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#19
Jan208, 10:25 AM

P: 28

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#20
Jan208, 10:52 AM

P: 3,967

Muccasen, thanks for replying to my controversial post in such a pleasant manner.
I have to pop out for a while, but would like to continue this discussion later. 


#21
Jan208, 01:21 PM

Astronomy
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PF Gold
P: 23,212

kev, this is good post, with points that are interesting to reply to
I see Muccasen already responded, and also that you said you had to go and would be away for a while. But I will reply to one or two things immediately. I see you have a conceptual problem with the INFINITE case: In the Friedmann model there IS NO PREFERRED DIRECTION to the expansion, and no preferred direction to any acceleration or deceleration of expansion. Expansion does NOT CONSIST OF GALAXIES MOVING THRU SPACE. Expansion is just a percentage increase in the distance between stationary points. GR teaches us to expect distance to change unless it is a distance locked in by atomic or other forceslike the crystal bonds of a metal rod, or the orbit dynamics of a planetary system. Within certain bound structures, forces keep distances from changing, but otherwise one should expect them to be expanding or contracting. The Friedmann equation is beautifully simple and you can see immediately how it governs the rate of expansion/contraction. It works either in the finite volume or infinite volume case and (as I said) doesn't need any force vector in any preferred direction. There's just this timedependent scalefactor a(t) and an ordinary differential equation that tells about the time derivative a'(t) and how it is determined. It is probably easier to understand in that form than in the form of words. Currently the percentage rate of expansion or more exactly the fractional rate (which you could write a'(t)/a(t) if you want) is one percent every 140 million years. You can see why it has to be. The left hand side of the Friedmann equation is actually the square of that very quantity(a'(t)/a(t))^{2}. Actually there are two Friedmann equations, both are simple and you should look at both. But for some reason everybody always talks about THE equation as if there were just one. the other one is about the second derivative of the scalefactor: a''(t). when you look at these two Friedmann equations the first thing you see is that they work for both the infinite volume case and the positive curved S^{3} case. that is made explicit with a parameter k which can be either +1, 0, or 1 You are right that expansion, as far as observations can tell us by fitting the data, was DECELERATING until some 4 or 5 billion years ago! That was when matter was denser and its effect prevailed over the cosmological constant. And then with expansion matter thinned out and the cosmological constant effect of acceleration began to prevail. But all this stuff works equally well in the Friedmann model whether you pick the infinite case or the finite case!. This doesn't need any preferred direction. And the expansion is not outwards from any particular point. Basically it is just GR effects involving the dynamics by which distances change (GR's dynamic geometry). But that is what the curvature data already suggested anyway. IIRC a 2007 Ned Wright paper presented a good fit for Omega as 1.011, and in a landmark 2006 paper by Spergel et al the errorbar all on the upside of 1something like [1.010, 1.041] at 68 percent confidence. Ned Wright's 1.011 falls in that range. http://physicsforums.com/showthread....02#post1558402 And that 1.011 would indicate an S^{3} circumference of some 800 billion LY, which was way more than the lowerbound that Cornish and Spergel came up with in 2004 (and later refined in 2006). So that much agrees. ==================== What I have still not given you is INTUITION for why an infinite flat volume that starts off expanding should ever slow down! The real reason is that GR is the most accurate theory of gravity at large scale that we have and if you buy GR then you buy Friedmann derived from it and you buy that in the interesting cases expansion slows down unless there is a cosmological constant. But this is unsatisfying. One still wants some intuition for why in the hell it happens. The puzzle is in the infinite flat case. There is no preferred direction, the net effect of everything should be zerowhy should expansion be slowed down? I hope someone else has some intuition about this. When I think about it I come up short on the intuition end. One thing to remember is that GR is a field theory and information only travels at a finite speed. So in some big region the local patch of geometry doesnt KNOW about the matter way out there. the effective distribution of matter in its past light cone could be very different from the instantaneous (approximately even) distribution. But I'm not sure enough about this. the problem of how you get intuition for the slowing down is interestingmaybe there is some obvious answer that i'm missing. We might hear from SpaceTiger or Garth about this. 


