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Space pushing on matter

  1. Mar 19, 2010 #1
    I've read that the expansion of the universe may be described in terms of space itself expanding (rather than galaxies rushing apart through space).

    If this is the case, does that mean that space is pushing galaxies apart? And if so, how is this consistent with space offering no resistive force to bodies in motion (i.e. a body moving at constant velocity will not be slowed as it moves through space)?


  2. jcsd
  3. Mar 19, 2010 #2


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    No, space is not pushing galaxies apart. They are just along for the ride, stationary in their part of an expanding universe.
  4. Mar 19, 2010 #3
    Russ' post above is correct .....
    His description is better than saying space is pushing galaxies....

    sitting at the middle of some galaxy you'd be moving along away from another distant galaxy without feeling a force of acceleration, as Russ implies.

    On the other hand, looking at two other galaxies moving apart [because space is expanding between them] they'd sure be moving apart as if something were "pushing" them apart...

    It's a crazy universe, what with space appearing out of "nothing" and virtual particles also popping in and out of existence all the time....sometimes becoming "real" (potentially observable)....all the while "dark energy" hiding out apparently almost invisible....
    Last edited: Mar 20, 2010
  5. Mar 20, 2010 #4


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    Dark energy claims the blame for expansion. Once it was determined expansion is not uniform, some form of unseen [dark] energy was the only logical explanation physicists had to fall back upon. This may not be an ideal solution, but, is the only one that satisfies the need for a semi-classical explanation for expansion. A little clever math revealed its contribution to the total energy-mass content of the universe is huge [~75%]. We reside in a strange universe.
    Last edited: Mar 20, 2010
  6. Mar 20, 2010 #5

    It's just this kind of phenomena, still not yet understood, that causes me to frequently post comments like "...most of the time most physicsts in history have been WRONG"....dark energy I think became a serious consideration in the 1990's for reasons Chronos implies.
    So once again when physicsts thought they were "cool" and understood so much, dark matter and dark energy revealed themselves and OOOOOOPS about 93% of matter and energy in the universe became a new mystery....on the other hand, IMHO that's just the kind of thing that makes physics so interesting...

    It's also a reason that I like to claim (without justification and to howls from those "more knowledgeable" here than I ) that spacetime IS something...how could it otherwise cause galaxies to move apart?? Which reminds me that we still don't know if spacetime is continuous or discrete...several threads here discussed that at length...it's continuous in GR and discrete in QM via Planck size theory.
  7. Mar 20, 2010 #6


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    We get discussions like this frequently. Russ is quite right, Darkmisc, but if you keep asking what the words mean you might find that you remain dissatisfied and puzzled nevertheless. Ordinary street-English plus intuition from highschool (Euclidean) geometry doesn't seem to work well at describing the math model that cosmologists use. There are various ways to try to translate the math model into words. I'll try one and you can see whether or not it works for you.

    Cosmologists have a wonderful math model: it's good for two reasons. There are millions of data points and the model fits the data amazingly well. It has only a handful of adjustable parameters, numbers determined by observation. You plug in the right halfdozen numbers and presto it fits millions of data. That's one reason. The other reason is that the model is derived from a theory of evolving geometry that precisely describes gravity, out to the farthest decimal place currently measurable. That's General Rel. It's a better theory of geometry than Euclid and a better theory of gravity than Newton. Someday we may have an improved quantum version of it, but for now we don't have anything better.

    Gen. Rel. has been tested on earth and in orbit and in the solar system and with binary neutron star observations and so on. It best describes how gravity and geometry work, of all the alternatives. People constantly challenge it with new tests, and propose alternatives and modifications of it---so far to no avail. It keeps passing all the tests.

    Trouble is, that the basic theory Gen Rel, and the cosmo model that is based on it, are neither one very intuitive if you insist on thinking like Euclid and Newton. Here's one way to venture out of that particular box:

    Cosmologists often refer to the cosmic microwave Background. Ancient light that was released by the hot fog of ancient matter at the earliest moment when it cooled enough to become transparent (while it was still approx. uniformly spread out and hadn't begun condensing into stars and stuff.)

    We can detect the Background radiation in all directions and measure it's temperature and map the slight variations in temperature. Anything moving with respect to Background will see a doppler hotspot ahead and a doppler coldspot behind. We know our speed and direction relative to Background and we can adjust for it, allow for it. So we can correct for slight motions relative to Background.

