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Thinking About Large-Scale Structure
Consider a hollow sphere of inert material moving inertially in
interstellar space. The volume of the sphere is on the order of 10^6 cubic centimeters , and inside we have enough helium atoms to yield an average density of on the order of 500 atoms per cc. We let the system equilibrate and observe the distribution of He atoms periodically. I envision that the distribution would be something I would unreservedly call homogeneous virtually every time the distribution was observed and recorded. Note: since this is a thought experiment we can do miraculous things like observe, with our educated imaginations, the distribution of atoms with high resolution. Now consider an analogy to the observable universe representing the hollow sphere and galaxies as the He atoms. Here our technical challenges of determining distributions are easier in some ways and harder in others. In this case we observe that there is a hierarchical clustering of galaxies, groups, clusters of groups, and superclusters. Most importantly we observe the Cosmic Web/Void structure that appears to dominate the large-scale structure within the observable universe. I have a hard time seeing the observed galactic distribution as even remotely "homogeneous", especially compared to our distribution of He atoms in the little toy model. However, astrophysicists routinely argue that the large-scale distribution of matter is statistically homogeneous, apparently like the distribution of He atoms. This confuses me. To me the two distributions are highly different and must be distinguished in any rigorous scientific description. I assume others might vigorously argue otherwise. So I present this thread as an opportunity to air ideas related to the inhomogeneity/homogeneity controversy. I would be most interested to hear why the galactic distribution deserves the term homogeneous, relative to the He atom distribution archetype. RLO Fractal Cosmology |
#2
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Thinking About Large-Scale Structure
On Friday, March 11, 2016 at 1:16:28 AM UTC-7, Robert L. Oldershaw wrote:
Consider a hollow sphere of inert material moving inertially in interstellar space. [Mod. note: rest of quoted article snipped -- mjh] For one thing, the He model really isn't homogeneous if you look at a small enough scale. There are individual knots of matter called atoms and then there are clouds of electrons and then there are really, really tiny knots of matter called nuclei and then there are protons and neutrons and then there are quarks and gluons. So one might ask the same question of a sphere with helium inside, yes? Gary |
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Thinking About Large-Scale Structure
On 3/11/2016 9:16 AM, Robert L. Oldershaw wrote:
Consider a hollow sphere of inert material moving inertially in interstellar space. The volume of the sphere is on the order of 10^6 cubic centimeters , and inside we have enough helium atoms to yield an average density of on the order of 500 atoms per cc. We let the system equilibrate and observe the distribution of He atoms periodically. Why don't you use H or Li atoms? Perhaps He would just not liquify at 2.73K cosmic background temperature, but many other atoms would cluster in droplets or condense at the sphere's wall. .. Now consider an analogy to the observable universe representing the hollow sphere and galaxies as the He atoms. Again, why He atoms? Galaxies look much more complicated. You could use some complex kind of molecule, perhaps! ... However, astrophysicists routinely argue that the large-scale distribution of matter is statistically homogeneous, apparently like the distribution of He atoms. I don't think they argue that, they say it is like a distribution of filaments. You really should use some more complex material than He to have a good analogy! Perhaps some molecules that form an aerogel, or proteins forming fibers, would somewhat better resemble the structure of the universe (but then agian, perhaps no molecules exists that would really do the job..) To me the two distributions are highly different and must be distinguished in any rigorous scientific description. Yes, the He atom distribution is very different from that of other types of atoms or molecules. (He is a rather special element, of course.) -- Jos |
#4
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Thinking About Large-Scale Structure
In article , "Robert L.
Oldershaw" writes: Consider a hollow sphere of inert material moving inertially in interstellar space. Now consider an analogy to the observable universe representing the hollow sphere and galaxies as the He atoms. I have a hard time seeing the observed galactic distribution as even remotely "homogeneous", especially compared to our distribution of He atoms in the little toy model. However, astrophysicists routinely argue that the large-scale distribution of matter is statistically homogeneous, apparently like the distribution of He atoms. This confuses me. To me the two distributions are highly different and must be distinguished in any rigorous scientific description. I assume others might vigorously argue otherwise. So I present this thread as an opportunity to air ideas related to the inhomogeneity/homogeneity controversy. I would be most interested to hear why the galactic distribution deserves the term homogeneous, relative to the He atom distribution archetype. Assuming (and that's a big assumption, considering quote-mining and other such devices in recent posts, but I'm willing to give you a chance) that this is an honest question: I think you really don't understand what is meant by "statistical homogeneity". Your admission that you have spent little time on statistics underpins this assessment. As George McVittie said, "The essence of cosmology is statistics.". If you read up on the history of statistics, you will see that many contributions were made by astronomers. I think a meaningful discussion is not possible unless it can be assumed that all parties understand basics statistics. |
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Thinking About Large-Scale Structure
On Friday, March 11, 2016 at 3:16:28 AM UTC-5, Robert L. Oldershaw wrote:
