Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Perceiving the expansion of the universe in daily life

  1. Jan 24, 2010 #1

    A few days ago I attended an astronomy lecture and while explaining the basic theoretical background of astronomy the lecturer compared the expansion of the universe from the big bang onwards to a balloon filling with air.

    Since the galaxies/stars are (loosely speaking) dots fixed on the surface of the balloon, they are not actually moving away from each other, rather the distance between them is increasing.

    If so why don't we perceive this in daily life? Supposedly, as I write these lines the distance between my keyboard and I is increasing...

  2. jcsd
  3. Jan 24, 2010 #2


    User Avatar
    Science Advisor

    The distance between you and your keyboard is not increasing.

    You should not think of dots on the balloon, but of ants... all moving around over the surface. The average distance between them is increasing in proportion how far away from each other they are... but they also have their own local movements.

    And the other big difference, of course, is that space is three dimensional, not two dimensional like a balloon's surface.

    Galaxies don't move freely; they attract each other with gravity. A group of gravitationally bound galaxies is no longer expanding apart from each other.

    One of the problems with the balloon analogy is that it gives the impression that space has some material substance that pulls on things to drive them apart. That's not actually true. You could imagine that skin of the balloon has no friction at all, and the ants are just sliding all over it, but this is starting to get too attached to the analogy for its own good.

    I am liking the balloon analogy less and less these days. One of the reasons people sometimes use this analogy is to help explain "curvature", but since space pretty much flat to our best attempts to measure it, this is not such a useful consideration any more. Or it could be used to explain how space could be finite in volume (like the skin of a balloon is finite in area) despite having no edge.

    The ants on the balloon are moving away from each other. The point is that there is no center point to this movement, and no edge or bound to how far they have gone.

    Cheers -- sylas
  4. Jan 24, 2010 #3
    I think I get it... basically everything (including space itself?) was given an initial velocity during the big bang and started moving away from each other.

    However galaxies that are massive enough/within a certain radius are 'gravitationally bound' meaning that their gravitational pull is strong enough to counter the continuous expansion of space.

    Is that about right?

    And if so, what exactly is 'empty space'?.. because once we claim that it can expand or collapse we make it 'real', give it 'substance' and yet to the best of my knowledge 'empty space' is a vacuum... and saying that a vacuum can expand or contract of its own accord without some additional medium for it to expand or contract into sounds, well, wrong... (and also forces the creation of an 'ether' in which 'space' exists... deja vu anyone?...)

    Anyways I'm sure I misunderstood something..
  5. Jan 24, 2010 #4


    User Avatar
    Science Advisor
    Gold Member

    The change in "distance" (loosely using this word, since it's not really well defined without some background in GR) is so small on the scales that we are used to, we wouldn't notice them at all.

    The expansion can only really be seen on the multiple galactic scale (millions of light years or more).
  6. Jan 24, 2010 #5

    Space is expanding because there is simply more space, but expanding space is not driving the accelerated expansion of the universe.
  7. Jan 24, 2010 #6


    User Avatar
    Science Advisor

    That's my understanding as well. The "including space itself" is the tricky bit, but yes.

    When you look at small enough regions of space (which are still humongous) that are large enough to show up the expansion of the universe but small enough that the cosmological recession velocity of any two galaxies moving away from each other is << c, then what you have looks very like a large cloud of stuff that is dispersing. Everything moves away from everything else and the mean distance between things increases.

    There are two ways in which this breaks down.
    • Over smaller scales, the local movements of galaxies become significant, and in particular for a cluster of galaxies that is gravitationally bound there's no expansion at all within the cluster. It's not that gravity is enough to "hold it against the dispersal". Rather, gravity HAS held it together and altered the movements of galaxies so that as a group they are not moving apart. There's nothing to make them move apart either** and so it is not a case of gravity holding it against dispersal. Gravity has stopped dispersal of the cluster and it simply isn't dispersing.
    • Over larger scales, the notions of distance we are used to start to fall apart. You start to need GR to describe space and distance and you start to find different notions of distance diverging from each other. You also find, on large scales, that there's no limit or edge to the galaxies or clusters of galaxies, and so the recession velocities end up being as large as you like, even much greater than the speed of light. And we can still see such galaxies, which is commonly a stumbling block for people first meeting up with this.

    **(Caveat. Dark energy works as a kind of pressure that pushes things apart, a bit like gravity acts as a force to pull things together. The former tends to accelerate expansion, and the latter tends to decelerate it. Without the effects of such forces, expansion would just continue at the same rate forever. These are forces that slow things down or speed them up. A cluster bound together by gravity is not expanding, except that dark energy may still be kind of "force" to push things apart. But note it is not "expansion" that is forcing things apart, but some energy effect that is forcing things apart, and which therefore accelerates expansion.)

    I'm pretty sure I misunderstand something too. Compared with some of the people here, I am an egg. But I still think I can say a few useful things at a comprehensible level, secure that if I go wrong there are folks here who can put me right.

    I very much like John Wheeler's famous two line summary of relativity.
    Matter tells space how to curve.
    Space tells matter how to move.

    This is also often repeated a bit more precisely, using "spacetime" rather than just "space".

    So if people think of space like the skin of a balloon dragging attached buttons along for the ride, they are missing something pretty crucial. "Spacetime" just gives the path along which things fall. Matter also determines what the "spacetime" looks like. So one should probably think just as well as of the skin of the balloon inflating because it is being dragged along by the expansion of the buttons.

    But you're right. Space isn't a material thing. It does have geometric properties, and it gets mixed up with time so that mathematically its better to describe the geometry of spacetime. And on small enough scales you can still get all the same results to very good accuracy using the older Newtonian ideas of gravity as a force between particles moving around in a space described with a Euclidean co-ordinate system. Even the expansion of the universe can be described like that, at least with regions where distance and velocity still behaves itself.

    Cheers -- sylas
  8. Jan 25, 2010 #7
    Surprisingly enough, I understand everyone's comments :-)

    And if someone could enlighten me with a description/definition of 'spacetime' I think that would just about wrap it up, otherwise I'll leave it in the 'inaccessible for laymen' section....

    Thanks anyways (especially to sylas)!
  9. Feb 2, 2010 #8
    Expansion of the universe could be likened to something being slingshot. E.g. if you had a ball in your hand and threw it, the speed, distance and tracjectory are contingent on how strong you are (what force you can apply), the mass of the ball and any opposing force (none in space). The more force the faster the object will move. However, the inherent make up of the object, or any bits and pieces in it or attached to it wont change.... So the big bang could be ananlogous to you and the earth (e.g.) could be likened to the ball...

    And they have worked out the rate at which the universe is exanding, but its going faster than the mass can explain. Hence the reason why the discovery of dark matter/dark energy is so insteresting cos they think it might account for the missing mass and correlate the mass of the universe with its rate of expansion....
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook