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

How could the universe expend directly after the big bang considering the total mass?

  1. Feb 13, 2008 #1

    I'm wondering, how could the universe continue to expend directly after the big bang considering the total amount of mass confined to the relatively small space.

    As an example consider that it takes 8 minutes for light to travel from the sun to earth. If at the beginning of time someone was to stand 150million km away from the big bang (distance from the sun to early) it would take 8 minutes for the first energy/particles to get to him. So consider the situation after 8 minutes. You have all the mass of the universe confined to the area within that sphear. Given a rough estimate of the total mass of the visible universe (excluding any dark matter) we have about 2E(52) KG. That means anyone standing 150M KM away would experience an acceleration of about 6E(19) m/sec [square]. At that intensity the gravitational field slows down any particle trying to escape it's field from the speed of light to 0 in 5E(-12) seconds. Considering that the rate of change of the gravitation field at that point is relatively slow (due to increase in radius), how could anything have escaped this force. Why didn't the universe just collapse onto itself?

    Let me know your thoughts.


    {as an addendum.. I used the mass of 2E52 for the total mass of the visible universe assuming our sun is an average sun, and our galaxy is an average galaxy).
    Last edited: Feb 13, 2008
  2. jcsd
  3. Feb 13, 2008 #2
    If you assume the universe is infinite, then the problem is avoided. Otherwise a considerable force is required to maintain the expansion of the universe
  4. Feb 13, 2008 #3


    User Avatar
    Staff Emeritus
    Science Advisor

    The problem you appear to have is that you are assuming that the big bang was an explosion of matter in space, like a normal explosion would be. However, this is not true. The big bang occurred at all points in space and since then it has not been matter that has been expanding into space, but space(time) itself that has been expanding.
  5. Feb 13, 2008 #4
    Thank you the explanation, I think I understand this concept, however I'm not sure I totally buy this explanation.
    Last edited: Feb 14, 2008
  6. Feb 13, 2008 #5
    But space-time expansion does not resolve the question of why the matter, being so concentrated in a small volume, does not gravitationally collapse upon itself -
  7. Feb 13, 2008 #6


    User Avatar
    Science Advisor

    Which point would the matter collapse to? The early universe is homogeneous (the same everywhere) so even though the density is high it is the same everywhere so there is no preferred point for everything to collapse to. You are still thinking as though there is a centre of a spherical Universe.
  8. Feb 14, 2008 #7
    Hello Yogi, I think the concept of space time expansion states that during the early universe the Hubble Constant - which is not actually a constant, but a constant varying with matter-energy density [Capital Omega] was VERY HIGH. The Hubble Constant was multiple magnitudes higher then the gravitation field of the early universe. As such even though the force of gravity would have been pulling everything towards the center on the mass, the space was expending much faster and as such the actual distances between the various mass particles was growing.

    You can think of this as two balloons (one inside the other). The outside balloon is the space time which is being expended from the outside (ie the pressure within it is falling). The inside balloon is the matter universe. The inside balloon is open and air is escaping from it [pressure is falling] (ie it should be getting smaller - as should the universe without space time expansion), however it is contained within the larger balloon which someone is making bigger from the outside, much more rapidly then the deflating balloon on the inside. Ie the pressure within the larger balloon is falling faster the the pressure within the smaller balloon, and as such the smaller balloon is ACTUALLY GETTING BIGGER, even though it is deflating. Hope this makes sense!

    However, as wonderful as all that sounds I don't buy it!
    Last edited: Feb 14, 2008
  9. Feb 14, 2008 #8
    Hi Wallace, even if we assume the early universe was homogeneous, then all the matter contained within that universe would have still had a center, and as such it would have to have a center of gravity, and as such without space time expansion it would have fallen onto itself.
  10. Feb 14, 2008 #9

    Don't forget, gravity travels at a finite speed. I think that inflation theory assumes that very early on the universe was much larger than the gravity crossing time. Thus, the universe wouldn't know where its own centre of gravity was. Each isolated region of the universe would be in gravitational equilibrium within its "gravity sphere". Of course, some of the matter near the edges (if edges exist at all) would be compacted and pulled in, but most of the universe wouldn't even be aware of those edges.
  11. Feb 14, 2008 #10
    Even if gravity travels at a finite speed, it would still be affected by space-time expansion, (If a particle can get to the edge of a universe in time t, so can gravity) and as such being at the edge of the universe you should still experience the effects of gravity, even for a particle which remains at the very center. This should be true, unless the theory states that gravity is not affected by space-time expansion, which I think would turn the theory on it's head.
  12. Feb 14, 2008 #11


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It is not too interesting to have to go over the same ground repeatedly, with these very common misconceptions. Maybe we need a system of sticky-threads that everybody should read, so we don't repeat so much.

    What you just said, to take that as an example, is a common misconception. You can't prove mathematically in the two most common cases which are Omega = 1 infinite and Omega > 1 finite. This is assuming, as you do, that the distribution of matter is homogeneous!

    In the Omega = 1 case space is infinite and matter is homog, so matter extends infinitely and obviously has no center-of-mass.

    In the Omega > 1 case space is S3, the distribution of matter is symmetric and again there is no center-of-mass.

    There is a sense in which you are right, if we relax the assumption of homogeneity (already assuming distances are NOT expanding.) If there is a static universe and random fluctuations are allowed to appear in the distribution of matter, then they will eventually grow and stuff will begin to collect. At several randomly located centers. There is still no overall center in either case.

