing and at one time ,only if artificially can be brought to order,as disorder is natural.where
    arises the questionof a sudden contraction of an already expanding universe?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook