Is the Observable Universe getting smaller?

Discussion in 'Pseudoscience Archive' started by Prof.Layman, Sep 28, 2012.

  1. Prof.Layman totally internally reflected Registered Senior Member

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    I was thinking about some descriptions or explanations popularized by physicist on television that explain why light cannot escape a black hole and some concepts involving the observable universe, and I came to a conclusion that the observable universe must be seen to be getting smaller.

    For starters, it has been said that the reason why light cannot escape a black hole is because the space itself around a black hole is traveling faster than the speed of light. So then the edge of the visable universe has galaxies that are traveling close to the speed of light. Then they do this because the space itself is expanding away from us close to the speed of light, and it has been said that this is okay because it is the space itself that is traveling faster than light. So then I assume that the reason why we cannot see galaxies past the edge of the visable universe is because the space itself is expanding away from us faster than the speed of light. So then what happens when these galaxies traveling away from us close to the speed of light end up traveling away from us faster than the speed of light with the expansion of space? They would no longer be able to be seen in our visable universe. Then since the rate of expansion is increasing, then the distance away from us where galaxies are traveling faster than the speed of light will become closer and closer to us. So then it seems like it would follow natuarally that the observable universe is getting smaller since the distance a galaxy would travel faster than the speed of light would become increasingly closer to us over time.

    It seems like a strange concept, but over the past few decades the age of the universe hasn't seemed to get any older. Could this cause our preception of the age of the universe to always appear to be getting younger?
     
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  3. origin Heading towards oblivion Valued Senior Member

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    Yes, the observable universe is getting smaller. All of the gravitaionally bound galaxies that are associated with the milky way will one day be all that can be seen and the rest of the universe will be dark.
     
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  5. hardalee Registered Senior Member

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    No, same radius less stuff.
     
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  7. Boris2 Valued Senior Member

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    increasing radius, less stuff. surely?
     
  8. Neverfly Banned Banned

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    It is said that light cannot escape a Black Hole because spacetime around the singularity is so warped that all straight lines lead back inside.
    Imagine a road that has a properly banked curve. You will drive through the curve at speed without having to turn the wheel- keep it straight- if properly banked.
    The banking of the curves around a black hole are so sharp that even in a straight line, light cannot leave.

    Which puts the rest of your speculations up for grabs.

    Please Register or Log in to view the hidden image!

     
  9. Prof.Layman totally internally reflected Registered Senior Member

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    But, could the radius at which you could have seen galaxies earlier in the universes history been larger at one time? The rate of expansion is exponential with distance, and the current rate of expansion of the outermost edges of the observable universe is close to the speed of light. Over time, distances closer to us from that edge will increase to the speed of light, since the rate of expansion is increasing per given distance. Then wouldn't we start to see less stuff from increasingly shorter distances? Or is the rate of expansion per given distance away from us constant?
     
  10. Prof.Layman totally internally reflected Registered Senior Member

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    I have also heard of this explanation, but it doesn't explain why we can't see outside of the observable universe. Could it just be coincidence that the edge of the observable universe is traveling close to the speed of light and that is the distance that we can see? Wouldn't a photon traveling from a distance further away from us than the observable universe in effect flat line from the amount of redshift? I think this explanation alone would leave why we can't ever see farther than the observable universe up for grabs, until somehow we could see an galaxy travel away from us faster than the speed of light. I don't think that could ever be the case. We would have to be really lucky to be living at the time of the age of the universe when it was just before the age that it would start sending galaxies away from each other close to the speed of light relative to us. Galaxies on opposite ends of the observable universe would already be traveling faster than the speed of light away from each other, so I don't think they would be able to observe each other since we wouldn't be able to observe a galaxy at that distance. Or it could imply that we are at the true center of the universe, lucky us.
     
  11. arfa brane call me arf Valued Senior Member

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    There seems to be a few misconceptions in this thread.
    To be part of the observable universe, distant objects need to be 'sending' light outwards, some of which reaches our location.
    If you assume there are distant objects whose light hasn't reached us, but will in the future, then we will see more than we can see now in that future.
    Also assume that the recessional velocities of those yet-to-be-observed objects is less than the speed of light (one of the postulates of relativity), so that the objects are not yet beyond a visible horizon. When the light from the most distant objects reaches us, then they may well be already beyond this horizon.