    But that random coagulation is inherently slow and the assumption of a static universe is unrealistic. What we see is an approximately homogeneous expanding system which has no center of gravity, and (extending the model back in time) never has had one.
    Last edited: Feb 14, 2008
  13. Feb 14, 2008 #12
    I agree, a system of sticky threads that everyone should read before being able to post is a very good idea (at least within a category of interest - such as cosmology.

    Further, I don't want to ask questions which I am sure have been answered before, however I haven't seen anything discussing this in particular, and as such you said:

    Omega = 1, meaning the universe will continue expanding however approaching an asymptote, correct? In this case at any point in time you still have an 'edge'.

    I don't understand this statement. No matter which Omega you take, at any point in time there is an 'edge' to the universe. If you stand at the edge you have matter on one side and nothing on the other side, and as such you must surely have a gravitational field. What I seem to have a problem grasping is the statement that there is no 'edge' I'm not sure how that can be.
  14. Feb 14, 2008 #13


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    Shems, with due respect I think you should be asking questions at this point, instead of making pronouncements. What i have seen of your posts involve naive misconceptions and may confuse newcomers to the forum.

    Here you are talking naively about an "edge" and a "center", which makes what you say nonsense from the standpoint of mainstream cosmology.
  15. Feb 14, 2008 #14


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    No Shems, you just do not understand thing one. In standard mainstream cosmology you do not have an edge anywhere. No place where there is matter on one side and nothing on the other. This misconception is, I would judge, the noobest of the noobie misconceptions that we ever encounter here at Forum. I strongly suggest that you ask questions for a while and stop asserting and arguing.
    Last edited: Feb 14, 2008
  16. Feb 14, 2008 #15
    Hi Marcus, I appreciate your answers and like how you answered with 'due respect' :). As such, can you point me to a 'Cosmology for Dummies book', or a 'Cosmology for Dummies' thread? I would like to understand how cosmologists can justify a universe with no end, which initiated from a big-bang. To me these two propositions seem contradictory, but as you say, I'm just a newbie!
  17. Feb 14, 2008 #16


    User Avatar
    Science Advisor

    Clear your mind Paduan.

    Now, what you have got to realise first and foremost is that what you think you have been told about what 'a big bang' is bears no resemblance to the scientific theory that shares the same name. The big bang was not an explosion, it did not happen at one point.

    The big bang theory at its base is simply this "the universe was hotter and denser in the past". That's it. There is no contradiction between this and homogeneity.

    You are adamant that there must be an edge and a centre of the Universe, even if it is homogeneous. This is a contradiction in terms! If there is a centre then that point is different from others and therefore the Universe is not homogeneous. If there is an edge the same reasoning applies.

    The observations we have, and the theories (General Relativity is the most important) tell us very strongly that the Universe is homogeneous and isotropic. We didn't make that up, the Universe told us. Science formulated a theory that was dubiously dubbed 'the big bang'. Popular media then described this as an explosion in complete contrast to everything the science says. Now, you have the explosion at a point picture and find that it contradicts what Marcus and others has been patiently trying to explain about what we think the Universe is like. Of course it does, because it is wrong, utterly and completely wrong. Your notion of what the big bang is has nothing to do with the scientific theory. Once you believe this you may make some progress.

    You can't hope to learn much from here unless you are prepared to leave the very basic misconceptions you've been exposed to so far at the door.
  18. Feb 14, 2008 #17
    Ah, but what if the Big Bang was not an explosion of any kind. What if there was an infinite amount of matter that had more matter around itself and the first infinite amount of matter actually condensed then IMPLODED instead of EXPLODING and causing the expansion of the universe. This would mean that possibly the universe WE know is actually and anti-verse meaning that our universe is actually surrounded by even more life (or no life, but at least matter) and we actually have no reckoning of the size of the universe or the fact that this is the only "-verse"
    Last edited: Feb 14, 2008
  19. Feb 14, 2008 #18


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    It is an old idea, all right. Lee Smolin wrote several papers about it in 1993-1995.
    And then he popularized it around 2001 with a book for general audience called
    The Life of the Cosmos.

    The way in which Smolin's presentation differs from yours is that it has some mathematical (non-verbal) underpinnings and it offers a way to TEST the idea by various astronomical observations.

    If you like the idea, then I suggest you get up to speed on it and, in particular study how Smolin proposed the idea could be observationally tested, and thus could be proven wrong (if it was wrong.) So far the idea has survived the empirical tests which were proposed, but it may yet turn out to be wrong.

    If you can read journal articles, it is cheaper than buying the book. The journal articles are free for download.
    Here is a 2006 Smolin article about this
    click where it says "download PDF" over on the right.
    this paper may not be the best but at the end it will have a bibliography with references to many of the previous writings
    including probably the papers around 1994 where he first launched the idea
    Last edited: Feb 14, 2008
  20. Feb 15, 2008 #19
    Hello Wallace, Marcus,

    Thanks for the insight. I actually never thought the universe had an edge.. I was trying to play a devil's advocate to the big-bang theory as I understood it. Apparently I totally mis-understood it! I though that because scientists were giving the universe an 'age' and because we knew redshift existed, that would imply a point from which things originated. Obviously I was wrong. I will read the papers quoted by marcus. This is good news to me because the current theory might actually make sense to me if I read it!

    Thanks for the clarifications.. this helped.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook