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Expanding Universe Question

  1. May 14, 2008 #1
    Assuming we are not at the center of the universe:

    If V is the vector from the center of the universe to us, then -V would be the vector from the center of the universe in the direction away from us. Wouldn't a star (or whatever) on the -V vector be accelerating away from us at a larger rate than other objects, and especially objects on the +V vector that are further away from the center of the universe than we are? This is obviously assuming that everything in the universe is expanding from a single point.

    Can we observe (through redshift or some other means) where the center of the universe is and where we are in relation to it?
  2. jcsd
  3. May 14, 2008 #2


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    You shouldn't assume that the universe is "expanding from one point" since this is not true. There is, in fact, no such place as the centre of the universe; the cosmological principle does not allow for it.

    Welcome to PF, by the way.
  4. May 14, 2008 #3
    Thanks... can you explain why the cosmological principle doesn't allow for a center of the universe?
  5. May 14, 2008 #4


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    The cosmological principle basically says that there exists no special place in the universe. Since a centre would be a special place, the cosmological principle tells us that such a centre does not exist.
  6. May 14, 2008 #5
    you cant point any place and say, big bang happened here..
  7. May 14, 2008 #6
    Why not ?
  8. May 14, 2008 #7
    Actually, you can point to anywhere and say "the big bang happened here".
  9. May 14, 2008 #8
    If there was a center (which there may not have been), we have no clue as to in which direction it may lie. However, we do have a good idea of its distance, and how long ago we separated from it.

  10. May 15, 2008 #9
    Hubble bubble ...

    The expansion of the spacetime manifold is the same everywhere. That is, the Hubble expansion (a local description) is the same everywhere, for a given stage of what might be referred as the Big Expansion of universe.
  11. May 15, 2008 #10
    You must not imagine that the big bang was like the explosion of some gigantic firecracker and that, in principle at least, you could have stood to one side and watched. There was no "one side" because the big bang represents the beginning of the spacetime itself. So, from the point of view of our present universe, there is no position in space to which you can point and say, "the big bang happened here." It happened everywhere.
    Moreover, there is no "before the big bang", because the time began with that creation event. In this context, the word "before" loses its meaning. We can, however, conjecture about what went on during succeeding intervals of time after the big bang.
  12. May 19, 2008 #11
    It's intuitively hard to understand that there need not be a "center" of the universe from which virtually everything is expanding. I have seen two graphic demonstrations, one by Alex Filippenko and another by some other professor, that adequately demonstrate the lack of a need to have a "center". I'm sure you would see that a "center" is not necessary if you could see these demonstrations; I'll try to use some words to describe one of these illustrations.

    Take a sheet of paper and draw many circles of various sizes on it, such that you can imagine that each of the circles is a galaxy. Now, use a copier and make a copy that is 15% larger. (If this second copy can be made on a viewgraph transparency, the effect is even more obvious). Now overlay the "expanded" universe on the other and notice what happens to where the galaxies are. No matter which two galaxies you use to match the locations (one from the first image and the other from the viewgraph slide image), all the other galaxies seem to be expanding from this hypothetical center. The meat of this is that you can't find any location on the first image from which a "center" can be defined such that the expansion isn't seen in the second image, regardless of the galaxie you chose to be the "center".

    I hope I've described this clearly enough that you may try it; it's quite eye-opening. But the central piont of this demonstration is to debunk our intuitive notion that a "center" has to exist; further, it easily demonstrates how every point can be considered the "center" in an expanding universe. Obviously, if the universe had edges and we could view an edge, there might be a preferential location, but most cosmologists I know discount the notion that the univese has an "edge" that we could ever discern.

    I hope this helps.
  13. May 19, 2008 #12
    I've often been confused about the relationship between time and the Big Bang. Over last several years, I've discussed it with others and have arrived at the following understanding: Time, FOR OUR UNIVERSE, did not exist before the Big Bang. But Time, for the HyperVerse or the Branes or whatever, must necessarily have existed before the instantiation of the Big Bang event. But this is time that will never be "viewable" by us (at least with technology and science we currently understand).

    Is my understanding consistent with yours?

