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The big crunch

  1. Feb 10, 2010 #1
    Hey everyone,

    I have a limited knowledge in this field, but am very interested in it and am able to understand the concepts.

    I wanted to talk about the 'big crunch' and one of the theories relating to it. In my studies I came across this theory, where if the big bang theory is correct and there is over a certain amount of mass in the universe, then everything will stop expanding at some point and then start coming back in on itself until it once again becomes a singularity.

    One of the theories I read about was that time would also reverse along with it, so we would all experience our lives in reverse, along with the rest of the universe. Is this feasible? Is it actually possible to reverse time?
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  3. Feb 10, 2010 #2


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    The Big Crunch scenario has been ruled out by current astronomical observations, so although we can talk about it in theory, we now know that it isn't something that will actually happen in our universe.

    The standard interpretation of bang/crunch cosmological models is that the thermodynamic arrow of time does not reverse itself. Because the laws of physics are symmetric with respect to time reversal (or very nearly so), it is certainly possible to have bang/crunch solutions that have exact time-reversal symmetry about the mid-point. However, such solutions would require incredibly precise fine-tuning. That is, if you're God, and you get to choose any solution for the universe at all, you have a *lot* more choices that don't have this symmetry than there are that do have it.

    So to imagine such a reverse-replay bang/crunch model, you have to accept two hypotheticals: (1) a hypothetical universe that, unlike ours, will experience a big crunch; and (2) a physical principle or some other unknown reason for the real universe to have exact time-reversal symmetry, which is in some sense extremely statistically unlikely.

    If you don't know about arrows of time, thermodynamics, etc., you could try two books: The First Three Minutes and A Brief History of Time.
  4. Feb 10, 2010 #3


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    Another good one which focuses on the arrow of time and discusses why physicists tend to reject the idea of a reversal of thermodynamics is https://www.amazon.com/Times-Arrow-Archimedes-Point-Directions/dp/0195117980 by Sean Carroll.
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  5. Feb 10, 2010 #4
    Thanks for your response. I was unaware the big crunch theory had been ruled out.

    I understand what you're saying, but what would the effects of this exact time-reversal symmetry be on entropy? I know that entropy is only ever able to go one way, but if the timeline is reversed would the direction of entropy change too?

    Or how about if we describe the timeline in a different manor. If time were simply the rearrangement of matter and energy, as both of these are conserved, and the matter and energy was rearranged to the state it was in, say a week ago, could we really say we travelled back in time as the entropy of the universe would surely still be in the state it was in. Hypothetically speaking of course.
  6. Feb 10, 2010 #5
    And thanks for the links to the books, I am familiar with some of the principles. Always good to read up on these things.

    Thanks again,

  7. Feb 10, 2010 #6


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    Entropy would reverse...from a statistical mechanics point of view, entropy reversal isn't impossible, just improbable (but that's assuming there's no physical principle which rules out solutions that don't have low entropy at the Big Crunch)
  8. Feb 10, 2010 #7


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    Also, if I remember correctly, I think Roger Penrose's Emporer's New Mind book discusses the big crunch and why the arrow of time would not turn backwards, even after the universe was collapsing on itself. And yes, it has to do with entropy for the most part.

    Well it may be too early to say that it's been completely ruled out, but it almost has.

    Before the late 1990s, it was a hot topic of physics to discuss whether there was enough mass in the universe to cause the big crunch, or if there wasn't enough such that the universe would keep expanding forever, albeit at a progressively slower rate. Either way, there was one assumption that people were making: Due to gravity, as time goes on, the universe is progressively expanding more slowly: It may or may not stop expanding one day and turn back on itself, but it is at least slowing down its expansion.

    Then some astronomical observations involving Type 1A supernova (used as standard candles) in the late 1990's shook things up. According to the observations, the universe's expansion is accelerating! Holy moly! As time goes on, the universe's expansion is not slowing down, but continually getting faster and faster.

    Since then, the Big Crunch is pretty much a non-issue for intense debate anymore. But these accelerating universe observations have sparked a lot of other discussions, topics and debates, such as the "Big Rip", re-introduction of the cosmological constant, dark energy, and questioning Einstein's general relativity at large distances. There isn't total agreement on any of these topics, but dark energy seems to be pretty popular at the moment.
  9. Feb 10, 2010 #8
    So if the universe is continually accelerating, whats giving it the energy to do so? It cant be gaining momentum from nothing, or is that where dark energy comes in? I am unfamiliar with dark energy.
  10. Feb 10, 2010 #9


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    Nobody really knows for sure at the moment. I guess that's why it's so exciting.

    "Dark energy" isn't really understood either. As a matter of fact, it's called "dark" energy, as opposed to some other kind of energy, simply because we don't understand it. It's possible that after a well described model is introduced and accepted, dark energy will be given another name, getting rid of the "dark" in the name, in favor of something more descriptive.

    I'm led to believe that most dark energy models so far have a property called "negative pressure." This is consistent with general relativity. The idea is that empty space contains this negative pressure.

    So what is negative pressure? Well, I haven't found an easy analogy, but I'll give it a shot anyway (btw, I'm not an expert in any of this either). Suppose you were to take a spring and compress it. You've added pressure to the spring when you compressed it, and you have also increased the potential energy of the spring. Since the potential energy of the spring increased, so has its gravitational attraction, since energy has gravitational attraction just like mass does, via E = mc2. Now release the spring so that its potential energy goes back to zero, and then let it go down some more. That's sort of like negative pressure. Note that this is where my analogy falls apart. I'm not saying pulling the spring apart is the same as negative pressure, because it's not; it's positive pressure, just in the other direction -- the spring's potential energy still increases even if you pull it apart. Try to imagine some sort of situation where the spring's potential energy is negative. You may not be able to imagine it, but that's the concept.

    So if empty space has negative pressure, then the gravity associated with it is also negative. Like anti-gravity.

    This anti-gravity is pretty negligible when comparing with the gravitational masses of bodies within a galaxy, or even nearby galaxies. But there is so much empty space out there between galaxy clusters that the anti-gravitation effects add up on very large scales.

    For clarity, I'd like to point out once again, that I do not consider myself an expert on this stuff. Also, from what I'm led to believe, the concept of dark energy is not completely accepted yet. There might be other explanations.
  11. Feb 11, 2010 #10
    Cool, thanks.

    So the mass in the universe is effectively being sucked outwards. It sort of makes sense, Im gonna do some reading up on this when I get a chance and see what I can find out.
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