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

Expansion of the Universe

  1. Sep 12, 2006 #1
    I was wondering how much we know these days about how the universe is expanding. Has space actually been stretching since the big bang? Or is new space being created somewhere? Or maybe the matter and energy of the universe is just expanding into space that already exists... Or is the matter and energy creating new space itself as it pushes outward?
  2. jcsd
  3. Sep 12, 2006 #2
    I believe I've read that space is stretching, but this somehow doesn't make sense to me. We can't percieve the stretching of space ourselves, can we? If everything is expanding, then we are too, but relative to everything else, we wouldn't notice any difference. So, how could we actually observe things getting farther away?
  4. Sep 12, 2006 #3
    Consider coins glued just in one point over a balloon; if you inflate the balloon what you see is the bi-dimensional view of how the universe is behaving. At least @ my understanding.


    Universe it's so simple that it's almost impossible for us to understand it
  5. Sep 12, 2006 #4
    Hmm... but what if instead of coins, you make little marker dots all in one area on the surface. As you blow up the balloon, the marker dots expand too. I guess the space between the dots will expand also, but would a stretched dot be able to percieve the difference? It would look porportionally the same, it seems like, wouldn't it?
  6. Sep 12, 2006 #5
    yes, but the universe seems not to follow this way. Don't ask me why...


    Universe it's so simple that it's almost impossible for us to understand it
  7. Sep 12, 2006 #6


    User Avatar
    Science Advisor

    Rulers (as well as other material stuff) do not co-expand with the universe. How would we, as you mensioned, otherwise be able to measure the expansion?
    The expansion only takes place on large scales where our universe can be considered homogenous and isotropic. For example the planet orbit radii in our solar system do not co-expand, and neither do our galaxy or even our local group of galaxies.
  8. Sep 12, 2006 #7
    Ok. Maybe space is expanding, but where there's matter, it isn't expanding as rapidly because of gravity. Matter stitches space together. Then expansion would cause space to be stretched more near matter, and increase gravity (or maybe space stretching outside of matter is responsible for gravity?). Did I read somewhere that gravitational constant may not be constant over time? Or was that debunked?

    This is too much speculation based on not enough knowledge, I know. I'll do some reading. Sorry...
  9. Sep 12, 2006 #8
    Ah... thanks for your response EL. I wrote my last one before I read it...
    So, the universe isn't expanding uniformly. I see..
  10. Sep 12, 2006 #9


    User Avatar
    Science Advisor

    The thing is that the expansion comes from a solution of the equations of General Relativity for a homogenous and isotropic matter distribution. And our universe is only homogenous and isotropic on very large scales. On smaller scales (like the solar system) where we see "clumpiness" in the matter distribution this expanding solution does not hold anymore. The solution is instead static and no expansion takes place.
    But on large enough scales, the expansion is uniform!
    Last edited: Sep 12, 2006
  11. Sep 12, 2006 #10


    User Avatar
    Gold Member

    It is not as complex as that. There is not a mystical connection between the amount of mass in a location and how much the space is expanding there.

    The balloon-with-pennies analogy is correct.

    The expansion of space is extremely weak, much weaker than any forces between atoms, molecules and even weaker than gravity. It acts everywhere, it just doesn't have an effect everywhere.

    The analogy to the balloon is that the glue holding the pennies onto the balloon is much weaker than the metal of the pennies. While the balloon does pull on the pennies (by way of the glue), the force has an inconsequential effect on them.
    Last edited: Sep 12, 2006
  12. Sep 12, 2006 #11


    User Avatar
    Staff Emeritus
    Science Advisor

    Several suggestions:

    First of all, try Ned Wright's cosmology FAQ. I've cut and pasted it here so you can get an idea of what the whole thing is, but the original has links for additional reading.


    The short version - think about observable differences (if any) between space expanding and the objects moving away from us. If you can't find any, a difference that makes no difference is no difference.

    Note also that the solar system is NOT expanding, no matter how you look at it.

    A final note: you might also want to try the cosmology forum.
  13. Sep 12, 2006 #12
    Awesome! Thanks for the references.

    Yes... I should have posted in the Cosmology forum. Actually, when I first posted, I was thinking I was going to ask a question about how light travels through space, but then realized I had questions about space first.
    And I now realize that I don't really know enough to be asking good questions yet! Thanks all...
  14. Sep 13, 2006 #13


    User Avatar
    Gold Member

    No, you're doing fine!

    The thing about asking is that, even if you don't get to the answer right away, people are happy to guide you there.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook