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Question for the enlightened

  1. Apr 8, 2008 #1
    I am just a curious observer of the cosmology debate, but I have a couple of questions on my mind: 1) Are f.i. galaxies actually expanding as a result of an expanding space? 2) Does the expansion of space have any implications? I am thinking since gravity is proportionally detriment to distance, could expansion not (if expansion is occurring inside galaxies as well) lead to the disintegration of galaxies?

    Best regards, Torbjoern, Norway.
  2. jcsd
  3. Apr 9, 2008 #2
    Hi Tbeer,
    1) No. Gravity keeps galaxies from being disintegrated by Hubble expansion.

    2) Eventually, when the acceleration of Dark Energy exceeds the acceleration of local gravity, galaxies indeed will disintigrate. If they still exist, because they probably will have collapsed into black holes before that time. Black holes eventually will dissipate through Hawking radiation.

  4. Apr 9, 2008 #3


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    Dark energy will not rip apart bound objects since the energy density of DE does not increase with time (unless it violates the weak energy condition, such a DE is dubbed 'phantom energy' and is thought to not be a good possibility). Bound objects are not expanding by definition and therefore will never enclose more DE than they do now and hence will remain bound.

    Likewise galaxies will not eventually collapse into Black Holes. Again, a stable bound object is just that, stable. Just as the Earth will not fall into the Sun given sufficient time galaxies will also not collapse into their central black holes even given infinite time.

    To answer the OP, the term 'expansion of space' does not refer to a causative force but merely is a convenient description of an expanding universe. In other words, 'space expands' simply because the Universe does and therefore everything in it is further apart but not the other way around, i.e. the expansion of the Universe isn't driven by some exotic property of space that drives things apart. If there is non-zero vacuum energy then this accelerates the rate of expansion, but this is not a requirement for the Universe to expand. The Universe expands because it did so in the past.

    Bound objects have ceased expanding and are stable objects. Hence at any moment you couldn't say they were expanding in the past and hence there is no reason for them to do so in the future.
    Last edited: Apr 9, 2008
  5. Apr 9, 2008 #4
    Hi Wallace,

    I agree that objects which are currently gravitationally bound will never be "torn apart" by DE. My mistake, and good explanation on your part.

    On the other hand, my understanding is that any spherically asymmetrical system (such as the collection of individual stars orbiting around the center of a galaxy) emit gravitational waves, and such emission causes them to lose angular momentum. Causing their orbits to converge towards the center over time, presumably eventually collapsing into a single black hole.

    I analogize this (hopefully correctly) to binary stars whose orbit with respect to each other has been definitely observed to collapse due to emission of gravitational waves. Do you agree?

    Last edited: Apr 9, 2008
  6. Apr 9, 2008 #5


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    The gravitational wave emission from the rotation of a galaxy is absolutely miniscule, so small I doubt anyone has ever bother to try and quantify it. The energy density is just so small and the mass distribution close enough to spherical (since dark matter haloes of galaxies and spherical) that this is unlikely to cause a galaxy to collapse even after many bazillions of bazillions of years.
  7. Apr 9, 2008 #6
    Hi Wallace,

    A "bazillion" years is a long time. Even if the gravitational wave energy emission is miniscule, eventually it must cause a galaxy to collapse. I agree that the time period is extremely long, but certainly not infinite. Galaxies must eventually collapse into black holes, unless something else happens to them first.

  8. Apr 9, 2008 #7
    More on the "Fate of the Universe" from Wikipedia:

    Planets fall or are flung from orbits: [tex]10^{15}[/tex] years
    Over time, the orbits of planets decay due to gravitational radiation or the planets are ejected from their local systems by gravitational perturbations.

    Stars fall or are flung from orbits: [tex]10^{16}[/tex] years
    The same scattering effect happens to stars and their remnants within galaxies, leaving mostly scattered stellar debris and supermassive black holes.

    The mighty supermassive black holes are all that remains of galaxies once all protons decay, but even these giants are not immortal.

    An estimated half of protons decay: [tex]10^{36}[/tex] years
    If estimates of the half-life of protons are correct, then one-half of all the free-floating matter in the Universe has been converted into gamma radiation and leptons through proton decay.

    All protons decay: [tex]10^{40}[/tex] years
    If estimates on the half-life of protons ([tex]10^{36}[/tex] years) are correct, then protons (and nucleonic neutrons as well) will undergo roughly 10,000 half-lives by the time the universe is [tex]10^{40}[/tex] years-old. To put this into perspective, there are an estimated [tex]10^{80}[/tex] protons currently in the Universe. This means the proton's numbers will be slashed in half 10,000 times by the time it is [tex]10^{40}[/tex] years-old. Hence, there will be roughly [tex]0.5^{10,000 }[/tex] as many protons remaining as there are today; that is, zero protons remaining in the Universe at the end of the Degenerate Age. Effectively, all matter would be contained within black holes, which are immune to proton decay, and leptons.

    The Black Hole Age - from [tex]10^{40}[/tex] years to [tex]10^{100}[/tex] years

    Black holes dominate: [tex]10^{40}[/tex] years
    Black holes continue to evaporate via Hawking radiation, but this process is very slow.