#22
Jan208, 02:25 PM

P: 28

Marcus:"lots more to say but I too have to go out"
We look forward to yr return!! 


#23
Jan308, 11:21 AM

P: 96

If the universe is homogeneous on the largest scales, then shouldn't other observer's red shifts produce at least an equal amount of blue shifts to red shifts for us? That assumption surely has not been verified. Why did Hubble and so many other scientists reject a nearby center when observations say otherwise?



#24
Jan308, 11:32 AM

P: 96

Thanks for the balloon explanation, but hear is the problem that I have with that model:
If space is expanding, then aren't lengths themselves expanding? What is the expansion relative too then? What I see happening are masses moving away from each other, nothing more. I think historically this has been interpreted as space expansion to fit the large scale homogeneity model. 


#25
Jan308, 11:46 AM

P: 96

*here



#26
Jan308, 12:38 PM

P: 15,319




#27
Jan308, 12:55 PM

P: 3,967

Thanks to Marcus, WFWINTERS and mucassen for all the interesting and informative responses. There are a lot issues to address here so I hope you don’t mind if I reply to just a few paraphrased points while I try and gather my thoughts. :P
Going back to Muccasen’s example of the raisin cake, what would an observer on the edge of an expanding but finite cake see? Light coming from raisins on the surface of the cake to either side of him would curve inwards towards the mass at the centre of the cake. The photons arriving at his location would appear to be coming from outside the cake so maybe even observes on the edge would not know they are on the edge unless they do some very careful measurements? This is similar to the aberration seen by a relativistic traveller that makes light from objects to the side of him appear to be coming from somewhere in front of him. I have read the ideas of Cantor on infinity and I can see how they apply to intangible things like real and imaginary numbers but when applied to physical objects like atoms or galaxies I have a problem. A spatially infinite universe with an even distribution of infinite mass gives means the universe is like a real incarnation of the Hilbert Hotel and what a messy thing that is! See http://www.ccs3.lanl.gov/megamath/w...y/inhotel.html Since mass density is a ratio of mass to volume it does not tell us if the mass or volume is infinite or finite. Omega is a ratio of mass density to critical energy density so I’m not sure if Omega tells us anything about whether the volume or mass if infinite or finite. The wikepedia article seems to imply the observed Omega>1 implies a finite radius (and finite mass?) for the universe. Thanks for pointing out the Friedmann equations to me. They look interesting but they are new to me and I will have to spend some time trying to understand them. 


#28
Jan308, 01:00 PM

P: 15,319

Having no edge or boundary does NOT automatically imply infinite distances. That is the point of the balloon analogy. 


#29
Jan308, 01:08 PM

P: 3,967




#30
Jan308, 01:58 PM

Astronomy
Sci Advisor
PF Gold
P: 23,212

You have to make allowances for bumpiness. On a perfect (not bumpy, not expanding) sphere they will meet. But in the real world, stars and galaxies deform the space around them so that geodesics are deflected. If the two travelers are not prevented by accelerating expansion from ever reaching each other then they will approximately meet. You ask:...and is that how our universe is modelled? Indeed in Quantum Cosmology some of the best modeling work that I know of uses spacetimes where the spatial sections are approximately S^{3}that is they are topological threespheres but they have the bumps and wrinkles you would naturally expect Amjorn and Loll's computer models are all of that type. they run sims of quantum universes that come into existence, grow, shrink, and disappear according to some simple rules. They use S^{3} spatial sections. If you want links, ask. Ashtekar group's computer models, many of them, are also of that type. that's where they replace the cosmological singularity with a bouncea prior universe which is spatially a "bumpy" threesphere collapses to Planck density and reexpands. They use S^{3} spatial sections. If you want links, ask. these are experimental quantum cosmology modelingI'm not saying the real universe spatial topology is S^{3} because we don't knowI'm saying what some leading modelbuilders do. You asked is it MODELED like that and the answer is certainly YES. Indeed the dominant classical model that working cosmologists use is LCDM and it comes mostly in two flavors as I saidOmega=1 flat infinite and Omega>1 finite with S^{3} spatial sections. So you see that kind of model whether you look at classical cosmology or the newer quantum cosmology approaches (especially since 2004 or 2005) 


