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B Infinite vs Expanding

  1. Feb 22, 2017 #1
    given all the great minds of physics, i still have trouble with this one.
    "the uverse is infinitely big"
    and
    "the uverse is expanding"

    my understanding is you cant have both, because to define expansion you need measurable differential on a boundary, thus expansion by definition infers changing finite regardless as to how fast the change is.

    so which is it?
     
  2. jcsd
  3. Feb 22, 2017 #2

    fresh_42

    Staff: Mentor

    It is important to stop thinking the universe is expanding into somewhere. The universe itself is somewhere. It is the same difference as with any, let's say surface, because we can imagine surfaces. If we only knew our earth and nothing about outer space, since the universe doesn't have an outer space, we would only knew the earth's surface. Why shouldn't it get bigger and bigger without any reference to getting bigger into something? The surface is all we have, and it might get bigger. O.k. not the earth as a solid rock, but for imagination it's sufficient.

    The universe is estimated to have an expansion of about 45 billion light years in each direction (see Wikipedia). But that doesn't mean it's shaped like a ball. We simply don't know how it is shaped. All we know is, that it is almost flat, so either it is really big, or indeed flat. And it doesn't need a boundary as our earth's surface doesn't have a boundary either. So infinitely big should better be read as without boundary.
     
  4. Feb 22, 2017 #3
    I tend to think of it as the universe is infinite, spatially, but the stuff in it is expanding. Love stuff.
     
  5. Feb 22, 2017 #4
    hi Blake
    so space in the space-time fabric is as any given time, infinite?
    and for me, i dont see how to apply "expanding" to "stuff". what i can see though is that all "stuff" is moving away from all other "stuff", but by that observation i do not conclude "expanding".

    example, lets take a large balloon, say 200x the size of the biggest weather balloon we know, inside there are a few atoms near the center and these atoms are moving away from each other at a rate of 1/1x10^100^100^100^100^100^100 m/1000yr, so from any point within the observation is that the "stuff" is just moving away from all other stuff, but you are not measuring any differential on any boundary since the boundaries that confine the stuff is the balloon, which is not changing.

    now, if we want to talk about the density of "stuff" across a space, that would be an observable decrease by definition when the "stuff" becomes more distant from the other "stuff" within the "infinite" space.

    if you blow up a balloon, the atoms up against the inside edge of the balloon are following the edge of the balloon, which infers that the edge of the balloon (aka boundary) has differential finite change of space, thus it is expanding.
     
    Last edited: Feb 22, 2017
  6. Feb 22, 2017 #5
    But the balloon itself is stuff. Your example assumes the universe, the actual space-time, has a boundary. Is space expanding to hold the stuff or is the stuff simply spreading out to fill the space?
     
  7. Feb 22, 2017 #6
    No, another definition of expansion is metric expansion. If the distance between any comoving points is increasing, then space is expanding. And this is what we observe (Hubble's law).
     
  8. Feb 22, 2017 #7

    phinds

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    Google "Hilbert's Hotel" for a discussion of how "expanding" and "infinite" are not in any way contradictory in the same context.
     
  9. Feb 22, 2017 #8
    i will lookup hilbets, but Re: your point, localized "space" itself is not expanding (the box around which all your "stuff" is), if it were then you would have decreasing "space" density.
    what you describe is nothing more than increasing distance between "stuff" within a confined non-changing "space".

    "space" itself could be expanding at the boundaries, but that is like an expanding balloon, which at any time t has finite boundaries = not infinite.
     
  10. Feb 22, 2017 #9

    phinds

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    Really? Are you sure about that?
     
  11. Feb 22, 2017 #10
    In Euclidean space and Galilean relativity, there isn't really any difference between expanding space (i.e. the coordinate axes compressing) and everything moving apart within that space. Because there wouldn't be any way of measuring the difference. But we don't have Galilean relativity. We have General Relativity. In special and general relativity, nothing can travel faster than c relative to a local observer. But, we can measure that the distance to faraway galaxies is increasing faster than c. This isn't possible if space were just a static grid and everything was moving apart on that grid. Because it would mean all these faraway galaxies are traveling faster than c. No, rather, they are traveling fairly slowly** in their local space, but all of space is being expanded. This is what the equations tell us.

    **relative to the cosmic microwave background
     
  12. Feb 22, 2017 #11
    In relativity, there's a concept called parallel transport. Let's consider what it means to have expanding space.
    Let point A be the origin of an inertial reference frame. Take an object at rest at point A and then move it to point B, far away. It is possible to keep track of all the accelerations that you have made on the object using an inertial guidance system, so in principle, it is possible to accelerate up to a velocity v, travel to B, and then decelerate back to velocity 0, relative to your original inertial motion which is calibrated to be 0 velocity.

