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

Universe center of mass?

  1. Oct 15, 2011 #1
    We are learning about centers of mass now and it made me wonder, does the universe have a center of mass? If so , what is its meaning? Is it place where "big bang" occurred?

    Also, on a related note, could the fact that the universe is expanding at an accelerated rate be explained by a net force acting on our universe?
     
  2. jcsd
  3. Oct 15, 2011 #2

    phinds

    User Avatar
    Gold Member
    2016 Award

    Since the universe has no center, it is not possible for it have a center of mass

    Universal expansion IS explained by a net force (or SOMETHING) which is called "dark energy".
     
  4. Oct 15, 2011 #3
    ohhh so thats what dark energy is :D

    And how do we know that the universe doesn't have center? If the universe is finite in volume, and if the amount of mass is finite, then there must be a center of mass?
     
  5. Oct 15, 2011 #4

    phinds

    User Avatar
    Gold Member
    2016 Award

    We don't know that the amount of mass is finite or that the universe is finite. If the universe IS finite, that does not imply a center.

    The current theory of cosmology says the universe, whether finite or infinite, has is homogenous and isotropic and therefore has no center and no boundary (no "edge").

    You would likely find it informative to read the FAQ section in cosmology.
     
  6. Oct 16, 2011 #5
    but if our universe evolved from the big bang (which consisted only of internal forces), then shouldn't the centre of mass lie at the place where the big bang occurred?
     
  7. Oct 16, 2011 #6
    In one sense, the big bang occurred everywhere in space. There is nowhere you can point to. Check out the balloon analogy. If the entire universe is the surface of the balloon, you cannot point to a place on that universe (again, just the surface) where the expansion started.
     
  8. Oct 16, 2011 #7

    phinds

    User Avatar
    Gold Member
    2016 Award

    Jack21222 has explained it correctly ... you would do well to read the FAQ in the Cosmology section ... since you don't yet have a handle on the basics of cosmology, you'll find it very helpful.
     
  9. Oct 16, 2011 #8
    Now I think about it though, I does seem odd that it doesn't have a centre of mass. I get the no centre of the universe, due to space curving in a circle, but newton's laws rely on something having a centre of mass, as far as I can remember. If you subject the universe to a force or something, how will it react?
     
  10. Oct 16, 2011 #9

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    The big bang did not occur in some vast ocean of empty space. It created space as well as matter. The cosmic microwave background [relic radiation from the big bang] is equidistant is all directions, so either we just happen to be smack in the center of the observable universe [improbable], or, the CMB is equidistant from every point in the universe [the accepted explanation].
     
  11. Oct 16, 2011 #10

    phinds

    User Avatar
    Gold Member
    2016 Award

    Uh ... "subject your universe to a force" Now THAT would be a good trick

    You can't apply Newton's law to the universe as a whole because it has no edge, no boundard, no OUTSIDE, so you can't subject it to a force.

    Space does not necessarily curve and if it does, it's not in a circle. It might just go on forever or it might wrap back on itself in some way but a circle is a 2D object, so impossible
     
  12. Oct 16, 2011 #11
    You could still find the centre of mass of the universe, if there was a finite amount of matter in it, and space itself wasn't so curved as to make it ambiguous. There are of course practical problems, one of them being that you only know the position of other galaxies in the past, and can only see to the horizon.
    But another problem is that a centre of mass is less meaningful in general relativity, because mass is equivalent to energy... so you'd want to sum the energy of the universe... but then I think that the total energy of the universe is 0:
     
    Last edited by a moderator: Sep 25, 2014
  13. Oct 16, 2011 #12

    DaveC426913

    User Avatar
    Gold Member

    No you could not, even with your premises.

    The most accurate thing to say is that everywhere is the centre of the universe.
     
  14. Oct 16, 2011 #13
    OK, I'll take your word since you didn't elaborate. Maybe you're referring to the universe appearing differently in different reference frames.
     
    Last edited: Oct 16, 2011
  15. Oct 16, 2011 #14

    DaveC426913

    User Avatar
    Gold Member

    If you were to choose a point and try to measure the distance to all matter, you would determine yourself to be at the centre. If you were to fly a billion light years west and do the same experiment, you would once again determine yourself to be at the centre.

    No matter where you are, you're at the centre.
     
  16. Oct 17, 2011 #15

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    Let's be fair, you are also at the temporal edge [most ancient region] of the universe no matter where you are. This often creates confusion. A galaxy a billion years distant was also a billion years younger when the light we now observe was emitted. That means their CMB was a billion years younger at that time. For observers in that galaxy, the universe 'now' is a billion years more ancient than the one we currently observe.
     
  17. Oct 17, 2011 #16
    Of course, it would be impossible to tell, if that hole was patched up and so smooth it resembled a sphere. But that doesn't mean there wasn't a place that started it.
     
  18. Oct 17, 2011 #17

    DaveC426913

    User Avatar
    Gold Member

    Are you stumbling over the idea that a balloon has a spigot? Are you unable to make the leap to a balloon that has no spigot?

    OK. Then imagine a soap bubble instead. Pretend it's being warmed by the sun so that the air inside it expands, causing hte soap bubble to expand without any netry point.

    Better?

    Yes. It does.
     
  19. Oct 17, 2011 #18
    That's only valid if the soap bubble always existed as a bubble. We're talking about t = 0 here, just before the balloon of space and time was created. Obviously, the universe didn't always exist as a "bubble" or whatever shape; there was an entry point purportedly caused by the Big Bang, the so called spigot.
     
  20. Oct 17, 2011 #19

    phinds

    User Avatar
    Gold Member
    2016 Award

    Thinking about it that way leads you to conclude that there was a point-origin for the big bang, but there wasn't.

    You also talk about t=0, but there is no current model that gives the slightest clue what was going on at t=0 and that makes your proposal personal opinion and overly speculative according to the rules of the forum.

    If you come up with a justifiable and falsifiable theory of what happened at t=0, you would make one hell of a big splash in physics, but your current proposal of a personal theory isn't going to do it.
     
  21. Oct 17, 2011 #20

    DaveC426913

    User Avatar
    Gold Member

    "Entry point"? What makes you think there was an entry point?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Universe center of mass?
  1. Center of the Universe? (Replies: 10)

  2. Center of mass of (Replies: 14)

Loading...