#31
Jan308, 02:37 PM

Astronomy
Sci Advisor
PF Gold
P: 23,212

spatial infinite means infinite spatial volume. it is mathematically convenient although it gives people philosophical vertigo the critical energy density is the density that must be surpassed in order to force the solution of Einstein/Friedmann equation to have finite spatial volume. So naturally since Omega is the RATIO, if Omega > 1 that means precisely that the observed density DOES surpass critical and therefore we must have finite spatial volume. aka positive curvature (one implies the other) I'm glad you are interested in these things. I find I am having to repeat myself a fair amount. What I have been telling you is manifest in the Friedmann equations, so they might be something good for you to study. It would help resolve your doubts perhaps. 


#32
Jan308, 04:59 PM

P: 3,967

Marcus,
I wouldn't mind seeing those links you mentioned, just out of interest. Can't promise I'll understand them ;) 


#33
Jan308, 06:56 PM

Astronomy
Sci Advisor
PF Gold
P: 23,212

want to join me in studying up on CDT in early 2008 then the articles to print off so you have hardcopy to scribble are: http://arxiv.org/abs/0711.0273 (21 pages) The Emergence of Spacetime, or, Quantum Gravity on Your Desktop "Is there an approach to quantum gravity which is conceptually simple, relies on very few fundamental physical principles and ingredients, emphasizes geometric (as opposed to algebraic) properties, comes with a definite numerical approximation scheme, and produces robust results, which go beyond showing mere internal consistency of the formalism? The answer is a resounding yes: it is the attempt to construct a nonperturbative theory of quantum gravity, valid on all scales, with the technique of socalled Causal Dynamical Triangulations. Despite its conceptual simplicity, the results obtained up to now are far from trivial. Most remarkable at this stage is perhaps the fully dynamical emergence of a classical background (and solution to the Einstein equations) from a nonperturbative sum over geometries, without putting in any preferred geometric background at the outset. In addition, there is concrete evidence for the presence of a fractal spacetime foam on Planckian distance scales. The availability of a computational framework provides builtin reality checks of the approach, whose importance can hardly be overestimated." http://arxiv.org/abs/0712.2485 (10 pages) Planckian Birth of the Quantum de Sitter Universe "We show that the quantum universe emerging from a nonperturbative, Lorentzian sumovergeometries can be described with high accuracy by a fourdimensional de Sitter spacetime. By a scaling analysis involving Newton's constant, we establish that the linear size of the quantum universes under study is in between 17 and 28 Planck lengths. Somewhat surprisingly, the measured quantum fluctuations around the de Sitter universe in this regime are to good approximation still describable semiclassically. The numerical evidence presented comes from a regularization of quantum gravity in terms of causal dynamical triangulations." http://arxiv.org/abs/grqc/0612104 Loop quantum cosmology of k=1 FRW models Abhay Ashtekar et al (Submitted on 18 Dec 2006) In this paper they do not put in the effect of a postive cosmological constant, so the universes eventually collapse. that wouldn't have to happenyou could have a contracting phase, a bounce, and then an expanding phase that went on expanding indefinitely. k=1 means the case with space looking like S^{3} FRW means Friedmann Robertson Walkerthe prevailing model in cosmology which has various flavors depending on what parameters you put in. With technical papers there is usually an understandable nontechnical introduction section and conclusions section. Not to worry about understanding the whole paper. Just get what you can out of it. And this is just to get a sample of people doing computer models 


#34
Jan408, 04:09 PM

P: 1

Hi sorry for barging in. Any idea if the Universe has at least a 2 dimensional geometry??



#35
Jan408, 04:22 PM

P: 28




#36
Jan408, 04:25 PM

P: 28




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