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

Universe is expanding like a balloon

  1. Sep 7, 2014 #1
    I've seen the example when people say the universe is expanding like the surface of a balloon. You have dots on that balloon which are galaxies and as you blow up that balloon they grow further away from each other. If the universe is expanding equally isn't there a centre which is constantly moving?
  2. jcsd
  3. Sep 7, 2014 #2


    User Avatar
    Science Advisor
    Gold Member

    If you think of the balloon's surface only as the universe(as the analogy requires you to do), then where would you say the centre can be?

    Have a read through this:
    It's one of the best explanations of the balloon analogy out there.

    We also have a FAQ in the cosmology section of the forums and sticky thread about the balloon analogy(therein). A forum search should net you a number of threads asking similar questions.

    But honestly, just read the Lineweaver-Davis article. It's great.
  4. Sep 7, 2014 #3
    Though the universe is 3-D, what I don't get is how it expands equally without having a centre. I have seen the raisin bread analogy, isn't there a centre to that raisin bread? The univserse is expanding equally in all directions, how is there no centre?
    Last edited: Sep 7, 2014
  5. Sep 7, 2014 #4


    User Avatar
    Science Advisor
    Gold Member

    All analogies are imperfect, and should not be considered valid representations of anything else than the narrow range of ideas they are meant to convey. They are bound to be flawed in one way or another, and the flaws ought to be always kept in mind, so as not to allow yourself to be confused. For a comprehensive, rigorous description, one needs to refer to math. There's only so much you can understand about the world in terms of everyday objects. Once you delve deeper, you just have to accept that e.g., elementary particles are not bouncing balls, electrons are not little planets orbiting the nucleus, space is not made of fabric, the universe is not a balloon etc.

    With that in mind, let's get back to the analogies.

    The raisin bread analogy aims to show how expanding space(dough) between the galaxies(raisins) equally everywhere can produce the observation of all raisins moving aways from each other, with the rate of recession being proportional to distance. While it's got the advantage of representing expansion in three dimensions, it does not aim to show the curvature of space at all.

    The balloon analogy does show that, but unlike the former, it is a 2-dimensional representation of the 3-dimensional space. It's the only way we can picture the curvature of space - by embedding lower-dimensional spaces in out familiar 3-d environment.
    But here, one needs to discard the notion of the 3rd dimension being of any physical significance. The embedding is not required mathematically for the 2-d surface to posses curvature. It's only a tool to let us picture the curving space.
    So, in the 2-dimensional world of the balloon's surface, there is no centre. Wherever you stand on the surface, you observe the same thing - all galaxies(dots) are receeding from you with rate proportional to distance.
    Once you start including the 3rd dimension, you've taken the analogy too far.

    I strongly advise you to read that article I linked to earlier. It's in its entirety about dispelling all the misconceptions that the balloon analogy comonly breeds. And while I could go on an on about explaining the same, I'm unlikely to do it with equal clarity and efficiency.
  6. Sep 7, 2014 #5


    User Avatar
    Gold Member

    I suggest that you check out the full discussion of the balloon analogy in my signature.

    The PDF that bandersnatch linked to is also good.
  7. Sep 7, 2014 #6
    say that matter has been shrinking/collasping/losing energy since its creation...if empty space remains constant but the galaxies are shrinking..would this not appear to be an explanding universe?
    "Some galaxies recede from us at 1,000 kilometers per second, others (those twice as distant) at 2,000 km/s"
    those galaxies twice as distant are younger, or atleast the light we observe is from a younger version of them--i'm positing that during that past period matter would have been more energetic and therefor shrinking twice as fast, so it looks like they are moving away from us twice as fast..if we could see the light of their present i'm betting we wouldn't notice any inflation at all..like our neighboring galaxies and whatnot. ridiculous notion ofcourse, as theres no way to prove if matter is collasping at a constant rate since the instruments themselves are made up of said matter. but i think it makes more since than dark energy, to me anyway
  8. Sep 7, 2014 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    I disagree. I think it is at least equally ridiculous as dark energy. Besides, this only redirects the question away from dark energy and expansion, to why a variable rate of shrinkage exists and how it works.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook