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Thread: Size of the Big Bang singularity?

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    Size of the Big Bang singularity?

    It is often said by laypersons and scientists alike that "all the matter in the universe originated from a single point", or words to that effect.

    How does that fit with the notion that the current entire universe is possibly/likely infinite in expanse?

    I figure that no matter how much an infinitely large volume is compressed, it can not become finite in expanse.

    Would it not be more accurate to say that the observable universe (being finite in expanse) originated from a single (very small, singularity-like) point, and the entire (infinitely large) universe originated not from a point but from an infinitely large 'primordial universe' that has the same properties as a singularity-like point wrt density (but not wrt size)?

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    Yes, or something along those lines.

    http://www.astro.ucla.edu/~wright/infpoint.html

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    Quote Originally Posted by noncryptic View Post
    How does that fit with the notion that the current entire universe is possibly/likely infinite in expanse?
    Indeed it's not entirely impossible based on current cosmological knowledge (despite some regulars insisting it is ) that the universe is spatially infinite in scope. Normally this is explained as Ned Wright does in the above link provided by speedfreek.

    Would it not be more accurate to say that the observable universe (being finite in expanse) originated from a single (very small, singularity-like) point, and the entire (infinitely large) universe originated not from a point but from an infinitely large 'primordial universe' that has the same properties as a singularity-like point wrt density (but not wrt size)?
    It is I think correct to say that we know for pretty dang certain that our observable universe originated in "single point" (singularity or not) of the Big Bang event, as most likely did a fairly large amount of more of the essentially same (sometimes known as "the uniform patch"). But as to if this inflation bubble was actually the origin of the entire universe or not, and if it was all there was at the time of the event or not, the answer must currently be a firm "dunno". Obviously there are many ideas regarding the issue but so far none of them appears to be entirely conclusive.
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    Thanks for the replies. I'm happy to see that i got the basics correct here.

    Since i started this thread, i suppose i can ask a few somewhat related questions about the nature of spacetime:

    Can curvature of spacetime due to the presence of mass be alternatively conceptualized as 'compression' of spacetime, in that this curvature increases the density of spacetime?

    Given that empty space is filled with "vacuum energy", and given that energy and mass are equivalent, does spacetime have (or cause) gravity (what i would call 'spacetime self-gravity')? Or is it that vacuum energy is more akin to photons wrt the energy it has, that is to say it has no (rest)mass and thus has no gravitational effect?


    Have i just reinvented "Dark Gravity"?

    Gravitational vacuum energy in our recently accelerating universe
    http://iopscience.iop.org/1742-6596/...161_012026.pdf

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    A couple of points to consider (excuse the pun) is that the term singularity is used to define a zero point. Its a mathematical term rather than a physical one. If the BB originated from a "singularity" then its initial size and density are irrellavent, since size and density would have no meaning at this point. Thats until space and time are created as this point expands, or if you like bursts into existence as we know it (for what ever reason), which we appear to observe today. If the universe originated from a "primordal atom" like state then maybe we could arrive at an approximation of what sort of size it might have been relative to what we observe today, possibly limited by the Planck scale.

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    Quote Originally Posted by noncryptic View Post
    It is often said by laypersons and scientists alike that "all the matter in the universe originated from a single point", or words to that effect.

    How does that fit with the notion that the current entire universe is possibly/likely infinite in expanse?

    I figure that no matter how much an infinitely large volume is compressed, it can not become finite in expanse.

    Would it not be more accurate to say that the observable universe (being finite in expanse) originated from a single (very small, singularity-like) point, and the entire (infinitely large) universe originated not from a point but from an infinitely large 'primordial universe' that has the same properties as a singularity-like point wrt density (but not wrt size)?
    You are correct. If the universe is infinite in size then it could not have ever been of finite size the escape to this is that a singularity is not of finite size. It could have transitioned from no size to infinite in size without ever having a finite size. That is basically how it was explained to me.

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    cryptic,

    ....Would it not be more accurate to say that the observable universe (being finite in expanse) originated from a single (very small, singularity-like) point, and the entire (infinitely large) universe originated not from a point but from an infinitely large 'primordial universe' that has the same properties as a singularity-like point wrt density (but not wrt size)?
    I think such a proposal would not be considered "more accurate." ------- Starting as ".... an infinitely large 'primordial universe' that has the same properties as a singularity-like point wrt density (but not wrt size)" (your quote), could not seemingly be categorized within the mainstream model. Although some BB cosmologists have suggested an infinite universe in expanse presently, such proposals would be considered minority hypotheses. As you suggest such a proposal might be logical but I think this idea would not fall under the present expanding universe BB umbrella.

