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Expanding universe.

  1. Apr 4, 2005 #1
    Before asking a qusetion let me put first the statement of big bang cosmolgy:
    "These are not galaxies which are going away from us in a fixed space, this is the space between galaxies which is opening up or stretching."

    Now my question is: In both the cases (either space stretch or galaxies move) what we see is the motion of galaxies, I want to ask that how to prove that this is the space which is stretching ?

    One answer could be that since we can see superluminal galaxies which is not allowed by special theory of relativity. So in order to expalin this we need the expansion of the space, because there is no upper limit on the rate of expansion of the space. Please let me know
    what are other arguments.

    Last edited: Apr 4, 2005
  2. jcsd
  3. Apr 4, 2005 #2
    Here's a thought experiment for you. If the size of the Universe is unchanging, why aren't there as many galazies moving towards us as away from us? It's not physically possible for every galaxy to be moving away from us unless 1) We're special -or- 2) The Universe itself is expanding. This doesn't *prove* the Universe is expanding, but it's one of the thought processes that led to the realization our Universe is expanding.
  4. Apr 4, 2005 #3
    What about the possibility that gravity becomes repulsive
    at large distances in that case (by the way this assumption has been used to solve the dark energy problem also) because there will be more galaxies at large distances so obviously there will be more galaxies going away from us.
  5. Apr 4, 2005 #4
    You're thinking in terms of expansion due to a force of one galaxy on another, and that the speed of recession is speed through fixed background of space. But that would make galaxies be moving faster than light through space. Try thinking as though galaxies are still with respect to their local space environment and that space itself is expanding.
  6. Apr 4, 2005 #5
    This is what I have also said. I am looking for some
    other arguments which can prove that it is actaully the
    space which is expanding.
  7. Apr 5, 2005 #6
    I don't think you'll find any. I see the necessity of 'stretching space' as coming from a certain choice of coordinates within general relativity, rather than being a physical effect. See http://www.chronon.org/Articles/stretchyspace.html
  8. Apr 5, 2005 #7


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    As Chronon said, you're not going to find any - at least none that are logical. Einstein believed in a static universe and he invented the cosmological constant to keep this steady-state universe stable. Edwin Hubble demonstrated that light from distant objects was redshifted in amounts roughly proportional to their distances. Hubble also believed that we live in a steady-state universe. Others looked at the redshift and said it was evidence of a Doppler-like effect, and that proved that the universe is expanding. There were some problems with this idea: Causally-disconnected parts of the universe (think of distant things on opposite sides of the sky) seemed to be very similar, with similar distributions and energies. To explain this, cosmologists invented inflation, in which every part of the universe was once in causal contact and then flew apart much faster than the speed of light. Another problem cropped up. If you interpret redshifts as recession velocities and extrapolate back to where everything must have been together (the Big Bang we all know and love :yuck: ) the universe is much too young to contain the very old objects we see around us. Cosmologists "solved" this quandry by inventing a cosmological acceleration, explaining that even though redshifts prove that the universe is expanding too fast, the Big Bang is still safe, because the universe was expanding much more slowly in the past, so there is plenty of time to accomodate the very old things we see. There is no explanation for where the energy to fuel this acceleration is coming from.

    Speaking of very old things: Right now, we know of 3 (perhaps more) quasars residing at redshift z~6.5. If redshift is strictly a function of cosmological recession, these powerhouses must have masses of several billion suns, and they must reside in galaxies massing around a trillion suns each. They also have metal contents equal to or higher than our own sun AND they existed this way over 13 billion years ago, when the BB universe was only a few hundred million years old. None of this seems to bother BB cosmologists, although it should bother anybody who takes a few minutes to think about it. Some really big questions are still out there to be addressed.
    Last edited: Apr 5, 2005
  9. Apr 5, 2005 #8
    Dear frinends,
    this all discussion is going fine. Let me mention that
    why I am worrying about this question.

    "Actually I have been thinking for last few years that is
    is there any meaning of space and time other than the
    space is what matter needs for existance and time is
    what we can say has passed after comparing initial
    and final appearence of a physical system."

    I think in this context you can distinguish the expasnion
    of space from the recession of galaxies only when you can
    define space independent from physical matter. This argument goes with time also. You can read about it here

  10. Apr 5, 2005 #9
    if these quasars are so massive, then wouldn't there be an addition redshift due to the great gravitational well that photons have to climb out of? Wouldn't this redshift be in addition to the redshift of recession?
  11. Apr 5, 2005 #10


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    Arp, the Burbidges, et al argue that quasars are black holes that are ejected from galaxies, and that they evolve with time. How can quasars possibly evolve? If you model a BH that is almost naked being shot out into the interstellar medium via radiation recoil or a multi-body slingshot effect, it initially will have an accretion zone that is very tiny, and light generated by matter infall will have to come to us from very close to its event horizon. As you know, at or below the event horizon, the light cannot escape and it has infinite redshift, and light emitted from close to the EH will be highly redshifted, proportional to the proximity to the EH. This is what a quasar looks like to us. As the black hole accretes matter from the IGM, its accretion zone grows larger, and the visible light that we see will be less and less redshifted. Eventually, the BH can accrete enough material to trigger star formation and then it will appear to us to be an AGN. If this model is correct, high-redshift quasars have intrinsic redshift that is in addition to the cosmological Hubble effect. This way, they do not have to be the most massive luminous monsters in the universe, and they won't pose the problems for standard cosmology that they currently do.
  12. Apr 5, 2005 #11
    What is IGM and AGN? Thanks.

  13. Apr 5, 2005 #12


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    Intergalactic medium (gas outside of clusters and galaxies) and active galactic nuclei (including quasars and Seyfert galaxies).
  14. Apr 5, 2005 #13


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    Time is a puzzle, and trying to divorce our definition of time from physical processes is not productive, as you noted, since that is how we experience and define time. We've also got to realize that that when we define space-time as a single construct and accept Einstein's idea that the speed of light in a vacuum is constant, we've locked time to a rigid, purely theoretical, speed of light. That can cause problems, because quantum theory tells us that "empty" vacuum is teeming with virtual particle pairs even at zero degrees Kelvin. This is the zero point energy (ZPE) field and light has to traverse this field as it crosses "empty" space. Every single field that we know can be polarized and can be densified or rarified, and this field is no different. Light has a wide range of speeds, depending on the refractive index of the material through which it is travelling, and we know from quantum physics that there is no such thing as a true vacuum in outer space. Where does that leave us? If this is true, light travels at different speeds as it travels through areas of space/time that are at different densities and/or polarities. Einstein never managed to reconcile himself with quantum physics, which is a great shame, because he had the brain-power to deal with these problems, and it probably prevented him from acheiving his ultimate goal - the unification of gravity with the other fundamental forces.
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