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FAQ-Is universe black hole: clarification request

  1. Mar 11, 2014 #1
    The FAQ states:
    "In our universe, we observe that space is not a vacuum, and tidal forces are nearly zero on cosmological distance scales (because the universe is homogeneous on these scales). "

    We observe Dark Energy. How do we know this accelerated recession of distant galaxies, which is related to distance to those galaxies, is NOT due to some kind of non-zero tidal forces?
    Does dark energy behave differently from the way we would expect tidal forces to behave? (At least in terms of how much the distance between two objects effects the rate at which they separate: is the relationship exponential, linear, etc..)

    I'm not clear on why the homogeneousness of the universe implies there are no tidal-forces. I could see why it would imply there are no non-homogeneous tidal forces, but couldn't it still have a homogenous (or nearly so) tidal force throughout the entire universe?

    I guess this is more of a LANGUAGE question. Regarding the homogeneousness of the universe: haven't we observed that the universe was denser in the past? Why doesn't this imply it is actually non-homogeneous? (Is it simply because there was a smooth, even transition between these two states everywhere?)
     
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  3. Mar 11, 2014 #2

    mfb

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    Homogeneousness is meant in space, not in time. The cosmological principle also includes isotropy - all (space) directions are the same. Tidal forces would violate this, they would give a preferred direction.
     
  4. Mar 11, 2014 #3
    OK, I suspect this follow-up question is inane, but I just don’t know enough general relativity to know...

    I want to argue that the past and future are in fact the "preferred directions" along which the tidal force could act, why would this be incorrect? Doesn't general relativity say the effect of gravity exists in space-time, not just space? So, is it theoretically impossible for a tidal force to be felt in the time coordinate only? (Keep in mind the original question in the FAQ: is the universe is a black hole. Isn't the radial dimension of a black hole, along which a tidal force is felt, very closely tied to the time dimension, within the event horizon?)
     
    Last edited: Mar 11, 2014
  5. Mar 12, 2014 #4

    mfb

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    This is meaningless without a proper definition of the terms. And then you won't get anything new.

    Measurements show the expansion of the universe is accelerating - this does not fit to black holes. In addition, black holes are objects in space, unlike a universe.
     
  6. Mar 12, 2014 #5
    Is it theoretically impossible for a tidal force to be felt in the time coordinate only?
    If the answer is yes, it is theorectically impossible: Please explain, and just ignore the rest of this post.

    >>This is meaningless without a proper definition of the terms. And then you won't get anything new.
    Let me rephrase: Why is the time coordinate NOT considered a valid pair of directions, in the proper definition of "preferred direction"?

    >>Measurements show the expansion of the universe is accelerating - this does not fit to black holes.
    Forgetting black holes: Does this measured accelerated expansion rate match an accelerated expansion rate due to tidal forces, acting along the time coordiante?

    >>In addition, black holes are objects in space, unlike a universe.
    So what does this imply, regarding tidal forces in the time dimension?
     
  7. Mar 12, 2014 #6

    mfb

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    Please define in a clear (mathematical?) way what "a tidal force in the time coordinate" means.

    Time has a special role anyway, it would be surprising if some statement is valid in both time and space directions.

    See the first part.
     
  8. Mar 12, 2014 #7
    >>Time has a special role anyway
    And I'm trying to treat it like just another dimension. Good point, that is probably what gave rise to (and invalidates) the notion below.

    >>Please define in a clear (mathematical?) way what "a tidal force in the time coordinate" means.

    Ok, I'll try, but keep in mind I'm just a monkey trying to mimic the right terminology here: In a model that has "a tidal force in the time coordinate", I'm saying space-time curves in the time dimension only. And that the rate of this curvature increases along the time-coordinate.

    Sample: We have two objects in this space-time model, which we will place at events with different time coordinates. The object with a larger time coordinate will be subject to a greater curvature of the time dimension, and (here is where I'm SURE I'm using the wrong terminology) "fall" into the future at a different rate than the object with the lower time coordinate. Since this is only relative, the object won’t be able to tell how "fast" it's moving through time, but it can see the other event and (via redshift change) detect it receding further away in time, at an accelerating rate (due to the tidal force effect).

    Hmmm, Reading this through, I'm afraid I imply yet another dimension for time to "curve into". And this dimension would not even be detectable to one of those objects. Bah, I didn't want to go there.
     
  9. Mar 12, 2014 #8
    Your right in what you wrote doesn't make sense. First off time is a rate of change of events. It is not a force. A force requires a carrier particle. Which is performed by bosons. Each force has a different boson. In order for time to be a force is to also have a corresponding boson. The only boson we have yet to detect is the graviton. This is due to the energy levels required to detect such. Our current technology comes no where close to the predicted energy levels. Each force also has a range of influence and relative strength that decreases with the range of influence. Hence the furthest range of influence force which is gravity is also the weakest. Where the strong nuclear short range of influence is the strongest. Time however is influenced by gravity locally GR has proven this influence.

    I would recommend studying GR, SR and cosmology before trying to develop your own theories. After all in order to change the rules your must first understand the reasoning behind the current rules and understanding
     
  10. Mar 14, 2014 #9

    mfb

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    Well, that does not work.
    Using the same set of words as an actual theory does not make a statement meaningful.

    That is not true.
    • Both the electromagnetic force and gravity have an infinite range, but the electromagnetic force is stronger by >30 orders of magnitude.
      We just don't note the electromagnetic force on large scales because most objects are very close to neutral.
    • The range of the weak force is shorter than the range of the strong force, but the latter is stronger. Actually, the range of the strong force is not strictly limited in range - just confinement makes sure that we do not have isolated charges hanging around so long-range forces don't happen.

    I agree.
     
  11. Mar 20, 2014 #10
    Sorry I was unclear, the only theory I am attempting to understand here is GR, i'm not trying create a theory.  It was my intention to ask about a model/test universe and see what the GR results for it were. (I thought this is is how we research and understand GR and it's implications.)

    I'm afraid I'm still unclear why the model I tried to define is wrong.  What part is "meaningless"?  Every single sentence?

    Regarding learning GR:  I Wish!  Despite my fascination with the subject,  I'm just not smart enough for that math. I'm kind of surprised you thought I might be, MOST humans are not.  So, I am relegated to posting on a board like this, and hoping someone wants to help me learn what I can. SR, I DO know, though admittidly poorly.
     
  12. Mar 20, 2014 #11

    mfb

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    This is indeed a method for research. But you have to learn the theory you want to research/test first. And this won't happen without the maths.

    Too many words without a clear meaning (I still have no idea what a "tidal force in time" would be, "curvature of time dimension" is nothing you have in GR, and so on), or without any visible connection to GR.

    I think many are, but they might underestimate the time others spend to learn it.
     
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