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    Quote Originally Posted by tnjrp View Post
    Indeed it's not entirely impossible based on current cosmological
    knowledge (despite some regulars insisting it is ) that the universe
    is spatially infinite in scope. Normally this is explained as Ned Wright
    does in the above link provided by speedfreek.
    I assert that the part of the Universe which evolved from the
    Big Bang and is currently participating in the cosmic expansion
    cannot be infinite. If the Universe is infinite, then only an
    infinitesimal part of the Universe evolved from the Big Bang.
    If the whole Universe evolved from the Big Bang, then the
    Universe is finite.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Yes, as per the paragaph you didn't quote I do believe we are in agreement.
    The dog, the dog, he's at it again!

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    Quote Originally Posted by noncryptic View Post
    Can curvature of spacetime due to the presence of mass be alternatively
    conceptualized as 'compression' of spacetime, in that this curvature
    increases the density of spacetime?
    The curvature both compresses and expands-- or stretches--
    spacetime. Diagrams of "gravity wells" show this graphically.
    You see them most often in connection with black holes. The
    presence of matter or any other form of energy stretches the
    space around it in a manner that is in some ways like a weight
    on a rubber sheet. (This is humorously ironic: It is an analogy
    which uses gravity to explain gravity.) The space stretches in
    the time dimension, which is the radial direction. (No, I'm not
    saying time points up and down, or anything like that. What
    I'm saying is at the limits of my understanding, so doubt what
    I can't explain because of my poor understanding!) However,
    the space compresses orthagonal to the radial direction. So
    everything falling into a black hole should be spaghettifiied:
    stretched radially and squeezed circumferentially. The volume
    of the infalling matter remains unchanged, as I understand it.

    Quote Originally Posted by noncryptic View Post
    Given that empty space is filled with "vacuum energy", and
    given that energy and mass are equivalent, does spacetime have
    (or cause) gravity (what i would call 'spacetime self-gravity')?
    Or is it that vacuum energy is more akin to photons wrt the
    energy it has, that is to say it has no (rest)mass and thus has
    no gravitational effect?
    All energy causes spacetime curvature, which is gravity.
    Very interesting question! I suspect that the density of
    the vacuum energy is far too low to cause significant
    spacetime curvature. But I dunno. On the scale of the
    entire Universe....

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Quote Originally Posted by Jeff Root View Post
    I suspect that the density of the vacuum energy is far too low to cause significant spacetime curvature.

    -- Jeff, in Minneapolis
    Wrt empty space i agree, but i wonder if things might work out a little different in the presence of mass.

    At any rate, it looks like i'm not the first to consider "gravitational properties of the vacuum energy".
    http://arxiv.org/abs/0711.0077



    Some thoughts:

    Spacetime that is curved due to the presence of mass has an increased density of the vacuum energy, which causes a stronger gravitational effect than uncurved spacetime.
    This curved spacetime then curves spacetime around the mass even more.
    In absence of some counter-acting force this would cause a run-away effect, in the presence of a counter-acting force (ie Dark Energy) an equilibrium is established (perhaps with the exception of the interior of Black Holes).

    The result then would be that the total gravitational effect caused by the presence of mass is larger than the gravitational effect caused by that mass alone; curved spacetime around a mass has a contributing gravitational effect.
    In other words: the strength of gravity that is measured in any particular case is not all the result of the mass of the object; part of it is due to 'self-gravity' of curved spacetime around the object.

    That effect would be stronger than the gravitational effect of uncurved spacetime, and at scales of clusters and superclusters (perhaps also at the scale of galaxies and SMBHs) it might be significant.




    Btw I have come to the conclusion that what i am trying to describe is not exactly the same as Dark Gravity ("weakening of gravity on the largest scales"), though it might be related. http://arxiv.org/PS_cache/arxiv/pdf/...711.0077v2.pdf (same paper that i link to above).
    Last edited by noncryptic; 2010-Oct-05 at 01:32 PM.

