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Big Crunch or Big Rip?

  1. Mar 18, 2007 #1
    I remember reading some time ago that in 2001, they discovered that the speed at which the universe was expanding was accelerating. According to current models, that would mean that the universe may expand forever and maybe even rip itself apart. But then, there is still some debate as to whether or not it will continue to accelerate.

    As of right now, this is how the future of the Universe looks:

    1) Galaxies will be so far apart that one day we won't be able to see any other than our own.

    2) Eventually, all stars will die, leaving only brown dwarfs, dead stars, and black holes.

    3) All matter will eventually decay and split into subatomic particles.

    4) Eventually, the only thing left will be black holes, and the temperature of the universe will go to absolute zero, reaching a state of complete disorder in which everything is the same temperature.
     
  2. jcsd
  3. Mar 20, 2007 #2

    Chronos

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    Sounds suspiciously like an initial equation of state problem.
     
  4. Mar 20, 2007 #3

    I don't see how everything will go to absolute zero or be at the same temperature. If dark energy continues to accelerate the universe's expansion, wouldn't there be local regions of the universe where there would always be photons to heat it.

    Many have suggested a big graviational crunch of the universe if dark energy diminishes over time and the universe's expansion is slowed to a such a point that gravity takes over as the primary force in with its negative attracting force. This means the universe would aggregate, eventually, into a giant mass that becomes smaller and smaller and therefore extremely dense. Some would speculate this extremely dense mass would then explode in a new big bang. And so the cycle goes...

    As a theory its an interesting philosophical viewpoint of the universe at large, but dark energy shows no signs of waning its influence on the universe presently, so the big crunch will be difficult for scientists to prove :)
     
  5. Mar 21, 2007 #4
    a la Penrose?
     
  6. Mar 21, 2007 #5
    I am not familiar with terms "ripping apart" in the context of cosmology.
    What kind of ripping apart are you refering to?
     
  7. Mar 22, 2007 #6

    Wallace

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    The 'big rip' occurs in cosmological models in which the scale factor of the universe goes to infinite size at a finite time. To be technical, this occurs if the limit of the equation of state of dark energy as t goes to infinity is less than -1. This is often dubbed 'phantom energy'.

    The way to think about the big rip is like this. Bound structures, like galaxies and clusters of galaxies, are not participating in the general expansion of the universe, as the local gravitational well they are in dominates over the global expansion. As the expansion of the universe accelerates in phantom cosmologies, the influence of the global expansion increases and the size of the local potential well needed to overcome this increases.

    Therefore, at first galaxy clusters become unbound, then galaxies themselves, then solar systems, stars, molecules, atoms, subatomic particles...

    Literally everything in the universe is ripped apart at a finite point in time. For phantom energy models that have not been ruled out to high significance by current data, from memory this point in roughly 30 Billion years from now, though it depends on a lot of different parameter values exactly when that will be.
     
  8. Mar 22, 2007 #7

    marcus

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    Wallace, thanks for an excellent clear statement.
    These days a fair amount of the cosmology literature seems to be about narrowing down the errorbars on w

    My impression is the better they measure, the more likely it looks that w is NOT less than -1. So "phantom energy" scenarios with w < -1 get taken less seriously now AFAICS than they were, say 3 or 4 years ago.

    I will try to fish up some links to recent observational work constraining w.
    If you know of anything especially good offhand, I'd appreciate it.

    As I understand it, the model that cosmologists favor these days, namely flat LambdaCDM, has w exactly equal to -1
    as it would be if the dark energy was a pure Lambda (cosmological constant) term.

    Here is one reference that talks about contraints on w, and cites recent observational work.
    http://arxiv.org/abs/astro-ph/0701584
    Constraints on Dark Energy from Supernovae, Gamma Ray Bursts, Acoustic Oscillations, Nucleosynthesis and Large Scale Structure and the Hubble constant
    Edward L. Wright (UCLA)
    17 pages, 8 figures. Submitted to the ApJ

    Of special interest is figure 7 on page 11.
    It looks like the best fit makes w around -0.9. so one is apt either to assume it is exactly -1 or, if not that, then greater than -1,
    so no big rip in either case.

    "The luminosity distance vs. redshift law is now measured using supernovae and gamma ray bursts, and the angular size distance is measured at the surface of last scattering by the CMB and at z = 0.35 by baryon acoustic oscillations. In this paper this data is fit to models for the equation of state with w = -1, w = const, and w(z) = w_0+w_a(1-a)...A flat LambdaCDM model is consistent with all the data."
     
    Last edited: Mar 22, 2007
  9. Mar 22, 2007 #8

    George Jones

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    When w = p/rho < -1, the possibility of a "big rip" appears. This means that the scale factor of the universe diverges at some finite cosmological time t, i.e., the universe rips itself apart at some finite time in the future.
     
