
#1
Aug411, 11:23 AM

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Since I see the Big Bang was the beginning of space; if space is infinite, does that mean that space can expand at an infinite rate?
(Thanks in advance from this layman; I've started Brian Greene's "Hidden Reality" and despite laymen being it's target audience, I'm stuck on this question.) 



#2
Aug411, 11:43 AM

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You have to be careful with popularizations. Many are written to make money, to sell to wide audience, rather than to honestly present the state of scientific observation and conjecture.
Most working cosmologists do not accept or use "multiverse" models. (Yet Greene's book is largely about "multiverse" speculation.) So far there is no scientific reason to favor infinite over finite space. Both versions of the standard cosmo model may be used in the same paper. So far they fit the data about equally well, so it is an open question. Keep both possibilities in mind, is the idea. (If space is infinite now, then it always was. And expansion started from infinite volume.) So far there is no scientific reason to suppose that space and time "began" with the start of expansion or any other known event. There are models which do not go back further than the start of expansion, and there are models which do go back further in time. And both fit the data equally well. As more and better data accumulate we will be better able to distinguish and see which models fit best. There is actually some progress being made in this currently! It's exciting, but it's not over yet. 



#3
Aug411, 12:20 PM

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You asked about expansion rate. What is the max?
This is mindstretching. The current expansion rate (as a percentage growth in distance per million years) is pretty much unimaginably slow. The early expansion rate (as percentage growth in distance per billionth of a second) is pretty much unimaginably rapidaccording to prevailing estimates. It's tough. The two rates almost do not fit on the same scale. I don't want to risk a big stretch (you just got here) so I will not say much about the early universe rate. I'll just tell you what we currently measure to be the expansion rate NOW. It is very slow. Distances in the universe (large scale and on average) are currently increasing at the rate of 1/140 of one percent every million years. That is a way of expressing a quantity called the "Hubble parameter" in ordinary familiar units. In the early universe, the Hubble parameter was vastly more rapid. One could say indecently and preposterously more rapid. We can't do anything about this. The best mathematical models, based on a law of gravity whose detailed predictions have been meticulolusly checked in the situations where we know how to check, tell us this. We are talking about a difference like between 10^{18} per second and 10^{43} per second. If your taste lies in the direction of such extreme numbers, keep on asking and I or somebody else can supply more detail about models of the early universe, and links to the actual research literature (not Brian Greene ) 



#4
Aug411, 01:16 PM

P: 7

max. rate of expansion of space?
Thank you very much for your wonderful replies, marcus.
I do understand that Green's book is largely speculative (and simplified), and I'm simply seeking a grasp of the bases of that speculation. (Speaking of "speculation", I guess I'm interested in it because it seems to me that quantum mechanics seems strange enough, that an encompassing theory might well be rather surprising.) Are the vast differences between current and past expansion percentage rates simply proportional to the distances involved? 



#5
Aug411, 01:38 PM

C. Spirit
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#6
Aug411, 02:22 PM

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#7
Aug411, 02:29 PM

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[tex]v = Hr[/tex] where [itex]r[/itex] is the distance to the object. So for a given, fixed rate of expansion (set by [itex]H[/itex]), we find that objects recede from us at a speed that is proportional to their distance from us. So to finally answer your question, in the real universe, the Hubble parameter is generally a function of time, and so the rate of expansion of the universe varies with time, but not location in a homogeneous universe. 



#8
Aug411, 07:34 PM

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Since this example uses a twodimensional object (the rubber sheet) to stand for (threedimensional) volume, does that mean that it's spacial expansion involves a dimension beyond three? 



#9
Aug411, 07:40 PM

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Had I stuck with my plan to take more math and major in physics decades ago, I'd have had a much better shot at fully grasping bapowell's reply (my fault, thank you bapowell)! 



#10
Aug411, 08:04 PM

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#11
Aug411, 08:06 PM

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#12
Aug411, 09:34 PM

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Perhaps my problem is that, in terms of area, "3D volume" and "space" seem equivalent to me. Which leaves me still wondering how "infinite volume" could expand. I'm sure it's just my limited imagination, I don't see how an infinite amount of volume can increase. Or maybe my problem is I don't understand "infinite". (In two dimensions, I think of "infinite" length as going on forever, endlessly. Such that infinity + 1 = infinity.) I promise if I can't get it this time, I'll stop wasting your time! 



#13
Aug511, 01:02 AM

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#14
Aug511, 01:19 AM

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The observable universe is temporally finite [~13.7 billion years] and contains a finite quantity of baryonic, nonbaryonic, spatial and energy components. It is hard enough to wrap your head around that without introducing indeterminate quantities of unobservable and exotic components.




#15
Aug611, 09:16 AM

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I apologize, I guess I needed to take more deep breaths. I hope there aren't too many like me who stumble in here needing clues, overwhelmed from reading pop physics. 



#16
Aug611, 12:17 PM

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Right. Don't think about expansion as some region necessarily increasing in volume (in a finite universe, this would indeed be the correct view). Instead, just think of expansion as the increase in size of the grid marks you paint throughout your volume. In general relativity, this is precisely the way expansion works  as an increase in distance between points drawn on the grid.




#17
Aug611, 05:21 PM

P: 7

I'd heard about (but not understood) the Big Rip hypothesis; it didn't occur to me that expansion was a factor at very small scales. I see that "the electromagnetic forces hold... molecules and atoms together" now (but perhaps not forever if "the energy density of dark energy increase[s] without bound"; then "Finally even atomic nuclei will be torn apart"). Knowing that electromagnetic forces now keep atom nuclei from being torn apart leaves me still wondering: that means the atoms aren't expanding at all yet, right? (I'm guessing that based upon atomic particles having quantum states.) I shudder to think how far off the rails I might be here. 



#18
Aug611, 07:00 PM

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