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    Quote Originally Posted by noncryptic View Post
    Given that empty space is filled with "vacuum energy", and given that energy and mass are equivalent, does spacetime have (or cause) gravity (what i would call 'spacetime self-gravity')? Or is it that vacuum energy is more akin to photons wrt the energy it has, that is to say it has no (rest)mass and thus has no gravitational effect?
    The former. The gravitational effect of dark energy is the main factor causing the shape of the visible Universe to be Euclidian.
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by noncryptic View Post
    This curved spacetime then curves spacetime around the mass even more.
    Well, I believe this is consistent with GR - "gravity gravitates". But here you're talking about a local situation - a large body and "its" gravity. The gravitational contribution of the curved space around it is going to be next to nothing. It's when you consider all the space in the visible universe, then the effect becomes significant.
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by Jeff Root View Post
    I assert that the part of the Universe which evolved from the
    Big Bang and is currently participating in the cosmic expansion
    cannot be infinite. If the Universe is infinite, then only an
    infinitesimal part of the Universe evolved from the Big Bang.
    If the whole Universe evolved from the Big Bang, then the
    Universe is finite.
    On what do you base this assertion?

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    I do think it is fairly mainstream to assume that the inflation speed, while it may have technically been superluminal at some point, was always finite so the Big Bang event or part thereof (let's call it Our Big Bang = OBB) that instantiated our observable universe couldn't have created a spatially infinite inflation bubble in a finite time. Thus it appears to be logical to conclude that if the entire universe was indeed created in OBB then it must be spatially finite.
    The dog, the dog, he's at it again!

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    What tnjrp said. If the cosmic expansion were infinite in
    extent, it would mean that parts of the Universe infinitely
    far apart from one another are causally connected to an
    event which occurred 13.7 billion years ago. That strikes
    me as being obviously impossible.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Quote Originally Posted by Cougar View Post
    Well, I believe this is consistent with GR - "gravity gravitates". But here you're talking about a local situation - a large body and "its" gravity. The gravitational contribution of the curved space around it is going to be next to nothing. It's when you consider all the space in the visible universe, then the effect becomes significant.
    So my idea is neither nutty nor ground-breaking.
    I'm still wondering though, is the effect also next to nothing on the scale of super clusters? I think so, but i'm not sure.


    Quote Originally Posted by tnjrp View Post
    Thus it appears to be logical to conclude that if the entire universe was indeed created in OBB then it must be spatially finite.
    If OBB is defined as "the BB event that instantiated our observable universe", and if the entire universe is finite but larger than our observable universe (the latter being very likely), then it seems logical that the entire universe was not created in OBB but in a larger BB event of which OBB was a part.


    Which goes back to one reason for my opening post: i have the impression that there is quite a bit of ambiguity in the use of terminology such as "the universe" and "the big bang". Presumably this is not a problem for scientists because they understand the context. Speaking from experience i can say that for laypersons it is a source of confusion.

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    Quote Originally Posted by noncryptic View Post
    If OBB is defined as "the BB event that instantiated our observable universe", and if the entire universe is finite but larger than our observable universe (the latter being very likely), then it seems logical that the entire universe was not created in OBB but in a larger BB event of which OBB was a part
    It's not immediately obvious to me how you get from the entire universe being bigger than the observable portion of it to "there must've been 'more stuff' besides that which was involved in OBB". The opposite, namely that "the entire universe was instantiated in OBB and is thus finite" is however fairly trivial.

    i have the impression that there is quite a bit of ambiguity in the use of terminology such as "the universe" and "the big bang". Presumably this is not a problem for scientists because they understand the context. Speaking from experience i can say that for laypersons it is a source of confusion.
    The confusion tends to arise from popularized science books and tv-shows I suppose, and I don't tink it's such a big problem in the actual scientific circles. The terms "universe" and "observable universe" or "multiverse" and "universe" for example do get sometimes used interchangeably in even scientific papers tho and a layperson reading them may not be able to determine this is being done as easily as a cosmologist.
    The dog, the dog, he's at it again!

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    Quote Originally Posted by tnjrp View Post
    It's not immediately obvious to me how you get from the entire universe being bigger than the observable portion of it to "there must've been 'more stuff' besides that which was involved in OBB".
    Maybe i am misunderstanding your defintion of "Our Big Bang".

    If OBB caused the observable portion of the universe, and the entire universe is larger than the observable portion of the universe, then i don't see how more than the observable portion of the universe could be caused by the BB event that caused the observable portion of the universe (OBB by your definition as i understand it).

    "the entire universe was instantiated in OBB and is thus finite"
    I don't see how that is possible if the entire universe is larger than our observable universe, and OBB is defined has having caused our observable universe.

    You seem to be saying that something of size x evolved into something that is y times larger than x, and at the same time also evolved into something that is y+z times larger than x. (where x, y, z are positive numbers).

