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Are galaxies fairly evenly distributed?

  1. Apr 29, 2010 #1
    Are galaxies fairly evenly distributed regardless of which direction we look out from? Also are the oldest stars and galaxies that we can view also relatively evenly distributed in all directions?

    I am asking because assuming that we are not in the center of the universe, I would imagine that in one direction we would see more stars and galaxies than we would in other directions (especially the opposite direction). Is this thinking correct?
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  3. Apr 29, 2010 #2


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    Galaxies are located in loose associations, locally. Galaxies are located in loose but larger clusters on larger scales. On even larger scales, galaxies are located in knots, filaments, and walls.

    On some scale, galaxies might appear to be evenly distributed, but I don't know that we have gotten to that point in verifiable observational astronomy. It's tempting to say that the Hubble deep field images show us a homogeneous universe everywhere we look, but it's pretty tough to discern structure out of such images. When we get so few photons per object, it's not a trivial task to construct some sort of spectroscopy and assign distances.
  4. Apr 29, 2010 #3


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    Galaxies are not evenly distributed. They are collected into large clusters of galaxies, with long "filaments" and "walls" of galaxies and clusters, and vast voids between them. However, on even larger scales the voids and walls do appear to be roughly evenly distributed, and about the same in every direction.

    There is one thing that is not the same in all directions; there is a "dipole" of the cosmic background radiation. It is redder in one direction of the sky and bluer in the other. This is a consequence of the motion of our own local group of galaxies, moving in a particular direction relative to the general background of the universe. The apparent velocity of our local group of galaxies is quite large, around 600 km/sec. In general, all the various galaxies we see are also moving with "peculiar" motion, meaning simply a movement relative to the overall background of the universe and in particular with respect to the background radiation. But I understand that our own local motions are at the high end of what is normal.

    Bottom line is that our our galaxy and our galactic neighbourhood is not particularly exceptional. On large scales, as far as we can tell, the universe appears to be homogeneous and isomorphic; not having any center or any preferred directions. But it does have structure and things are moving around, so it isn't uniform.

    I really recommend An Atlas of the Universe, which gives an idea of what the universe looks like from our location, at all scales from the immediate stellar neighbourhood of nearby stars out to the scale of the observable universe. Gorgeous pictures, clear diagrams, and easy to navigate. (Added in edit... As turbo-1 suggests, the very largest scales involve a fair amount of guess work.)

    Cheers -- sylas
    Last edited: Apr 29, 2010
  5. Apr 30, 2010 #4


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    All observational evidense to date strongly suggests galaxies and matter are evenly dispersed throughout the universe. WMAP is the best evidence to date.
  6. Apr 30, 2010 #5


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    WMAP shows that the cosmic background radiation is evenly distributed; and this is our earliest view of the gas which then later became stars and galaxies and astronomers.

    WMAP also shows the "ripples", which are seeds of structure, or the lack of even distribution on the scale of galaxies, which has been mentioned previously. So yes, WMAP is good evidence for large scale homogeneity... with the important quibble that it is not an even distribution on the scale of galaxies at all! Galaxies themselves are not evenly distributed at all... they are clustered, and the clusters are clustered. But if you go to larger scales, you do have homogeneity.

    One of the interesting questions in deep space astronomy is to identify the largest scales of inhomogeneity. Give the extreme smoothness of cosmic background radiation, we expect a bound on the size of structures. There's been time for material to clump into the filaments and walls and voids which have been mentioned, but it's limited time. So there would be a problem if structure was found on sufficiently large scales.

    The observations on that point are not all cut and dried! Models for how matter clumps together also have to take into account the effects of dark matter, which also clumps. There have been occasional reports of apparent early structure which is on scales larger than would be expected, but I am not entirely sure of the status of these at present.

    One topic I looked at some years ago was the "Francis Filament", which was at that time considered to be something of a problem by at least some researchers.
    See also an old thread on the subject, [thread=147473]Explaining the structure of Galaxy "Filaments"[/thread].

    Cheers -- sylas
    Last edited: Apr 30, 2010
  7. Apr 30, 2010 #6
    Thanks all for your replies. Knowing that the universe is homogeneous from our perspective, there's something I'm still confused about, and maybe the question put another way may help me understand. Is there an edge to the universe? If we could freeze the universe in its current state and then travel in space in any direction, would we eventually come to a point where there were no more stars in front of us?
  8. Apr 30, 2010 #7
    That's unknown, but if the inflationary model is correct and you could travel Faster Than Light, you should be able to reach the edge of the universe. What that means, is anyone's guess, and it's impossible to exceed c anyway.
  9. Apr 30, 2010 #8


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    There is no "edge" to the universe in conventional cosmology. It may be that there is some kind of edge well beyond the bounds of what we can see (well beyond the observable universe). But we can be pretty darned sure that if we could some how get to the edge of the observable universe it would all continue to look pretty much like what we see from here.

    We can't be so sure that we can continue to that indefinitely.

    The relevance of inflation is that if there was any discontinuity or edge at which things start to look a lot different, then that boundary has been removed far beyond the bounds of the observable universe.

    We are, roughly speaking, in something like the position of a space capsule that has made an emergency landing in the ocean of an unknown world. The horizon marks the limits of what we can see, but the basic similarity of everything we can see in all directions suggests that most likely there there's ocean extending beyond the horizon. We might be on a world covered entirely with ocean, or there might be land (or ice) somewhere beyond the horizon.

    The horizon is analogous to the observable universe, and the ocean covered world is analogous to the simplest cosmological models.

    The analogy is imperfect; in particular there is a reasonable evidential basis for considering that an inflationary epoch applied to the space we inhabit, and that it has stretched out the stuff of the universe well beyond the horizon of the observable universe. So it is a bit more than merely presuming that things continue the same beyond the horizon. But we don't have any good basis for extending that indefinitely, or for confidence that all the universe is like what we see.

    Cheers -- sylas
  10. Apr 30, 2010 #9
    No. There is no edge of universe. Much like surface of Earth does not have any edge, and all points are equal (there is no special point).
  11. Apr 30, 2010 #10
    Of course, as we DO know that recession speeds and a positive cosmological constant completely preclude such a trip in the first place. The only time I've really read about what the hypersurface of a universe would "look" like, is a very speculative article about inter-universal collisions. Did I say speculative? I meant: Vastly speculative.
  12. Apr 30, 2010 #11
    Thanks again for your replies. If there is no edge to the universe, or put another way if there is no place in the universe where the stars and galaxies "end", then I think there can only be two conclusions.

    1) There are an infinite number of stars and galaxies.


    2) The universe loops and eventually stars and galaxies would start to repeat.

    Do you agree that these are the only possibilities? Does current theory favor one of these?
  13. Apr 30, 2010 #12


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    Yes, those are the two possibilities. There's no good direct observational basis for choosing one over the other, as far as I know.
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