    So the observable (matter in the) universe is increasing, not decreasing. But eventually, universal expansion will reduce the number of visible objects, it's a sort of contest between recessional velocities and the speed of light.
     
  12. Prof.Layman totally internally reflected Registered Senior Member

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    A lot of statements in more recent television programs seems to suggest that this postulate doesn't apply to galaxies or the expansion of the universe, and galaxies in the universe can expand away from each other faster than the speed of light because it is the space that is expanding faster than light. I don't think that we could ever observe an object that is already traveling faster than the speed of light away from us. Addition of velocities could never give an accurate description of the even rate of expansion across the observable universe. Each galaxies velocity would be dependant on the other galaxy that is doing the measuring. For example, if you added the velocity of the galaxy to the "left" of the milky way and added that to the velocity of the galaxy to the "right" of the milky way that was on opposite ends of the observable universe, you would end up finding that one of the two galaxies wasn't traveling very much faster away from the other than we are. But, it is still traveling close to the speed of light away from us relative to where we are, and not traveling close to the same speed we are next to us...
     
  13. Neverfly Banned Banned

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    It's not supposed to. The two topics are not solved by the same solution, now are they? You cannot see outside of the Observable Universe because light from that spacetime has not reached us. That is all there is to it. It's that far away (Due to expansion)
    This isn't accurate. Spacetime there is not moving even close to the speed of light. You're referring to the observed cumulative effect we see at this distance due to the great distance between us and that point. It accumulates to resemble that speed from our perspective but if we went (by magic) to that point, spacetime would be moving at a regular rate as it does in our neighborhood.

    The rest of your post appears to be based on the misconceptions above which once corrected, should solve any issues you had with those speculations.

    Forget Discovery Channel, ok? Not the most reliable source... Spacetime is NOT traveling anywhere in the Universe at the speed of light.
    The expansion of the Universe is only occurring in vast expanses of empty space where it cannot be overwhelmed by gravitational forces.
    As a cumulative effect, it makes that trailing edge at the furthest point we can see appear to move at a speed that the space at that point is not moving at.
     
  14. hardalee Registered Senior Member

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    As space expands, the expansion exceeds the speed of light. When this happens, the light from the areas that have exceeded the speed of light due to this expansion will never reach us. Those areas will be lost to our observation forever though they will still exist. Eventually, only our local group of galaxies that are bound together by gravity will be visible. We can still see the edge or close to it, but the other galaxies will be lost to us and us to them.

    This assumes our local group will exist at that time, which is whole other question.
     
  15. origin Heading towards oblivion Valued Senior Member

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    Yep, the local group will probably be some sort of super galaxy in the far distant future.
     
  16. Prof.Layman totally internally reflected Registered Senior Member

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    So are you claiming that the speed of galaxies at the edge of the visable universe is not close to the speed of light? Or are you claiming that the speed of those galaxies is not determined by the expansion of space itself? I don't think we would have to travel by magic to a point on the edge of the visable universe in order to know what it is like there. I think it would look the same as if we assumed we where right here now, since there is no absolute frame of reference. At the current rate of expansion we could one day leave their light cone, and the galaxies further away from it than we are would never enter their light cone. And the galaxies closer to it than we are could leave their light cone still. This is because if a galaxy is allowed to travel FTL relative to another galaxy it would end up traveling a greater distance than the current light cone. So then wouldn't the light cone become smaller if the distance away from us that had objects traveling FTL became closer to us?

    I think the only conception that leads to this is that we can't observe light being emmitted by an object that is traveling FTL with the expansion of space. If we assume that the observable universe is its given size only because of the age of the universe, then it leaves open the question of if another galaxy traveling FTL can be observed. That we couldn't verify for some time, even though the galaxies on the edge of the visable universe are traveling close to the speed of light. But, if another galaxy traveling FTL cannot be observed than the visable universe could have looked much the same way it does now for some time, since the galaxies close to the edge of the visable universe are traveling close to the speed of light already. So it raises the question of if the observation of these galaxies will start dropping off the visable spectrum at closer and closer distances over time, and if so how long would it have been doing this?
     
