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Space time mass

  1. Mar 28, 2007 #1

    wolram

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    Has space time any mass?

    If space can expand, then it suggests
    that (some thing) is feeding the expansion, if so then that (thing)
    may be the missing mass .
     
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  3. Mar 28, 2007 #2
    In general relativity space-time is not some entity that exists by itself, but instead it is the combined gravitational field of all particles.
    General relativity does not operate on the background called space-time.

    With regards to "expansion", I am not a big fan of this term, in fact I think it is a misnomer.

    So perhaps people who find themselves more qualified could comment on "expansion".
     
    Last edited: Mar 29, 2007
  4. Mar 29, 2007 #3
    So if you could ever escape the gravity of all the particles in the universe, you would be "outside" the universe?
     
  5. Mar 29, 2007 #4

    Wallace

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    The short (and long) answer is no.

    Space can be said to expand since the distance between two observers who are at rest in their local inertial frame increases. The thing that drives this expansion however is simply the energy in the universe. The expansion of space is an effect rather than a cause. The reason that the universe expands is that is did so in the past. The nature of the energy in the universe (the matter, dark energy if it exists, radiation etc) governs the acceleration of the universe, but the only reason the universe is expanding is because it did so in the past. Inflation (or whatever someone comes up with that is a better theory) is the ultimate reason the universe is expanding.

    So no, there is nothing 'feeding' the expansion that is currently unaccounted for. Space-time dosn't have mass but it responds to the presence of mass by curving etc. The way in which it does this is accounted for in our current standard theory.
     
  6. Mar 29, 2007 #5

    vld

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    According to Zeldovitch and Novikov (in their book "The Structure and Evolution of the Universe"), the net mass of the universe is zero. However, if mass is equivalent to energy then, according to the current theories, locally vacuum contains enormous amounts of energy (hence - mass?)
     
  7. Mar 29, 2007 #6

    wolram

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    If expansion is driven by the energy in the universe, then surly as space
    grows then the energy will be dilluted, even now it should be getting less
    per volume, if so then maybe the properties of space change over long periods, unless that is there is some limitless energy source.
     
  8. Mar 29, 2007 #7

    marcus

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    Normally I find myself understanding and agreeing even down to level of nuance, with what you say in your posts. But I have what may be a slightly different take on the meaning of this popularized phrase.

    For me the clearest way to picture it is that what increases is the distance between two observers who are each at rest with respect to the Hubble flow

    Is there some reason I should not be thinking this way (is it circular, or misleading in some respect?)
    Should I substitute for my idea the idea of two observes each at rest in local inertial frame?
    ==================

    In any case "space expands" is a phrase used to commicate something to lay readers. and it is a concise phrase to use in casual conversation.

    and I think what it means is that distances between stationary things increase.
    (stationary meaning "at rest" in a sense defined various possible ways, with respect to CMB, or hubbleflow, or inertial frame, or whatever)

    what seems to infuriate people is that as children they got used to the idea that the distance between two stationary things should not increase---but alas it seems in the real world it typically does:smile: ---so one has to get over the enraging fact of it being different from the childhood experience of nearby things.
    But after all there is no logical or philosophical reason why the distance shouldn't change! We have no right to expect it to stay always the same.
    Still people kvetch and kvetch about this.:smile:
     
  9. Mar 29, 2007 #8

    marcus

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    This sounds very reasonable to me!

    I am not a big fan of the phrase "space expands" either.
    It is a popularization and it gets across the idea, but it is a bit crude and could be called misleading, or a "misnomer" as you say.

    the philosophical bedrock, I think, is simply that largescale distances increase.

    largescale distance does not behave like smallscale distance that is locked onto a crystal or piece of metal or rock, or the surface of the earth.
    those distances are anchored to molecular and atomic forces in material.
    so a ruler, which is crystaline metal, stays the same length.

    but largescale distance is not anchored to a crystal or a rock or a dynamical system of bound planets or whatever. it is over vast reaches of space between unbound entitites. It can and does increase.

    And the change in distance can affect the wavelengths of light.

    If one could say all that in two words, in casual conversation, then one wouldnt have to resort to a misleading summary like "space expands".

    it's all in the distance function (which, as you say, is the gravitational field itself: gravity = geometry)
     
  10. Mar 29, 2007 #9

    marcus

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    Wolram, space is not a substance.
    The fact that people say "space expands" suggests to people that it is a substance or behaves like a material, which is probably wrong. I think it is wrong.

    I believe Aristotle pointed this out. I think he said that space was not a material, but rather it was the relationships between things. Leibniz agreed with Aristotle nearly 2000 years later. Its how thinking people normally thought.

    In Aristotle's view, which has been the standard one thru most of European history, space is the sum total of all the geometrical relations between things

    basically it's the catalog of all the angles and distances and this-between-that and what is beside what, or inside, or outside.

    and, interestingly enough, EINSTEIN CONFIRMED THIS OLD TRADITIONAL IDEA because he replaced space by the idea of a distance function, i.e. a METRIC, which is basically just what I said.

    a metric is a catalog of all the geometrical information about relations between things.

    Einstein said that points of spacetime dont have reality. what has reality is the relationships between events summarized by the metric---and the METRIC IS THE GRAVIATIONAL FIELD. that really is all the gravitational field turns out to be in Gen Rel. it turns out to be the metric---the distance function between things that defines geometry.