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    Quote Originally Posted by tnjrp View Post
    I do think it is fairly mainstream to assume that the inflation speed, while it may have technically been superluminal at some point, was always finite so the Big Bang event or part thereof (let's call it Our Big Bang = OBB) that instantiated our observable universe couldn't have created a spatially infinite inflation bubble in a finite time. Thus it appears to be logical to conclude that if the entire universe was indeed created in OBB then it must be spatially finite.
    But what if the pre-inflationary universe was already infinite in extent and full of the same stuff as our pre-inflationary volume?

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    You tell us! What do you think would happen?

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    I would expect that the pre-inflationary volume would have expanded! I mean, what else could it do if it was all in a super-dense state?

    If it was full of the same stuff as our part was, I can't see how it could contract, but perhaps I am missing something, or making unreasonable assumptions.

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    When would it start expanding? Say, at five locations, A, B, C,
    D, and E, where locations A and B are a nanometre apart, location
    C is a millimetre away from them, location D is a kilometre from
    them, and location E is a gigametre from them.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Quote Originally Posted by Jeff Root View Post
    When would it start expanding? Say, at five locations, A, B, C,
    D, and E, where locations A and B are a nanometre apart, location
    C is a millimetre away from them, location D is a kilometre from
    them, and location E is a gigametre from them.
    Well, I am way out of my comfort zone here, but as far as I can tell, that all depends on why it started expanding.

    But it seems logical to assume that if the whole universe was similar to the parts we can see, then it might undergo similar processes, for similar reasons.

    Do the contents of the universe need to know that conditions elsewhere are the same before they react to what is going on locally? If everywhere is similar (subject to quantum fluctuations, of course!), then surely if our part expanded, it might have all have acted in pretty much the same way, even if it were infinite.

    Those quantum fluctuations are thought to have introduced the seeds of large scale structure in our universe, but only after they were stretched out by the inflationary epoch to introduce that slight inhomogeneity. So perhaps in an infinite universe the quantum fluctuations elsewhere would cause very different end results outside of our observable universe, but surely that is the case however much larger the fundamental domain is, rather than being relevant to the issue of an infinite fundamental domain. But unless there were large differences in density across that domain before inflation, I do not see how the contents would not start off by expanding, like parts we see did.

    Can anyone provide a source that says the Big-Bang universe cannot be infinite?

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    Quote Originally Posted by speedfreek View Post
    Can anyone provide a source that says the Big-Bang universe cannot be infinite?
    I doubt it.

    When would it start expanding?
    One of the pieces of evidence for the inflationary period is the homogeneity of the observable universe. It is therefore deduced to have come from a sufficiently small volume that it was all causally connected.

    It is assumed that that homogoneity extends beyond what we can see (if only because it would be rather implausible for the bit we can see to be different from the rest). However, IF the universe is infinite, there is no reason to assume that all of it is equally homogenous. In which case there is (as far as a I know - and like speedfreak I am outside my area of expertise) no problem with different parts of the (infinite) dense primordial universe starting to expand (bang) at different times.

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    Quote Originally Posted by speedfreek View Post
    Quote Originally Posted by Jeff Root View Post
    When would it start expanding? Say, at five locations, A, B, C,
    D, and E, where locations A and B are a nanometre apart, location
    C is a millimetre away from them, location D is a kilometre from
    them, and location E is a gigametre from them.
    Well, I am way out of my comfort zone here, but as far as I can
    tell, that all depends on why it started expanding.
    I'm interested in a general answer which shouldn't depend
    on a specific cause.

    Quote Originally Posted by speedfreek View Post
    But it seems logical to assume that if the whole universe was
    similar to the parts we can see, then it might undergo similar
    processes, for similar reasons.
    I will agree with that. My question is, "When?"

    Quote Originally Posted by speedfreek View Post
    Do the contents of the universe need to know that conditions
    elsewhere are the same before they react to what is going on
    locally?
    I will stipulate that they do not.

    Quote Originally Posted by speedfreek View Post
    If everywhere is similar (subject to quantum fluctuations, of course!),
    then surely if our part expanded, it might have all have acted in pretty
    much the same way, even if it were infinite.
    Yes. But when would it do that?