  14. May 20, 2008 #13
    Einstein initially proposed a forth physical dimension to space. In this hypothesis if you were to travel in a straight line you would end up where you started. According to this "curved space" idea there could be no center to the universe. Some Alternative cosmologies to the BB that also ascribe to a finite universe in size, assert that the center of the universe would be the center of its mass. These theories are called flat-space theories. No theories that I know of believe that the center of the universe could be found. This is because for a finite size universe it could be countless millions of times larger than what we can observe.
  15. Jun 23, 2008 #14
    Hello All,

    I'm new to this forum, and compared to all of you here, I would be viewed as an elementrary student if I tried to hold a conversation in this subject, so please try to make any responses as elementary as possible.

    I seem to have a problem trying to comprehend the following; Is the universe flat or round? If (hypothetically) we could travel at many billions times the speed of light, would we fall off the edge, or end up right back where we started? If there is an edge to the universe, is there anything outside (beyond) this edge? If not, what is nothing? If the universe is round, and expanding, shouldn't there be some kind of outer edge? Shouldn't there be something outside of this edge? White, Black, Conciousness, Another Universe, God?

    Could it be possible that our universe is simply a photon (or similarly sized particle) in some much, much, larger universe that we have not yet or possibly never will detect?

    Last question... If time and space or spacetime was created at the big bang, was this (ball of matter/energy) or whatever the big bang expanded into just floating around in "Nothing?" until it exploded and expanded into the universe we are aware of today? What was outside of this planck sized? ball of mass?

    I look forward to hearing everyone's thoughts, and please try to take it easy on the technical aspects, like I said before, I'm wouldn't even say that I'm a Novice in this field yet.
    Last edited: Jun 23, 2008
  16. Jun 24, 2008 #15
    Re: Hubble bubble ...

    Would such manifold still be equally expanding in a locality where there is gravitational contraction, such as for a cluster etc.? Would it help to focus herein on gravitation as curvature?

    If there is more concentrated mass, then more curvature. But couldn't such curved surface still be equally expanding (i.e. stretching}?
  17. Jul 5, 2008 #16
    There universe is not "flat or round" but this better be stated as "curved or not curved (ie. 'flat')".
    Measurements can not really tell, which means either that it is flat or almost flat, which indicate either an infinite universe, or very large (magitudes larger then the Hubble sphere, the observable universe).

    The universe, wether finite or infinite, has no edges or boundaries.
    If you squeeze the size of the universe in one direction, making it effectively a 2D space, it would fit around a sphere (like the surface of earth). The radius of that sphere is very large or infinite.

    There is no "outside" of the universe.
    There might be higher dimensions, but that is another topic (string theory suggests that at very small -atomic- scales, there might be another 7 space dimensions).

    There has also been suggested that the universe is in fact a fractal, containing copies of itself.

    Unless there is a way of testing this, that is hard to say.

    The suggestion is that none of that took place, and time and space were already there. Although the very idea that Big Bang = simultanious creation of time,space and matter has been popularized, and even some cosmologists promoted it, this turns out to be not the case.
    It is understadable that this misconception arise, since it follows from the Einstein equations that there is a singularity at time = 0. (*)
    Yet, General Relativity itself happens to break also near that point, and then we also have to deal with Quantum Mechanics. These theories are both fundamental but they are not compatible, which therefore makes it necessary to establish a new theoretical framework.
    So this makes the situation very much more complicated, and you can not simply draw the line back untill you have a precise (dimensionless) point, because the theoretic framework can not handle that situation.
    But this idea nevertheless has stuck.
    What realy happened, is still theory in progress
    Currently the idea is that spacetime itself inflated (= fast expansion) in the early phase, and after some rapid expansion the field that drove this inflation decayed, and the energy released as particles and radiation. Then normal expansion took over, decoupling, and matter formed (quarks, atoms, etc.).
    When universe was sufficiently cooled and less dense photons were free to move which is what we see now as Cosmic Microwave Background Radiation.
    Inflation theory was successfull because it explained why the universe was so large (because of the initial very rapid expansion) and homogeneous (the rapid expansion got rid of all the inhomogeneities).

    Compare this for example with Newtonian physics. If the distance between two point masses becomes zero, there is also a singularity. In reality however that never happens, as real masses are not zero, and also because there is the electromagnetic force.

    A good start for learning more about the Big bang theory is Ned Wright's cosmology page.
    Last edited: Jul 5, 2008
  18. Aug 14, 2008 #17

    In an infinite universe, the statement "no boundaries" makes perfect sense, but in a finite universe, the statement "no boundaries" makes "no sense" to me. How can something that is finite not have boundaries? Did you mean; even if we live in a finite universe, due to the laws of physics and "our perspective," that there is no possible way of ever reaching this boundary or "edge", therefore it is unreasonable to even ask such a question?

