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Sky in a Galaxy.

  1. Jun 4, 2008 #1
    Lets suppose I live in a planet that is located in the tip of a galaxy that is ubicated at the most distant point from our universe. What would I see if I look up to the sky at night(looking away from the center of my galaxy)?

    Manuel.
     
  2. jcsd
  3. Jun 4, 2008 #2

    russ_watters

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    I'm not sure what you mean by "the most distant point from our universe". However, if you looked away from the center of the galaxy with your naked eye, you'd see nothing. If you used a telescope, you'd see the sky was completely filled with galaxies (a la the Hubble Deep Field).
     
  4. Jun 4, 2008 #3
    Wouldn't that depend on the rotation of the planet? ..or is it that the stars "outside" of the galaxy would be so faint?
     
  5. Jun 5, 2008 #4

    mgb_phys

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    In general there are no stars outside a galaxy, so the only objects you would see are other galaxies. Becuase of the distances involved other galaxies are rather faint. the nearest galaxy to us M31 is pretty close and is only just visible to the naked eye - and that would be the brightest star in your sky.
     
  6. Jun 5, 2008 #5
    The question goes again, what would I see in the sky at night if I happen to live in a planet that is located at the tip of Galaxy (not our milky way), but another Galaxy that happens to be at the most distant possible point away from the center of our Universe? (of course, supposing that I am not looking towards the galaxy I am ubicated, but "away" from it, since I am in the tip of that galaxy). Then question is applied with all kind of telescopes incuding radiotelescopes.
     
  7. Jun 5, 2008 #6
    That makes sense, but what if the planets rotation its self caused the planet to face the center of the galaxy then away. I've attached a quick diagram of what I'm trying to say.

    The universe doesn't have a "center". That is a common misconception.

    I would assume you'd be able to see the CMB and other distant galaxies as Mgb_phys stated. It would also depend on what type of telescope you are using.
     

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  8. Jun 5, 2008 #7

    turbo

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    These authors found that the stellar disk of NGC 300 extends to ~10 radii - much larger than previously thought, so collecting visible light with our current instruments leaves us "blind" to a lot of what is out there. Also, what you see is highly dependent on what wavelengths you observe in. We assume that the universe is homogeneous and isotropic at large scales, meaning that apart from small overdensities such as clusters and chains, galaxies are smoothly distributed, and the night sky from your theoretical planet would look remarkably like the one we see from Earth.

    http://arxiv.org/abs/astro-ph/0602573
     
  9. Jun 5, 2008 #8

    DaveC426913

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    Once I thought was a bad typo but twice is no accident...

    What is an ubicated?



    (thinks hard to himself:

    'ubi'

    semper ubi sub ubi = always wear underwear

    ubi = wear
    ubi = where?

    ubicated = wherecated?
    = located??

    Tests:
    "...a galaxy that is located at the most distant point ..."
    "...not looking towards the galaxy I am located, but "away" from it..."
    Pass!

    Does marssal speak Latin?

    )
     
    Last edited: Jun 5, 2008
  10. Jun 5, 2008 #9

    mgb_phys

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    Then half the time you would have a sky with a faint milky-way (depending on the galaxy's orientation to you) and half the time an empty sky. A bit like having a full or new moon.

    Even on Earth which is about 2/3 of the way to the edge the galaxy is pretty faint unless you are on a dark site so from a planet on the edge of the halo it wouldn't be half the sky full of stars.
     
  11. Jun 5, 2008 #10

    DaveC426913

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    I think this may get at the crux of the OP's question. The OP seems to want to know what we would see if we looked outward from "the edge of the universe". Resolving this misunderstanding will likely resolve the OP's question.
     
  12. Jun 5, 2008 #11

    turbo

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    ubicated is Italian for "located", I think
     
  13. Jun 5, 2008 #12

    DaveC426913

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    So 'tis, so 'tis!
     
  14. Jun 5, 2008 #13

    djeitnstine

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    yes we know, and to the OP we need a few millenia to know if our universe is even finite in size.
     
  15. Jun 5, 2008 #14
    Thanks for the replies. Yes, english is not my native languaje, it is spanish (sorry for the location/ubication confusion). The OP question is as some of you have said, What would I be able to see if I point all kinds of telescopes away from my galaxy that happens to be at "the edge of the universe?". I also can conceive the idea of universe without a center if we do not beleive in the big-bang. But since must of the beleivers "beleive" in the big-bang, then would have to mention "center of the universe" as a possible reference point.

    marrsal.
     
  16. Jun 5, 2008 #15

    russ_watters

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    No, as said above, that is a common misconception about the Big Bang. It didn't happen at one point, expanding outward, it happened everywhere. It is similar to the concept of a center of a spherical surface, but in 3d instead of 2d. The British may have constructed the longitude scale to put themselves at 0 degrees, but that doesn't make them the central point on earth's surface.

    The fact that the Hubble Deep Field and Ultra Deep Field show that the sky is literally filled with galaxies regardless of where we look supports the Big Bang theory, it does not refute it.

    http://hubblesite.org/newscenter/archive/releases/2004/07/
     
  17. Jun 5, 2008 #16
    Are you saying that because there is no center, then there is no "edge", therefore I can never point with an hypothetical telescope from an hypothetical planet looking away from an hypothetical galaxy located (not ubicated) at the "edge" of our universe? The explanation of the 3D spherical surface was not very convincing to apply in to our universe and big-bang theory.

    marrsal.
     
