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Universe Expanding

  1. Jul 14, 2009 #1
    When it is referred that the Universe is expanding/accelerating outward, do they mean to say that space is expanding or that matter is accelerating outward through space?
     
  2. jcsd
  3. Jul 14, 2009 #2

    Danger

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    The space itself is expanding, carrying the matter with it.
     
  4. Jul 14, 2009 #3
    Since you can't observe space directly (you can only observe measuring sticks) , all the direct astronomical observations that we use to justify the statement "the universe is expanding" are based on light radiated by moving objects.

    The primary theory behind the expansion of the universe is Einstein's General Relativity. In terms of the theory, it is space itself that is expanding, analogous to what would happen if our universe was a 2-d surface of a balloon and the balloon was expanding.

    Therefore, if you believe General Relativity, then it is space itself that is expanding. If, however, you want to strictly limit ourselves to what we observe, we can only observe measuring sticks.

    At a slightly more detailed level, here are two explanations for the cosmological redshift:

    (1) Objects are moving away from us and so the frequency of emitted light is lower (redshifted). This is just like an ambulance moving away from you producing a lower tone from it's siren.

    (2) In the time that the light has been traveling from the object, space has expanded , increasing the wavelength of the light, which decreases the frequency, leaving it redshifted.

    In general relativity the answer is unambiguously (2). It turns out (1) and (2) predict different results, and (2) is the correct explanation of the cosmological redshift using general relativity.
     
  5. Jul 14, 2009 #4
    I see (to the extent I can)...thanks for the replies. But since it is space that is expanding, does that still imply that we might experience relativistic effects, or do we need to be moving through space for that to happen?
     
  6. Jul 14, 2009 #5

    russ_watters

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    There is no such thing as "moving through space" and no, there are no relativistic effects associated with expanding space. There are actually galaxies that appear to be moving faster than C.
     
  7. Jul 14, 2009 #6
    Thanks for the answer.
    Then what is there?
     
  8. Jul 14, 2009 #7
    I'm not totally sure what Russ is getting at, but from a spacetime perspective, moving through space without moving at all through time would correspond to infinite velocity, and that is impossible in relativity.
     
  9. Jul 15, 2009 #8
    I see. Thanks again for the reply.
    An additional question: if space is expanding, doesn't that necessarily imply that there should be a higher-dimensional medium for it to expand (thus, we have indirect evidence for higher dimensions)?
     
  10. Jul 15, 2009 #9

    russ_watters

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    Space is not an object nor an absolute reference frame, so there is nothing for one to measure a velocity relative to. Velocity is measured between objects.
     
  11. Jul 15, 2009 #10
    Then what is space and what is meant by it expanding?
     
  12. Jul 15, 2009 #11

    sylas

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    I think the notion that matter is "carried by" the expansion of space is a bit misleading. (Reply #2 above.)

    I like this definition: space is that which is measured by rulers and clocks. This is the converse of Einstein's operational definition of time: that which is measured by clocks. Spacetime is that which is measured by rulers and clocks.

    Expanding space means that distances between things are increasing. We speak of expansion of the universe because on sufficiently large scales, the distance between any two objects is increasing. Since everything is moving further from everything else (on sufficiently large scales) you can equally say that the space between everything (on sufficiently large scales) is increasing. But I think it may be slightly better to say that space is expanded by the motions of objects, rather than objects are carried by the expansion of space. It's a bit like noting that spacetime is curved by massive objects.

    You can also say that volumes are increasing. If you try to define a volume for a region of space, on sufficiently large scales, that volume is increasing.

    Cheers -- sylas
     
    Last edited: Jul 15, 2009
  13. Jul 16, 2009 #12

    Ich

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    Yes, that's what one can say and usually does say.
    The unambiguous meaning of it is that distances increase with time (with some subtleties concerning definitions and exact values). Every statement that goes beyond this fact, like space dragging objects with it or otherwise, is necessarily interpretation or mental picture, not fact. Interpreations may be useful or not, and one may pick in each event the one that is least misleading.
     
  14. Jul 16, 2009 #13

    Chronos

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    General relativity is not so easily understood as special relativity. GR forces you to divorce many principles of SR when you look at the big picture. SR is only a special case of GR. Does that help? Nothing in SR forbids an expanding universe. Pretty much everything in GR demands it. The speed of expansion is irrelevant.
     
  15. Jul 16, 2009 #14
    From all this, it seems that there is no stable definition for motion...

    If I'm right, SR implies that all motion is evaluated relative to some reference that is also evaluated relative to another reference and so on. So then, as Ich said, there isn't too much against this interpretation:
    Still...if space is the one expanding...shouldn't it need to expand through a higher-dimensional medium and is that implied anywhere in GR or is it just interpretation?
     
    Last edited: Jul 16, 2009
  16. Jul 16, 2009 #15

    sylas

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    No, there's no need and no implication for higher dimensions.

    As for motion having no stable definition; I think it is more correct to say that when we describe things with conventional language or analogies, descriptions are incomplete. The maths of it is unambiguous. There are various ways you can measure a recession velocity, with different co-ordinates or distance definitions. If this is done with full rigour -- and this certainly can be done -- then there is no ambiguity about meanings or transformations between co-ordinates or alternative definitions.

    Cheers -- sylas
     
  17. Jul 16, 2009 #16

    Ich

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    I'd say Gear300 is correct: there is no undisputable definition of "motion" in GR, if the concerned objects are at some distance.
    You know thar SR has no concept of absolute speed, there is only relative motion. That's true also in GR, so I don't like the idea of "motion through space" - even if it is useful for specific calculations, and even though it works generally and can be defined appropriately.
    In GR, even relative velocity between distant objects depends on how you define it. Even worse, relative velocities are not easy to calculate, so it's common to take some coordinate values and call them velocity.
    But, of course, if you pick one definition and stick to it, everything will work out mathematically.
     
  18. Jul 16, 2009 #17
    'Space' only exists as a measure of the separation of 'objects'.

    The measurement is determined relative to a 'standard' -- how this standard is chosen can vary.

    The definition of an 'object' can vary.

    My summation.
     
  19. Jul 16, 2009 #18

    sylas

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    Exactly. This is what I meant when I spoke of using different co-ordinates and transformations.
     
  20. Jul 17, 2009 #19

    Chronos

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    The universe does not easily submit to human descriptions. All we know is it is really big.
     
  21. Jul 18, 2009 #20
    In looking over the posts, in general, I think it presumptious to exclude some of the interpretations of space based upon what we think we know and what we know we don't know. Historically, Einstein premised his GR theory on the fact that the universe was static - and this was one of his primary arguments he used to criticise de Sitter's model - it wasn't static. GR is based upon the ad hoc postualate that inert matter affects static space - it really doesn't imply expansion per se. Then there are the statements by Einstein in his later years that embrace the notion of space as being more than just the distance between things - that it was a part of the universe that functioned in such a way as to make accelerated motion relative just as is inertial motion - that is, a reactionary force would be exerted upon a static mass if the entire universe (including its space) is accelerated, just as would be the case if the mass were accelerated relative to to the content of the universe.

    Presently, the search is to find the energy needed to bring omega equal to one - and if this turns out to be hidden in the spatial stress or in the gravitational fields, then space cannot be relegated to nothing And as one one more point, the notion matter creation during inflation depends upon the energy released by an expanding negative pressure space.

    In summary, I think it is good to keep some options open about what space could be - there seems to be a mainstream urgency to surpress interrogation in this regard

    Who was it that said: "Cosmologists are frequently wrong, but never in doubt"
     
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