    I don't know if this is what you were trying to say or not, but this subject is something that I am very intereted in, so let me know your thoughts.

    Regarding the higher dimensions; this may take a second to sink in, but what would be the exact opposite of these supposed "higher dimensions" higher = smaller than we can find? or invisible to the human eye? What if there were "lower dimensions" that were so massive, that we could not begin to comprehend while "trapped?" within our known universe? (I know that this is an unreasonable question to even ask, but what the hell?)

    I think that I may be looking at things from too much of a philosophical viewpoint as oposed to a scientific viewpoint. If anyone out there would like to respond to my questions posted here, or previously asked in this forum, it would be greatly appreciated.

    Thanks in Advance
  19. Aug 14, 2008 #18
    I'm not going to say that you are wrong about spacetime beginning with the big bang, but I am curious as to your sources for believing this. Have you done any research on this theory lately? I am finding nothing but speculation regarding this kind of thinking.
  20. Aug 14, 2008 #19


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    No. Jeez, we've got two of these going at once! We need a sticky for this.

    Ask yourself this: does a plane flying around the earth ever reach an edge of the earth?

    The same concept applies to space, but in 3d instead of 2d.
  21. Aug 14, 2008 #20


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    Certainly, from within a uniformly expanding homogeneous universe, any point will perceive itself to be the centre of expansion. Equally, modelling the universe as an explosion would appear to create some major problems given the measured uniformity of expansion and absence of blast waves, e.g. acoustic peaks? So my questions are not intending to challenge the collective wisdom that generally supports both statement above, but I would like to break the issues of an expanding universe into a number of questions that relate to the nature of a homogeneous expansion and the implication of there being no centre. We might initially start from the simplicity of a Newtonian approximation, where the `force` of gravity is defined as:

    [tex]F = GMm/r^2[/tex]

    The implication of this equation is that [r] is typically the radius from the centre of gravity of a larger mass [M]. However, on the very large scale of the universe, where homogeneity is assumed, there appears to be an implicit assumption that the universe can have no centre of gravity. Another theorem of Newton’s explains how an infinite universe may have no centre of gravity, e.g. Newton’s shells, but it is not clear to me how a finite universe avoids this concept.

    What I mean by uniform expansion is the assumption that each unit volume of space expands in time in accordance with the Hubble constant [H], although the actual rate may change with time. The other interesting thing about this expansion is that it does not work on all scales, such that atomic nuclei don’t expand, atoms don’t expand; neither do solar system or entire galaxies, only the large-scale space between galaxies. If so, is it conceptually possible to define a ‘force of expansion’, i.e. it is less that the gravitational force holding a star within a galaxy but less the gravitational force between two distant galaxies?

    While modelling the big bang as an explosion is more than problematic, it is noted that a Newtonian derivation of the Friedmann equation can be based on the conservation of energy of a unit mass with respect to a centre of gravity [M]. This appears analogous to an explosion and also leads to questions concerning the violation of SR, i.e. the mass can end up travelling faster than light with respect to [M]. However, the approach appears to produce a reasonable approximation of the expansion of the universe, at least, within the matter-dominated era. Again, this is just an observation for clarification not an assertion.

    If I define a spherical volume of space, which separates two galaxies (A) and (B), these galaxies can end up moving away from each other with a velocity > [c]. The issue of SR violations is said to be resolved because in a localised frame, the speed of light is always [c] and the velocity of the galaxy is < [c]. I am assuming that this frame of reference also resolves the issue of energy and momentum associated with SR?

    Finally, in the absence of any centre of gravity, must I assume that any slow down of the expansion of the universe, now thought to be cause by dark energy, is due to localised gravitational effects. For example, our 2 galaxies (A) & (B) are being pushed apart by expanding space, but does the gravitational attraction between the 2 galaxies still effectively slow the observed rate of expansion perceived by an observer? While I can picture this model with just 2 objects, in reality, all mass objects in the universe are interconnected, given the caveat of gravitation propagation equal to [c], such that the net gravitational force on our 2 example galaxies might still be zero, even under expansion?

    Appreciate any clarification of these issues.
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