Consider a hollow sphere of inert material moving inertially in interstellar space. The volume of the sphere is on the order of 10^6 ..... A better way to think about it is a metal foam: https://en.wikipedia.org/wiki/Metal_foam On the size scale of the foam cell and cell wall, obviously there are major inhomogeneities in the density. The density of metal in the walls is much higher than the air in the voids. However, on the size scale much larger than the cell size, the foam is actually quite uniform. One can weigh different large chunks and get very nearly the same density. Pretty much the same for the observable universe. At the small scale, walls and voids with extremes in density variations. At the large scale, measureably uniform. Note I said measureably uniform. It's possible to measure the clumpiness of galaxies on different observational lenghth scales, and beyond a few hundred Mpc, there is very little detectable clumpiness. CM |
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Thinking About Large-Scale Structure
On 3/13/2016 10:26 AM, Craig Markwardt wrote:
On Friday, March 11, 2016 at 3:16:28 AM UTC-5, Robert L. Oldershaw wrote: Consider a hollow sphere of inert material moving inertially in interstellar space. The volume of the sphere is on the order of 10^6 .... A better way to think about it is a metal foam: https://en.wikipedia.org/wiki/Metal_foam On the size scale of the foam cell and cell wall, obviously there are major inhomogeneities in the density. The density of metal in the walls is much higher than the air in the voids. However, on the size scale much larger than the cell size, the foam is actually quite uniform. One can weigh different large chunks and get very nearly the same density. Pretty much the same for the observable universe. At the small scale, walls and voids with extremes in density variations. At the large scale, measureably uniform. Note I said measureably uniform. It's possible to measure the clumpiness of galaxies on different observational lenghth scales, and beyond a few hundred Mpc, there is very little detectable clumpiness. If this Atlas of The Universe is accurate: http://www.atlasoftheuniverse.com/universe.html then there's a nice resemblance with 3D graphene foam: http://acsmaterial.com/product.asp?cid=99&id=126 And on the "Atlas" you can nicely zoom in, After two zoom levels you just see the Virgo supercluster and the homogeneity is clearly gone. -- Jos |
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Thinking About Large-Scale Structure
On Sunday, March 13, 2016 at 5:16:44 AM UTC-4, Gary Harnagel wrote:
For one thing, the He model really isn't homogeneous if you look at a small enough scale. There are individual knots of matter called atoms and then there are clouds of electrons and then there are really, really tiny knots of matter called nuclei and then there are protons and neutrons and then there are quarks and gluons. So one might ask the same question of a sphere with helium inside, yes? Thanks, Gary, but in the intended analogy the He atoms are analogous to the galaxies, and galaxies have nuclei and central supermassive black holes. Also galaxies are surrounded by large halos which we could say are analogous to electron "clouds. I am NOT saying that the analogy is an exact one, OR EVEN CLOSE, but it is reasonably useful for discussion purposes. We are not talking primarily about the internal physics of the fundamental "particles" on the two radically different scales, but rather their DISTRIBUTIONS. The analogy does not involve sub-nuclear phenomena. It is an unimportant of off-topic matter, but quarks and gluons have never been observed to my satisfaction. RLO http://www3.amherst.edu/~rloldershaw |
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Thinking About Large-Scale Structure
On Sunday, March 13, 2016 at 5:18:36 AM UTC-4, Jos Bergervoet wrote:
To me the two distributions are highly different and must be distinguished in any rigorous scientific description. Yes, the He atom distribution is very different from that of other types of atoms or molecules. (He is a rather special element, of course.) I know this is a subtle difference for some to grasp, but I am not trying to reproduce galactic scale structure with the He analogy. I am specifically choosing an atomic scale system that *avoids* most of the complexities you mention because I want to compare the *distributions* of the particles and *only* the spatial distributions of them. I want to compare what I think of as an archetypal homogeneous distribution with what I think of as an archetypal inhomogeneous distribution. Hope this helps. |
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Thinking About Large-Scale Structure
On Sunday, March 13, 2016 at 5:19:16 AM UTC-4, Phillip Helbig (undress to reply) wrote:
Assuming (and that's a big assumption, considering quote-mining and other such devices in recent posts, but I'm willing to give you a chance) that this is an honest question: I think you really don't understand what is meant by "statistical homogeneity". Your admission that you have spent little time on statistics underpins this assessment. As George McVittie said, "The essence of cosmology is statistics.". If you read up on the history of statistics, you will see that many contributions were made by astronomers. I think a meaningful discussion is not possible unless it can be assumed that all parties understand basics statistics. Your condescension is truly princely. However, I was not encouraging you to avoid substantive discussion but to speak plainly on a simple question. And here is the question once again for all to see: Given these two very different distributions, are we allowed to say they are the same, or are we scientifically required to distinguish the physical characteristics of the two distributions? Can you offer us a direct answer? RLO http//www3.amherst.edu/~rloldershaw |
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Thinking About Large-Scale Structure
On Sunday, March 13, 2016 at 5:26:47 AM UTC-4, Craig Markwardt wrote:
Pretty much the same for the observable universe. At the small scale, walls and voids with extremes in density variations. At the large scale, measureably uniform. Note I said measureably uniform. It's possible to measure the clumpiness of galaxies on different observational lenghth scales, and beyond a few hundred Mpc, there is very little detectable clumpiness. I would direct you to the newest paper, http://arxiv.org/abs/1603.03260 , on this issue that I have cited in my 3/13/16 post to the thread entitled something like "Largest Structure...". Are we really confident that we can claim that "there is very little detectable clumpiness"..."beyond a few hundred Mpc"? Is this due to the fact that the lumpiness is not there, or is it due to the fact that we have trouble seeing it? Can we *confidently* decide which is the case with existing data, or are we engaging in wishful thinking? RLO http://www3.amherst.edu/~rloldershaw |
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