  10. Mar 22, 2007 #9

    Wallace

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    Marcus, you are correct in suggesting that a lot of cosmology today focuses on finding out about w, and yes constraints have tightened in recent years though the next leap in constraints will have to wait for next generation surveys, roughly 5 years away at best.

    There are a lot of theoretical issues with w <-1 dark energy as well, as it violates a string of physical principles, so it's not just observations that disfavor it.

    None the less, phantom energy is not ruled out by the current data to any meaningful significance, though few cosmologists would bet on it turning out to be the winning dark energy candidate!

    The LCDM model is the most favored model given the current data as it fits the data well with relatively few parameters, 6 in total describing the entire universe, pretty neat huh! Extending the LCDM model to 7 parameters by allowing w to be free to find its best fit value gives a best fit value of not quite -1. However, using statistical tests it is possible to judge whether the data is 'good enough' to justify the additional parameter. More parameters give you more freedom to adjust the model to fit the data so you need to be sure that the fit has improved sufficiently to justify including the additional parameter.

    At present, letting w go free dosn't pass this test, the data just isn't good enough to justify it, basically saying we can't yet say for sure what value w has. w=-1 is a good choice from a theoretical point of view and since it is roughly consistent with the data the LCDM model is the current best guess. Don't be surprised if this changes in the future however!
     
  11. Mar 23, 2007 #10
    Very nice post, let me see if I understand it correctly.

    Are you saying that general expansion of the Universe, guided by dark energy's acceleration, will one day rip apart all objects in the Universe, namely, clusters, galaxies, solar systems, and then even matter itself?

    So, the strong nuclear force binding nucleons would be usurped by the expansionary force of DE?

    I am having difficulty imagining this, if I understand it correctly.
     
  12. Mar 24, 2007 #11
    What exactly is supposed to happen via the strong force? I know that when I yank two quarks apart, the energy density between them increases until they 'pop' into two quark pairs. What is the mechanism that stops this in 'Big Rip'?
     
  13. Mar 25, 2007 #12

    Wallace

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    Not quite, remember that a Big Rip will only occur if dark energy has a particular property, namely that its equation of state is less than -1. This is not ruled out by current data, however an equation of state of greater than -1 is preferred by the data. So the best bet is that a Big Rip will not occur however that bet is far from certain. Plus there are the theoretical problems with the existence of energy with an EOS of less than -1.

    In terms of what happens to particles held together by the strong force, remember that we do not have a quantum theory of gravity so it gets a bit murky. What we can say is that the equations of general relativity govern the expansion of the universe, and they predict that in the moment of the Big Rip an infinite Four Force is required to prevent the distance between any two objects becoming infinite.

    Now in general relativity we can describe the other forces on nature phenomenologically as providing four forces. For instance, when gravitational tidal forces rip you apart as you plunge into a black hole, the tidal force must overcome the electrostatic forces holding you together (i.e. the chemical bonds between the atoms in your body). The same occurs in a Big Rip, essentially the Force ripping you apart is a gravitational tidal force that becomes infinite, overcoming any finite force provided by the strong force or any other that is holding to particles together.

    Now there are actually several problems with this. The Big Rip is a singularity, and as such we should be suspicious of the solution. Just like the more well known singularity solutions to general relativity (the Big Bang and Black Holes) we know that general relativity does not give us the complete picture and we need quantum gravity to reveal the full details. The reason for this is that GR deals well with large scale things where gravity dominates and QM effects can be treated in bulk rather than individually (i.e. treat the other forces and just Four Forces between particles) and QM deals with small scale things where gravity plays essentially no part. When gravity is important on a small scale then neither theory is valid! This is true for the three singular solutions mentioned above.

    So when a cosmologist (such as myself) says that in a Big Rip everything is torn apart right down to the smallest particles, be aware of the caveat that we do not have the complete theory to describe this situation.

    I hope that help more than it confused :redface:
     
  14. Mar 28, 2007 #13

    Thank you for a most interesting reply. I understand that the tidal gravity force during the Big Rip is sufficiently large to overcome the 'glue' of the 4 Forces.

    I am having great difficulty digesting one sentence in particular as seen above. What do you mean by an 'infinite Four Force', and what do you define Four Force as? I am thinking of the four forces in nature, but I cannot understand the sentence as such.

    Thanks, I appreciate your nice in depth post Wallace, keep up the good work in solving our Universe.:smile: :smile: :smile:
     
  15. Mar 28, 2007 #14

    Wallace

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    Ahh oops, yes I see how that could confuse easily!

    So, there are four forces in nature, but this is not what I was referring to at all. In relativity you have 4 dimensions and hence 4 dimensional versions of well known 3 dimensional quantities. For instance there is 4-veloctity, 4-acceleration and also the 4-force. So this is just a force (any force) described 4 dimensionally.

    I hope that clears it up!
     
  16. Mar 28, 2007 #15

    George Jones

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    Later, this was

    Sorry, Wallace; I somehow missed your post.
     
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