  18. Jun 5, 2008 #17

    russ_watters

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    Convincing or not, that's how it works. It's something you'll just need to accept. Though the evidence that comes with it really should help it make more sense.
     
  19. Jun 5, 2008 #18
    Would love to read something about that EVIDENCE. I am strong beleiver of a science based on sense, than evidence based on non sense. I want to think you have answered my question, but I still feel that I have not had it answered. ( I hope somebody has understood it).

    marrsal.
     
  20. Jun 5, 2008 #19

    russ_watters

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  21. Jun 6, 2008 #20

    DaveC426913

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    Imagine yourself sitting on a giant inflated balloon. The balloon is sealed, it has no nipple, but the sun is warming its insides so it is expanding. Now try to find a spot on the surface of the balloon that you could call "the centre" i.e. where everything is expanding away from you and the "edge of the expansion" is farthest from you in all directions.

    There is no such place. Or more accurately, everywhere is the centre.

    Note that that balloon did start off very tiny, hypothetically at a point, but on the expanded balloon there is no such single point. Note also that, while the balloon has no edge-beyond- which-there-is-no-more-balloon (it is unbounded), the balloon is still finite in size.
     
  22. Jun 6, 2008 #21

    DaveC426913

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    The evidence is all around us. Everywhere we cast our telescopes we see a homogeny of mass density. We do not see a centre-from-which-everything-is-expanding, nor do we see an edge.
     
  23. Jun 6, 2008 #22
    I have mentioned on my previous post that "The explanation of the 3D spherical surface was not very convincing to apply in to our universe and big-bang theory". And I said that, because why/when and who has decided to exclude the "air inside and outside the baloon" to be part of the universe. Even so, if we decide to exclude this "air (in/out of the baloon)", the "matter" that makes the baloon should have a certain "thickness". I perfectly understand the concept of expansion in all directions (if you are an atom embedded in the baloon material), but still it should be possible to live in one of these atoms/galaxies that are located in the "surface" of the baloon/universe and look "up".

    Manuel
     
  24. Jun 6, 2008 #23

    russ_watters

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    You must understand, as said above, that the expanding balloon is a 2D analogy to a 3d phenomena. Most people are incapable of actually imagining the 3d equivalent. But perhaps this will help: the surface of a balloon is 2d, and is curved in a 3rd dimension. Space is 3d, but curved in a 4th (or more....) dimension.
    You are attributing characteristics to the analogy that the analogy does not have. Ie, mathematically, the surface of a sphere has no thickness. It is 2 dimensional.

    Perhaps we should turn this around, though: You think there is an edge. Surely, you must have a reason for this belief. You asked for evidence that there is no edge (and it was provided). Surely there must be some evidence that you've seen that leads you to the conclusion that there is. So....what is it? Be careful, though: We've already provided you with the applicable evidence. The interpretation is simple. Please do not make a claim about the evidence that is factually untrue. Make sure you have looked at the evidence provided. It is extremely frustrating to us when people make such claims and haven't even checked the evidence provided for them.

    For example, if there was an edge, you would think that the HUDF would show galaxies at 1 billion light years, 2 billion light years, 3 billion light years, then none any further, indicating an edge at 3 billion light years distant. But it doesn't. It shows galaxies right up to the limit of its sensitivity.
     
    Last edited: Jun 6, 2008
  25. Jun 6, 2008 #24
    Thanks for the answer. I did look at the evidence provided. I can meditate about it years and years and probably get convinced. However, the hole concept of the evidence given does not fill up my vision. If you say: "matematically the sphere has no thickness", then I have to agree with that. But, I am not talking about mathematics. Then in your second paragraph, it appears you accepted the hypothetical concept of "edge". And said that the HUDF "shows galaxies right up to the limit of its sensitivity" . How sure are you about that? It would make my hands sweat if I type something like that. On regards, of the "evidence that I have seen". I have not seen it on TV or read about it, It is something I have just deeply thougth about. Look up at the sky, and the answer is there. Are you going to say that the "matter" exploded by a supernovas has no outer edge?.

    I know it is very frustrating for a mathematician to read "non-sense" sentences of somebody not dedicated to this fascinating field. But it is more frustating to be somebody who thinks about this matters every day an not being able to discuss them with somebody who senses the need to know.

    Manuel.
     
  26. Jun 6, 2008 #25
    As you're not a native english speaker I think that this might be a mis-interpretation. What he's saying is that for as far as HUDF can detect ANYTHING, it detects everything. It sees no edge except for the edge created by it's own limitations.

    As much as I love trying to reason things out(and I really do), for the last hundred years or so, how the universe works and evolves has been beyond the reach of philosophy.

    This analogy doesn't really correspond to the standard model. In a super nova you're looking at a 3d occurrence in a 3d environment. The universe expanding is usually seen as a 3d occurrence in a 4+d environment. The difference is just like the balloon analogy; that was a 2d occurrence in a 3d enviroment.

    I'll clarify first off: I'm no mathematician, nor do I even hold a degree in any physics related field. I'm just a physics undergrad that enjoys this stuff.

    As such, I'm not frustrated at all. Being able to have this conversation makes me think about it more, which helps me understand it more thoroughly.

    -QQ
     
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