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Universe Expanding - Effects On Gravity?

  1. Jan 15, 2014 #1
    Hi,

    I joined this forum because I love learning new things about our universe and I often have questions like the one I'm about to ask. Also I would like to state that I haven't taken any physics classes so terms and advanced talk will be beyond me however, because I am so interested in this topic these things either come rather easily to me or I research until I figure it out.

    So my question is about our expanding universe. I know that our universe is expanding and I know that it is space expanding and not the objects in space moving. I also know that certain areas of space are expanding faster than the speed of light.

    What really confuses me is how space stretching has no effect on gravity itself. Every documentary I've ever seen explains a star in space as a marble on a sheet held tight in the air (the marble makes an indent in the sheet and if you place something much smaller next to it say a an air soft pellet it will fall in towards the marble). Well, with space stretching I imagine the star would be making a bigger impact on space because it is stretched thinner. I'm sure the simple explanation is that the marble and a sheet is simply an analogy and spacetime is actually very different but I still think this deserves an explanation. Does a star's gravity not have a bigger impact on spacetime because it's stretched (for lack of a better term) tighter meaning it's thinner but also more difficult to act on because it's tighter?
     
  2. jcsd
  3. Jan 15, 2014 #2

    phinds

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    The "stretching sheet" analogy is a VERY misleading analogy. First of all, it's a 2D representation of a 3D phenomenon. Second, space does not "stretch" as though it were something tangible. In fact, it's a bit misleading to say space is "expanding". Google "metric expansion" for a discussion of this. Basically, it's just that distances get greater.

    Space doesn't "thin out" in some way that would cause the mass of an object to have more gravitational attraction. The gravitational attraction of any object is just based on its mass.

    There are some further comments about all this in the link in my signature.
     
  4. Jan 15, 2014 #3
    A problem I've always had with the "marble on a rubber sheet" analogy is that the "indentation" in the sheet that represents a gravity well, is only there because of the force of gravity outside the scope of the analogy acting on the marble.

    The analogy won't work, if one is in free-fall.
     
  5. Jan 15, 2014 #4
    Thank you both for the quick replies.

    @Phinds thank you for confirming that the analogy is flawed. So if space isn't stretching in anyway and the distance between the objects simply becomes more, that leads me to the what is space? question. I have a terribly difficult time contemplating space being intangible, isn't there a relation to dark matter/energy, that is why it's thought that the universe won't eventually come back to a singularity correct? I might just be getting lost in my own head here but space needs to be something or else what was before the universe and space? Space is, therefore space could not have been...

    @ckirmser I've always had the same issue as well as the issue of spacetime being 3-d/4-d while the sheet is 2-d. But I don't have any other way to understand it.
     
  6. Jan 15, 2014 #5

    phinds

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    "What is space" is a somewhat contentious and widely discussed topic. Do a forum search.
     
  7. Jan 15, 2014 #6
    Will do, thanks for the input.
     
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