  17. origin Heading towards oblivion Valued Senior Member

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    I went back and read what I wrote and I misspoke, in that I agreed that the observable universe is getting smaller. I like hardalee's simple statement "same radius less stuff". That is neglecting the acceleration of the expansion, if the expansion increases significantly then the distance at which the recession velocity exceeds the speed of light will dicrease the radius of the observable universe.

    Here is a useful guide to some of the points raised here.
     
  18. Neverfly Banned Banned

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    Yes, that is what I'm claiming. Galaxies (matter/mass) are not moving FTL at the edge of the observable Universe. Spacetime is moving faster than light. It's a relative measure. If you magically popped into that galaxy at the edge and looked at the Milky Way, the Milky way would appear to be receeding FTL from you. It's a relative observation.

    I am absolutely making that claim.

    It is a cumulative effect. This cumulative effect is of spacetime expanding between galaxies, between galactic clusters. Not within galaxies or clusters of them. Expansion occurs where it is not overwhelmed by gravity. These vast expanses of empty space, none are moving FTL. Only when you cover extreme distances does it add up to superluminal speeds. But no particular point is moving superluminally relative to any point near it.
     
  19. Prof.Layman totally internally reflected Registered Senior Member

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    If you where in a galaxy close to the edge of the visable universe, the Milky Way would appear to be traveling close to the speed of light. Then the galaxies on the other side of the Milky Way would appear to be traveling FTL. But, then you wouldn't be able to see those galaxies unless the radius of the visable universe was larger than it is from inside of the Milky Way. So if the visable universe was the same size from the view of both galaxies than you wouldn't be able to see a galaxy that was traveling FTL. There is no reason why an observer should view the visable universe as being a different size from these different galaxies. So in order for the visabele universe to be the same size from every galaxy, the limit to how far you could see would be the distance other galaxies where traveling FTL.
     
  20. Neverfly Banned Banned

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    Prof.Layman, I apologize. I blame the example of a Black Hole and it's comparison, but either way, I misunderstood you and jumped the gun.
    Given what you said here, you seem to fully understand it's a cumulative relative observation. That said, I (unqualified as I am) can find no fault in your proposal.
     
  21. Sarkus Hippomonstrosesquippedalo phobe Valued Senior Member

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    Keep in mind that galaxies do not travel FTL - they only appear to do so because of the expansion of the space between them.
    If space expands then even static objects will appear to move further apart without actually moving.
    And if space expands uniformly then more distant objects will appear to move away faster than closer objects.


    As for the size of the observable universe... my understanding is that it's getting larger, but with fewer observable objects in it.
    The current size of the observable universe is thought to be a radius of 47 billion LY or so.
    What this means is that objects that are currently 47 billion LY away are now observable to us. The light that we observe from them was emitted from those objects when they were probably no more than a few 10s or 100s of millions of LY away from us, but that light has taken almost 14 billion years to reach us due to the expansion of space.
    So the scenario at the time the universe became observable (i.e. when the universe became opaque, some 400k years after the BB) the object was, say, 100m LY away. It emitted light. That light headed toward us but rather than taking 100m years it took almost 14 billion years because the space was being expanded. The light has just reached us, but the object that emitted it is now c.47 billion LY away (due to the continuing expansion of space).

    At least that is my understanding of it.

    The observable universe therefore becomes larger as we go on, but there is a limit to its size... i.e. some objects that were so far away from us at the time the universe became opaque will never become observable, because our understanding of the expansion of space suggests that light emitted at that time will never reach us. The increase in the size of the observable universe is slowing down, and will taper toward static after an infinite time.
    The objects in the universe will actually get fewer and fewer... if the apparent expansion of the universe is as currently understood.
    The objects that we are only just seeing now, that were 100LY away when they emitted the light and are now 47 billion LY away... the light they are now emitting is unlikely (I say unlikely as I have not done the math, but understand this to be the case) to ever reach us, as those objects are now appearing to move away faster than the light travels toward us.

    So I hope this helps answer the OP, as far as I understand things.
     
  22. Prof.Layman totally internally reflected Registered Senior Member

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    Then maybe it is a matter of ones definition of observable universe. Maybe it is not the observable universe that is getting smaller but just the universes event horizon. Not the predicted destination of galaxies we can see now, but just the distance of the locations of galaxies how we see them in the state they where in when they emmitted that light to us.
     
  23. Farsight

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