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Question Concerning the Rate of Expansion

  1. Jan 25, 2016 #1
    I admit that I do not have much knowledge pertaining to relativity and space expansion, however I do know that all speeds are relative excluding light.
    This makes sense to me, however a new article published says that "Space itself is pulling apart at the seams, expanding at a rate of 74.3 plus or minus 2.1 kilometers (46.2 plus or minus 1.3 miles) per second per megaparsec".
    I don't understand this, as I thought that there couldn't be a universal constant at which something is transpiring at, excluding light.

    Perhaps space is just like light and does not have a relative speed but rather a universal speed. Again, not informed on issues concerning the subject but just that's what pops up in my mind.

    I know that you are probably tired with mainstream articles but this is where I derived the quote from: http://www.space.com/17884-universe-expansion-speed-hubble-constant.html
  2. jcsd
  3. Jan 25, 2016 #2


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    That information you got is a bit misleading/confusing.

    First, "space expanding" is a pop-science description. As you will see if you Google "metric expansion" the actual physics is just that things are getting farther apart. Space is not a "thing" that can "stretch"/"expand"/whatever, it's just a framework in which stuff exists.

    It is good that they talk about a "rate" of expansion, but then they give an example that is a particular speed and that is confusing/misleading. What it is, is that things are getting farther apart by about 1/144th of a percent per million years.

    So locally (out in deep space away from gravitationally bound objects such as galactic clusters) things are moving so slowly on small scales that it might as well not be happening. Here's what you'll find in the article linked to in my signature:

    On cosmological scales, however, it has a massive effect. For example, objects at the edge of our Observable Universe are receding from us at about 3c.
  4. Jan 25, 2016 #3
    Thanks for the response; your answer made sense to me.
    However although it makes sense, it kind of amplified my confusion.
    You said that
    I derived from this that it's just universal fraction at which things are becoming farther apart and the speed is dependent upon their distances.

    For example:

    Two objects that are 1 inch away from each other would expand their distance between them 1/144% in 1 million years - so 1/144% of an inch in one million years.

    Conversely for objects that are light years apart, even 1/144% is a lot.

    So I don't understand how they came up with their speed. It seems like it should be totally relative to how far an objects distance is from another object.
  5. Jan 25, 2016 #4
    I'm not sure why they used this particular phrase, space is expanding at the rate they mentioned, not tearing apart.

    I don't know what this means, can you clarify?

    The expansion rate of space is not constant or "universal," it is actually accelerating at a rate quantified by the cosmological "scale factor."

    From: https://en.wikipedia.org/wiki/Scale_factor_(cosmology)

    "Current evidence suggests that the expansion rate of the universe is accelerating, which means that the second derivative of the scale factor 4ea4cfa40e1e42c224abc7f40e7b84e6.png is positive, or equivalently that the first derivative c623f787b4b98468b2d7aab6237d0e6f.png is increasing over time.[4] This also implies that any given galaxy recedes from us with increasing speed over time, i.e. for that galaxy ad0442e72773bf13db1da437e69869ef.png is increasing with time."
  6. Jan 25, 2016 #5


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    The relative speed of distant objects in cosmology (general relativity) is fundamentally ill defined. I am not sure how much it will help you, but I will attempt a non-poetic statement of the referenced facts (you will see no reference to space as an 'thing' in this description, nor to expansion of space):

    Consider you have a bunch of observers such that each sees the rest of the universe as the same in all directions, and has done so back to when the universe first became transparent to light. Note, that an observer passing one of these observers, moving inertially at some non-trivial speed, will see the universe as NOT the same in all directions. Note that each of these observers that sees isotropy are themselves moving inertially.

    Next, define a global time coordinate by the time on each of these observer's clocks since the universe became transparent. At some time since this initial time synch (of transparency), each observer measures the distance to a very nearby observer of this family (perhaps using radar ranging). All of these measurements are transmitted to all other observers. The process is repeated periodically. The the accumulated distance (between any pair of such observers) at common global time coordinates is what grows by the indicated formula. Also, note that essentially all galaxies are approximately observers of this type.

    These are the observational facts predicted by general relativistic cosmological models. Describing this as expansion of space is poetry, not part of the actual formal theory. Space is not a component of the models, nor is expansion of space. They are interpretations added to the actual formalism of the theory that some people find useful.
  7. Jan 25, 2016 #6
    Yeah sorry, it could have been better phrased.
    I meant that speeds and motions do not have a objectively true universal speed, and that they're all just relative to who's perspective your watching through. I then followed up by saying that light was the only exception, which has a universal speed at which observers perceive it to be.
  8. Jan 25, 2016 #7
    So it seems that my definitions may be the reason for confusion. Perhaps, as you guys have said, to say that space is expanding is misleading. Rather, it should be stated that the amount of space between two objects is increasing.

    I would also appreciate it if you could give me clarification regarding the following claim.
    Space does not increase at a speed but rather at a fraction between two distances. This fraction is 1/144% per million years. Due to it being a fraction, this means that distances that are far away from each other would experience more space being created in between them, and therefore a relative speed can be formulated for the distance.

    If what I posted above is true, then I don't see how it's possible to derive an absolute speed such as they did.
  9. Jan 26, 2016 #8


    Staff: Mentor

    They didn't give an absolute speed. They quoted a number whose units are kilometers per second per megaparsec. Kilometers and megaparsecs are both units of distance, so the actual physical units of the number are just inverse seconds, i.e., a percentage rate of expansion. (If you convert the kilometers to megaparsecs, and "per second" to "per million years", you will get the number phinds quoted.)
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