Space expansion in our perspective?

  1. So if space is expanding everywhere, does this mean the space in my room is expanding?

    I'm assuming that its expanding so slow because of the small scale, but is there still a measurable amount of space expansion occurring in my room in any time interval?

    If i were to wait a few billion years, would the space in my room be noticeably bigger or would i be expanding at the same rate and not notice a difference?

    P.S. (I like explanations and details.)

    Thx :)
  2. jcsd
  3. Ich

    Ich 1,931
    Science Advisor

    There is observational evidence that galaxy clusters are moving away from each other. They do so with a velocity proportional to their distance.
    One can choose an expanding coordinate system such that all those comoving clusters have constant coordinate values. Since they all are in relative motion, their proper distance increases nonetheless. This increase of distance while keeping constant coordinate positions is dubbed "Space expansion". If you'd use a different coordinate system, you would call the same thing "velocity".
    The walls of your room are not comoving. They are where they are, without relative motion. The distance between them is not increasing.
    In the expanding space paradigm, you'd say they have a inward peculiar velocity that cancels the expansion. That's just a complicated way to say that they don't move.
  4. Our planet will remain the same size. So will our galaxy. It is the space between galaxies that are expanding.
    cheers, BT
  5. So space is expanding locally, but it is being canceled by inward peculiar velocity? What is inward peculiar velocity?
  6. Ich

    Ich 1,931
    Science Advisor

    In expanding coordinates, one often says that comoving things are not moving, that the increasing distance is due to expansion of space. Peculiar velocity is what's left when you subtract that part. It's everything that does not behave exactly like a homogeneous expanding universe.
    So things without relative motion (zero change in distance), like the walls of your room, would be described in a funny way: you subtract the supposed motion due to expansion from zero, and are left with a negative value. You call that an inward peculiar motion.

    This kind of description is matematically well defined, and it's useful for modeling the universe. But it's a real nuisance if applied to things that are simply not expanding, it makes you think all kind of weird stuff is going on, when simply those things have no relative velocity.
  7. Isnt the answer simply that atoms and objects are bound together by force?
  8. DaveC426913

    DaveC426913 16,534
    Gold Member

    This is how I illustrate it:

    The expanding universe is an expanding balloon. Galaxies are shown as pennies glued all over the balloon.

    As the balloon expands, the distance between the pennies increase, yet the pennies do not increase in size. Why? Because the forces holding the atoms of the penny together utterly dwarf the forces of the balloon pulling it apart.
  9. Thanks for the great replies. :)

    So the space in my room is expanding, however the 4 fundamental forces can resist the force of expansion.

    Galaxies that are far enough apart don't have the required gravitational force to resist the expansion and therefore are accelerating apart.

    Is this correct thinking?
  10. Ich

    Ich 1,931
    Science Advisor


    There is no physical property of empty space called expansion. Expanding space is locally exactly the same as nonexpanding space. The difference is in the description, not in space.
    If the walls of your room don't have relative velocity (which I hope is true), they are not expanding. That's it. There's no force needed to prevent "expansion".

    There may be air in your room, or dark matter, or dark energy. Those things gravitate and would attract or repulse the walls. To withstand this attraction or repulsion, the wall must exert a minuscule force. But it doesn't matter whether the universe is expanding or not.
    Last edited: Jun 10, 2010
  11. Space is not expanding uniformly.

    I simply can't make sense of what you are saying. Expanding space is locally exactly the same as nonexpanding space? How does that work?

    Where does relative velocity enter the realm of space expansion? Our best guess is that dark energy has something to do with the possible negative energy density of the universe. The acceleration of galaxies away from us is interpreted as the effect of this negative energy density force overcoming the attractive force of gravity.
  12. DaveC426913

    DaveC426913 16,534
    Gold Member

    I did not imply it was.

    Space is not doing anything. The idea of the "fabric" of space and what it might be doing - as anything other than an analogy - is misbegotten.
  13. Misbegotten? The idea of the fabric of spacetime is nothing more than an analogy? I'd love to see the proof from which you extract that.
  14. Ich

    Ich 1,931
    Science Advisor

    There is nothing, absolutely nothing, in space that could betray "expansion". The concept of velocity does not apply to space. Objects can move, space can't.
    Spacetime can be curved, and this curvature can be measured locally. Spacetime curvature is not expansion. Flat, empty space may be "expanding". It's just a matter of the coordinates you use.
    Well, those galaxy clusters are moving away, aren't they? And their velocity is proportional to distance, at least in our vicinity (say, some GLy out).
    You will encounter people that tell you "they are not actually moving, it's rather space expanding" or so. Ask them how they can tell expansion from motion. If there's a galaxy some 100 MLy away, what measurement establishes that this galaxy is not moving away?
    They are actually moving away from us, and that motion is called expansion.
    Yes. I explicitly mentioned DE and its implications. But acceleration is not expansion. Expansion is velocity, the biggest part of it caused a long time ago by inflation.
  15. This link might help understand what Ich is saying or at least clarifies this confusion that has affected not only lay people but experts too (BTW I found the link in this forum)
  16. DaveC426913

    DaveC426913 16,534
    Gold Member

    The onus is on you to show the idea has substance and is not merely an analogy.
  17. I believe we must all recognize we are discussing the interpretation of observation. I put no such onus on myself, I merely contend that we cannot simply dismiss such concepts as "the fabric of spacetime". The onus is in fact on you to disprove it, as you are the one who claims it misbegotten.

    Yes, galaxies are receding from us. It is the idea that they are receding at an ever faster pace that requires further analysis. If I throw a baseball in space, it recedes away from me at a constant velocity. The galaxies, as of what I last heard, are accelerating away. The acceleration is why, it would seem to me, you cannot simply say that expanding space is the same as nonexpanding space. Maybe locally they are too similar to discern from one another, but the very idea of expansion implies more than a constant velocity of recession.

    And this is where the possibility of DE comes in. Different interpretations are always out there.
  18. DaveC426913

    DaveC426913 16,534
    Gold Member

    It can't be dismissed because it was never present. This 'model of space as a fabric' would first have to be more than merely an analogy.

    So, my question then becomes "What do you mean when you talk about this 'fabric of space'? Can you please point me to some literature where this model is used in anything but an analagous way?"
  19. The way I've understood this is that the "fabric" is just merely the unification of space and time dimensions into Spacetime. To say this model was never presented... i'm confused. The literature is all around us...
  20. DaveC426913

    DaveC426913 16,534
    Gold Member

    It is an analogy, to help us understand such things as curvature of space. But analogies break down; you cannot infer from an analogy back to real life.

    Likewise, the expanding balloon analogy of the universe is an analogy; it helps us visualize how we can appear to be at the centre of our universe when in fact we are not in any such privileged position. But you cannot infer from the analogy such things as 'OK, if the balloon is a sphere, then it has a centre, right?' No. The analogy breaks down. The universe isn't really a balloon.

    Likewise, there is no actual property of space that is expanding and carrying things with it. The only things that can move are matter and energy. We measure expansion of the universe by measuring the increasing distances between objects. The space between the objects has nothing to do with it.
  21. Ok.. then by all means rewrite the books on Relativity. What do you propose then, instead of the dimensions of space and time? As humans we defined ourselves to be moving through dimensions. It's not an analogy of anything..

    Your last sentence is quite a bold statement, the truth is that we don't necesarily know, that is just what most agree on. That's the concept of dark energy.
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