    Now, after decelerating back to "0 velocity", is the object at B moving relative to point A? We could infer this motion by sending radio signals back and forth. What you would be measuring is the distance between B and A. And if space is expanding, that distance is growing, even though your inertial guidance system is telling you that you aren't moving.
     
  13. Feb 22, 2017 #12
    The Balloon Analogy shows that when you inflate the balloon, the stuff on the surface of the balloon will expand. It isn't talking about the atoms inside it. So if you have point X and point Y on the surface of the balloon, the distance between point X and point Y will grow during the time that the balloon is being inflated. This is expansion on the 2 dimensional level. The problem with this analogy is that it only works on a 2 dimensional scale. Enter: the universe... 3 spatial dimensions, not to mention the time dimension which collectively make up spacetime. It would be VERY challenging to come up with an analogy to fit spacetime expansion.

    It has been stated that the galaxies themselves are getting further apart, but they themselves are NOT moving. Khashishi explained this well in post #10. So to go back to the Balloon Analogy, imagine that point X and point Y are galaxies. They themselves are not moving. The expansion of the balloon is moving them, which translates to the expansion of the universe moving the galaxies farther apart.
     
  14. Feb 22, 2017 #13
    ok, i see some good info for me to chomp on.
    but, balloon analogy of the particles of the balloon itself, you are the observing differential in boundary, thus would be saying that "space" is not infinite, but rather finite at any given time. however, if i look at that in 2-dimensional view, from surface, i can see where it look like the stretching or "expanding" of space.

    i see Suskind u-toob video for this example, a simple cut rubberband with a few equally spaced dots on it (not unlike balloon analogy, but 1-dim), stretch the rubber in linear fashion and the dots get further apart yet there is no actual movement of the dots relative to their dot location on the rubber. but most things will disappear if you stretch then for a long time, so how can "space" itself stretch continuously at speed of c ?
     
  15. Feb 22, 2017 #14
    What do you mean by they disappear?
     
  16. Feb 22, 2017 #15
    i mean, stretch something, stretch it more, more more, what eventually happens? take the individual atoms and increase the space between them at a constant rate, what you wind up with is a single atom with all the rest infinitely far away.

    "space" doesn't seem to work like that, it can stretch at the speed of light for infinite amount of time? So does "space" just fill itself in when it gets thin? do blackholes form because the forces created there are strong enough to rupture "space"? and this odd "space" will stretch like that no matter where the observer is, an observer in this "space" seems to alway be at the center point of the 3D expansion, yet, if that's the case then we have conflict when analyzing the data from two observers because when combined the "space" would seem to be expanding towards each other, how can that be?

    example, me at xyz, joe at x+1 y-2 z+3, to me space is expanding in 3D as if I am the center, for joe, space is expanding in 3D as if he was the center, but that means space is expanding towards me and joe at the same time. if this model is false then the expansion of "space" has a singularity origin, so where is that?
     
    Last edited: Feb 22, 2017
  17. Feb 22, 2017 #16
    You have all these preconceptions about what space is, and that's why it doesn't make sense to you. Space is not a substance that can get thin. It is just a mathematical framework for assigning positions and measuring distances between things or events. We describe space in terms of the topology and the metric. You need to forget everything and just start reading. All your questions are of the form: if this then that. how can that be? Well, the answer is that your deductions are based on some incorrect assumptions. If you find a contradiction between two things you have read, then please ask, and we'll try and figure it out. But if there's a contradiction between something you read and one of your preconceptions, your preconception is probably off.

    In relativity, you don't have a universal grid with nice x, y, z coordinates that work for everybody. Space is curved, so parallel lines may diverge or converge in some places. What you can do is create local x, y, and z coordinates for a specific observer for a small area around the observer. And you can have another set for another observer. You can calculate how to transform between the coordinates, but you'll find that one set of coordinates don't work well for the whole universe.
     
  18. Feb 22, 2017 #17
    This is not why black holes form.
    There is not a single spot in the universe where you can say the universe started, according to Big Bang Theory.
     
  19. Feb 22, 2017 #18
    Khashishi

    ok, take for example two local observers A&B who each have access to hubble, each observer looks at different distant objects to determine that those objects are moving away from each other, and, that they themselves (objects) are moving very slowly but their apparent differential speed is quite high, thus each observer concluded that it must be space that is expanding, however, when each observer makes their reference the distant object it would appear that all the other "stuff" is moving away from that reference point, but observer B sees the same thing in another place, so wouldn't the apparent directions of expansion conflict?

    also, if space itself was a thing of infinitely tight folds, like a roll of paper towel unrolling onto table and it folds back and forth on top of itself, you could have infinite distance in a infinitely small space ? the u verse as we see it, could possibly be within the smallest thing ever.
     
  20. Feb 22, 2017 #19

    phinds

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    You really need to pay attention to what's being said to you:

     
  21. Feb 22, 2017 #20
    yeah yeah, i know, i know that i dont know, just me throwing up some wild questions.
     
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