    As far as we can tell, the random motions of galaxies relative to Background are all very small compared to the rate that largescale distances between the different clusters of galaxies are expanding. So small that we can just neglect the random motions. The millions of galaxies we see can be taken as approximately stationary with respect to background.

    So at this point we need a visualization. You have to be able to wrap your mind around the idea of clusters of galaxies which are at rest and the distances between them are increasing. Oh, and in Gen Rel space is not a substance. It has no objective existence. What exists is geometry, and geometry is dynamic changeable---it somehow interacts with matter without itself being a material. Geometry is a network of relationships between events, it somehow exists without being a material substance. OK OK enough philosophy. We need a visualization so you can picture galaxies at rest with distances between them increasing.

    So google "wright balloon model" and watch it. The latitude longitude of the galaxies does not change---they are stationary. But the little wiggly photons do move, always at the same speed of like, say, one millimeter per second across the surface.


    This is a 2D analogy. All existence is concentrated on the 2D surface of the balloon. Any creatures living there are 2D creatures. Only the mathematical surface exists. There is no rubber, no air inside the balloon, no room outside around the balloon, only the surface. There are no directions off the surface that a 2D critter can point its finger at. After you watch, try to imagine being in the 3D analog of this. Maybe in fact we are.

    All the photons are moving the same constant speed of say 1 mm per second but after 30 seconds a photon will be much farther than 30 mm from its galaxy of origin, of course, because of expansion. It always travels the same speed but it gets away from its starting point at a higher rate than that.

    That's a first lesson in expansion geometry. Nobody is pushing. Its just the way solutions of the Gen Rel equations are. Realistic solutions to the basic equation of gravity/geometry just do either expand or contract. We just have to get used to it. If you don't like it, try to invent an alternative mathematics of gravity/geometry that still fits the data but where distances between stationary observers DON'T increase. People have tried.

    If this approach is working for you, let me know and ask some more questions and I'll continue. If not, try a different intuitive route. Like keep asking Russ questions and get his take, or Chalnoth. There are several possible coherent viewpoints.
    Last edited: Mar 20, 2010
  8. Mar 24, 2010 #7
    I suddenly feel the need to throw in a little question that bothers me for quite a while:

    How is this expansion understood? I could think of two models:
    1) Space itself stretches, such as if you looked from the outside (a non-stretching outside), the units of distance become larger relative to the (non-stretching) observer.
    2) Space moves apart, and "new" space forms in between. Our outside observer then wouldn't notice any stretching of units, but with time more and more units would emerge between any two points.

    I would, naively, favor #2, since to my understanding "simply" stretching would leave the immersed observer to the impression that nothing ever changed, and thus there shouldn't be any redshift, if we take for granted that light travels in the immersed observer's system, as opposed to the external observer's - which would lead to further interesting thoughts to say the least.

    But since I am no physicist, I wouldn't really know which one it is. Could anyone shed a droplet of visible photons on that?
  9. Mar 24, 2010 #8

    Speaking about space as a substance is not particularly helpful, neither provides good analogy.
  10. Mar 24, 2010 #9
    Actually, I wasn't. I am thinking of "space" a bit like as the space between ruler marks. Don't confuse that with the material the ruler is made of - just the space as a concept in which to measure distance.

    I am aware that "inserting new space" (option 2) is a strange idea, if space isn't thought of as "something", but I am fully aware of this, and willing to accept that for now, since it isn't really important for my question.
  11. Mar 25, 2010 #10


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    That's good.
    One more thing:
    The idea of motion may not be applied to space. You hear somtimes motion through space, or motion of space, but this is just a reification of a certain coordinate sytem.
    Expansion is the phenomenon that different things - e.g. galaxy clusters - have increasing mutual distance, such that the process looks the same at every place in the universe.
    So, expansion means that things are moving away from each other in a certain way. You can define certain cosmological time and space coordinates such that the space coordinate of all these things does not change with time. Their distance is then expressed as the difference in space coordinate times a scale factor. If the scale factor increases with time, we speak of expansion.
    This description does not define wheter space is being stretched or inserted, such notions have no physical meaning.
    Distances are increasing. Normally that's called motion, in this uniform case it's called expansion.
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