    Quote Originally Posted by speedfreek View Post
    Those quantum fluctuations are thought to have introduced the
    seeds of large scale structure in our universe, but only after they
    were stretched out by the inflationary epoch to introduce that
    slight inhomogeneity. So perhaps in an infinite universe the
    quantum fluctuations elsewhere would cause very different
    end results outside of our observable universe, but surely that
    is the case however much larger the fundamental domain is,
    rather than being relevant to the issue of an infinite fundamental
    domain. But unless there were large differences in density across
    that domain before inflation, I do not see how the contents
    would not start off by expanding, like parts we see did.
    I think the details of inhomogenaity you refer to are not relevant
    to the question at hand. When would this expansion begin at the
    five locations I suggested within the infinite Universe?

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Quote Originally Posted by Jeff Root View Post
    I think the details of inhomogenaity you refer to are not relevant
    to the question at hand. When would this expansion begin at the
    five locations I suggested within the infinite Universe?


    Seeing as simultaneity is relative in our universe, so we cannot say that any two events that happen at different locations can be thought to be simultaneous in any absolute sense, I don't know how to answer that question.



    If it seems like I am being evasive, all I can say is I don't know how to begin to define a global simultaneity convention for the fundamental domain of the universe! Does "at the same time" have any meaning in this context? What does the idea that different parts of an infinite universe might expand at different times really mean anyway?

    If there are parts of the universe beyond our particle horizon that started to expand later, or earlier, than the parts within, why is that relevant to the question of whether our big-bang universe is infinte? If some parts started out a lot less dense are we to assume they might contract, relative to the denser areas, or expand anyway, due to inflation or a cosmological constant? Might inflation or a cosmological constant be global, even in an infinite universe? Does it matter if the infinite universe contains parts that are expanding, other parts that are contracting, and parts where the two states overlap or meet? Perhaps, way beyond our observable universe, in one direction the next part of the universe is contracting, but in the other direction it has been expanding even longer? (I have no idea if any of this is possible)

    If, at scales much larger than our observable universe, the universe is contracting in parts that have never been causally connected to right here, but are causally connected to parts of the universe that are causally connected to parts of the universe etc etc... that are eventually causally connected to parts of our observable universe, is it not all part of one big infinite universe?

    I am afraid I have more questions than answers!

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    That's okay.

    My goal is the understanding that two places in the Universe are
    extremely unlikely to have nearly identical conditions unless
    something caused them both to have those conditions. That is,
    they have been in causal contact. The time we have been talking
    about is prior to the hypothesized Inflation. (The subject of the
    thread is "Size of the Big Bang singularity", so we know we are
    discussing the very earliest moment of the Big Bang.)

    If the Universe is infinite in extent, and all of it has similar
    conditions, as you suggested in post #20, then we have a big
    problem of how the conditions got that way. Not just a large
    volume of nearly identical conditions, but an infinite volume
    in which the conditions are nearly identical and changing at
    an enormously rapid rate, with nothing at all preceeding it to
    make it that way.

    I'm not saying that we don't know a mechanism that could
    have made it that way-- I'm saying that there is no time for
    any such mechanism to exist. Such a mechanism cannot
    exist unless it ignores causality and operates on an infinite
    volume simultaneously.

    Simple alternatives to this absurd situation are that the
    Universe is not infinite, or that the Big Bang involved only
    an infinitesimal portion of an infinite Universe.

    I just do not see any possibility at all that the Big Bang
    could affect an infinite volume in finite time.

    -- Jeff, in Minneapolis
    http://www.FreeMars.org/jeff/

    "I find astronomy very interesting, but I wouldn't if I thought we
    were just going to sit here and look." -- "Van Rijn"

    "The other planets? Well, they just happen to be there, but the
    point of rockets is to explore them!" -- Kai Yeves

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    Quote Originally Posted by Jeff Root View Post
    What tnjrp said. If the cosmic expansion were infinite in
    extent, it would mean that parts of the Universe infinitely
    far apart from one another are causally connected to an
    event which occurred 13.7 billion years ago. That strikes
    me as being obviously impossible.

    -- Jeff, in Minneapolis
    problem with this thinking is thinking there was any time in the singularity. So it is more that the universe popped into infinity. I guess you can think of it as always being infinite in nature but at T0 by definition there was no time so size didn't have meaning at that point.

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    Quote Originally Posted by Jeff Root View Post
    When would it start expanding? Say, at five locations, A, B, C,
    D, and E, where locations A and B are a nanometre apart, location
    C is a millimetre away from them, location D is a kilometre from
    them, and location E is a gigametre from them.

    -- Jeff, in Minneapolis
    A,B,C,D & E

    Expansion is a property of space and not isolated to a particular point in space. That is what the data shows in my view.

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    Last Post: 2005-Jan-17, 04:02 PM

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