    Black holes disintegrate: [tex]10^{100}[/tex] years
    Few—if any—black holes remain; virtually all matter is now converted into photons.

    Last edited: Apr 9, 2008
  9. Apr 10, 2008 #8
    I wonder why it is that ALL the pop-sci shows on this subject say the opposite: that eventually the big "rip" will tear apart not only galaxies, but every atom in the universe. Doesn't this assertion directly contradict all observations...that galaxies tend to cluster and anything bound in a gravitaional cluster exhibits no expansion?

    Cosmology can be so confusing in this way, as you can get a completely different scenario depending on the cosmologist that is giving it....

    Yeah I know, you all keep saying that "pop-sci" is full of misconceptions, but this IS the "voice" of the cosmology community to the general public.
  10. Apr 10, 2008 #9
    I do not see any problems with cosmologists holding different scientific opinions. But I think it is a problem when cosmologists presents these opinions as a cut and dried theory that is beyond any question.

    Bound objects? Well the FRW metric does not describe bound objects. I would not know how cosmologists can make definitive statements about those objects. The FRW metric describes a universe with only comoving objects. You cannot simply add bound objects to that model. First obvious issue would be additional gravitational radiation.
    Last edited: Apr 10, 2008
  11. Apr 10, 2008 #10


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    The 'Big Rip' will happen if and only if there is 'phantom' dark energy. This model for dark energy is still not ruled out by observational data, but is not favored over more reasonable models either. Given the choice between 'the whole Universe will tear itself apart in an gigantic space kabloomie!' or 'the Universe will slowly fizzle out in a slow drawn out process' which do you think pop-sci doco's are going to cover?
  12. Apr 11, 2008 #11
    I hear it all the time: "every atom in the universe will eventually be ripped apart" sounds like a statement of fact to me. It's mainly Neal Degrasse Tyson (or something like that) who is on almost every pop-sci show spouting this crap (along with others).

    By "bound", I was referring to gravitational clustering (and picking up the term from what Wallace posted). There is no expansion within these areas...quite the opposite. How would this matter ever be pulled apart by dark energy in the future when it is much more dense, if it is not being done so now?

    Unless they are saying that eventually these areas will condense to a SMBH and dissipate through Hawking radiation? But that's not the same as atoms being ripped apart by DE....
    I dunno, I hope everyone gets on the same page at some point in my lifetime and starts making sense....because this "Big Rip" scenerio doesn't make a "rip" of sense. :)
  13. Apr 13, 2008 #12


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    Really, when asking what the ultimate fate of the galaxies is, I think we agree they are gravitationally bound and thus will not experience expansion due to DE. Then it simply becomes a question of long-term galaxy dynamics, which is an area in which we don't have much data... After all, they're all still around.
  14. Apr 14, 2008 #13
    You seem to exclude the possibility that while expansion factor is much lower than the gravitational attraction it still is a factor.

    Feel free to demonstrate, by for instance by using the FRW or more complex models, that expansion is only a factor in empty space. Are you perhaps saying that the cosmological constant is selective and only applies for empty space?
  15. Apr 14, 2008 #14


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    Please re-read post #3 which explains why bound systems in a FRW universe with a cosmological constant do not expand.
  16. Apr 14, 2008 #15
    I do not see anything explained in that posting.

    An FRW metric models only co-moving objects. One cannot simply take the freedom to stitch in other things.
  17. Apr 14, 2008 #16


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    Of course you can, as long as the other things are perturbations that are relatively small in amplitude. Using the Newtonianly perturbed FRW metric (assuming the weak field limit) bound structures can be very well described. This is what cosmological N-body simulations do. Given the size of the perturbations seen in the Universe, there seems no reason that this would not be a good approximation.

    Of course there exists many papers that claim this perturbative process fails, giving rise to 'apparent' acceleration etc however there are equally many papers describing in detail why the standard approach works in the regime that it is intended to be applied. The current consensus is certainly strongly in favour of perturbation theory working fine.

    If you don't want to use perturbative solutions to problems, your going to have to throw away a hell of a lot more of science that just the entire field of modern cosmology.
  18. Apr 14, 2008 #17

    Please feel free to demonstrate how the "much lower" factor of expansion would ever overcome the much stronger gravitational attraction of clustered galaxies and rip everything apart.
  19. Apr 14, 2008 #18


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    In the same sense that this penny I'm tugging on with both hands, while undergoing a force trying to tear it apart, does not do so. Its interatomic bonds easily overwhelm my puny strength. Another example is that of the space shuttle, which is not pulled apart by the Earth's gravity while in orbit. It's structure is much stronger.

    Nor does it mean that "eventually" the penny or space shuttle will get pulled apart. One force simply overwhelms the other. Period.

    A galaxy is held together by gravity. The expansion of space is overwhelmed. The galaxy (and anything else that is bound with a like or a greater force) does not expand. Period.

    The only thing weak enough to have expansion come into play is the extremely weak gravitational force between galaxies.
    Last edited: Apr 14, 2008
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