    :biggrin: knowing you, I guess you will scream and find that completely impossible to swallow. there is no space. there is only relationships.

    but you and I and everybody is stuck by one simple fact: nobody has a theory that WORKS better than Gen Rel to predict gravity stuff. And in that theory there is no space
    there is only the metric distance relationships
    and in Gen Rel solutions----specific metrics that come out as solutions of the equations DISTANCES CHANGE typically rather than staying the same.

    so here, at PF, half the time half the people are screaming----that is what many of the posts amount to. people dont seem able to accept that distances change. but they are stuck with there being only one model of gravity that works

    and big bang (or more recently big bounce) cosmology is one of the inescapable consequences of this model of gravity that works

    (works with exquisite precision, tested over and over again!)

    this is our situation, Wolram, are you going to be one of the screamers?
    or will you try to live with the idea that distances between stationary things can increase?
     
    Last edited: Mar 29, 2007
  11. Mar 29, 2007 #10

    Wallace

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    Spot on! This is exactly what happens. The density of the universe was much higher in the past and will be lower in the future. As the universe expands the energy within it is indeed diluted. The 'properties' of space-time are determined by the energy residing in it, so indeed the properties of space-time change as the universe expands and the energy within it decreases in density.

    I don't disagree with you here at all, in fact what could 'an observer at rest in local inertial frame' be other than an observer at rest with respect to the Hubble flow? My statement was just a slightly more fancy way of saying the same thing, though perhaps I didn't explain it well enough.
     
  12. Mar 29, 2007 #11

    wolram

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    Marcus, knowing you, I guess you will scream and find that completely impossible to swallow. there is no space. there is only relationships.


    Maybe not scream, but the idea that we live in a (metric) is to artificial an existance, unless some (thing )controls that metric. :smile:
     
  13. Mar 29, 2007 #12

    Wallace

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    Marcus, I had another read of what I wrote and I think I missed an important point. What I meant to/should of said was "the distance between two observers who are at rest with respect to the cosmic fluid in their local inertial frame increases". This now sounds even more like your statement, so I think we agree, once I explained myself properly!

    Wolram I don't understand what your last post was trying to say at all? Could you clarify?
     
  14. Mar 29, 2007 #13

    marcus

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    Wallace, watch out for Wolram. He has a British sense of humor that one doesnt always get. maybe you are British yourself and understand. keen independent mind. asks basic original somewhat strangesounding questions

    I never really know what he's driving at but always has some interesting aspect.

    Wallace everytime I reflect on it I am very glad you are showing up and posting at this cosmology board. It makes it a totally better place to be.
     
  15. Mar 29, 2007 #14

    wolram

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    Sorry Wallace, i try to understand things in my own way, Marcus is
    generous, While it seems easy for some to accept that we live in a
    universe governed by dark matter, dark energy and this thing called
    space time (a thing that has only a mathmatical description) afaik, i
    find it most difficult, may be when maths can describe smell or color
    i might find it easier.
     
  16. Mar 29, 2007 #15

    Wallace

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    I think people overestimate how happy 'mainstream' cosmologists are with living in a universe full of the entities we label dark matter and dark energy. Most cosmologist are in fact very uncomfortable with this, but that's what the evidence points to. The reason so much effort goes into say, direct dark matter detection, or SN or other surveys to probe dark energy is the very desire to either find out what they are or prove they aren't there!

    Many things in science were postulated to exist without being directly observed. Some of these were subsequently confirmed to exist, some were not.

    The other thing you mention, that space-time just a mathematical description, is also a more common cause for concern than you might think. On the other hand, what is physics if not a mathematical model for the universe? Maths can describe both smell and colour, just as it describes space-time. If you want to go beyond a mathematical description then physics may not be the place to look!
     
  17. Mar 29, 2007 #16

    wolram

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    Thankyou Wallace, like Marcus you are very kind and helpfull.
     
  18. Mar 30, 2007 #17
    Please excuse this interruption to a really informative exchange, but I have a much more basic question. If the observed increase in distance is a change in a metric, would it be consistent to argue that the change could be caused by a distortion (as mass causes a distortion, but with a possibly different cause and a far less localized effect) so that the path that light follows gets longer? That is, would a model that does not involve the translation of objects, but just a longer path for light meet the requirements? Does that make sense?
     
  19. Mar 30, 2007 #18

    Wallace

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    There is probably an easier way of answering your question in the negative, but here is my answer:

    The change in the metric with time (specifically the scale factor a(t) ) has an important role in the structure formation of the universe. So with a different expansion history described by a different function for a(t) the clustering of material in the universe is different. This would not be the case if the expansion only affected light.
     
  20. Mar 31, 2007 #19
    Wallace,
    Thank you for the reply. I actually think I understand your point (although I do not have any sense for the functional characteristics of a(t)). Is there more you can say about a(t) or references that can help a non-cosmologist understand?
     
  21. Mar 31, 2007 #20

    Wallace

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    Try this paper. It is a review of some different dark energy models. If the text and maths is too dry and academic have a look at the figures. They show the a(t) evolution for a variety of DE models. Note this paper actually uses R(t) instead of a but they are essentially the same thing.
     
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