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#21
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Advantage Inhomogeneity
On Wednesday, February 10, 2016 at 3:54:43 PM UTC-5, Phillip Helbig (undres=
s to reply) wrote: In article , Jos Bergervoet =20 No; I don't think all agree on this. By definition, a fractal=20 distribution doesn't "stop" at some scale... =20 The only question is whether the sequence will *resume*, at some scale larger than what we can see today. =20 ...and then resume again. =20 (1) This assertion that fractal models can only work in the naive and simplistic way you say they must is patently false. (2) Continuous self-similarity is a mathematical myth that is not realized in nature. Even Mandelbrot's classic fractal examples like the Koch curve/snowflake are constructed by discrete iterations and so the self-similarity cannot be rigorously continuous. (3) In nature self-similarity is discrete self-similarity. One can see that in the myriad examples of turbulence. In any real fluid undergoing turbulent motions there are preferred scales in the distribution of vortices, that depend on the physical properties of the fluid and the forcing details. (4) In nature hierarchies also tend to be stratified (i.e., discrete rather than continuous in the distribution of levels). Try observing nature instead of relying on Platonic fictions. RLO http:/www3.amherst.edu/~rloldershaw Fractal Cosmology [[Mod. note -- I will note that continuous and/or discrete self-similarity in gravitational collapse has been and continues to be a major research area in general relativity. See http://www.livingreviews.org/lrr-2007-5 for a detailed review. However, while these are solutions of the Einstein or Einstein+matter equations, they require "fine-tuned" initial data and so form a set of measure 0 in the space of initial data. They are thus (very) unlikely to occur in nature. -- jt]] |
#22
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Advantage Inhomogeneity
On Wednesday, February 10, 2016 at 10:17:17 AM UTC-5, Jos Bergervoet wrote:
That was not my impression. I think we all agree that this sequence exists, but seems to stop a few hundred Mpc (i.e. from that point on no larger structures have been seen.) The only question is whether the sequence will *resume*, at some scale larger than what we can see today. My point is that it could go either way. Just like at the other end of the scale, the elementary particles that we know *might* be truly elemental, but could also turn out to be composite. Indeed, it could very much "go either way" on either cosmic or microscopic scales, or both. So why do people make the thoroughly unscientific assertion that nature's hierarchy MUST have cutoffs at both "ends"? That is purely a theoretical assumption and a lot of hype. Exactly my point too! A turnover at a few hundred Mpc is not at all a matter of fact. Plenty of authors have published papers claiming structure on the Gpc scale [see my post on the vast quasar cluster posted here not long ago, or the latest Planck results on intrinsic dipole anisotropy at very large scale. Just repeating the questionable turnover assumptions over and over, as many LCDM proponents do, will not make those assumptions true. Citing only evidence that suggests a cutoff at a few hundred Mpc, and ignoring the published evidence for structure on far larger scales, is not the way to proceed scientifically. If anyone wants to see published papers on Gpc scale structure, simply do an arxiv.org search on "very large scale structure" or "cosmological inhomogeneity" or "cosmological anisotropy". If anyone wants to subsequently claim that I cannot back up what I say with published research, THEN I will be happy to provide specific documentation to show that the claim is quite incorrect. But first give me the scientific freedom of speech to make my general argument, and then let's see who can counter it scientifically. [Mod. note: reformatted. Scientific practice, which _all_ participants in the group are encouraged to follow, is to provide references to support your claim at the time you make it -- mjh] RLO http://www3.amherst.edu/~rloldershaw Fractal Cosmology |
#23
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Advantage Inhomogeneity
In article , "Robert L.
Oldershaw" writes: That was not my impression. I think we all agree that this sequence exists, but seems to stop a few hundred Mpc (i.e. from that point on no larger structures have been seen.) The only question is whether the sequence will *resume*, at some scale larger than what we can see today. My point is that it could go either way. Just like at the other end of the scale, the elementary particles that we know *might* be truly elemental, but could also turn out to be composite. Indeed, it could very much "go either way" on either cosmic or microscopic scales, or both. So why do people make the thoroughly unscientific assertion that nature's hierarchy MUST have cutoffs at both "ends"? Who makes such claims? Most people say "let's see what we find". It seems that YOU make the claim that there MUST NOT be any cutoff, e.g. fractal cosmology. This is based not on observation, but on your theory. A turnover at a few hundred Mpc is not at all a matter of fact. Plenty of authors have published papers claiming structure on the Gpc scale [see my post on the vast quasar cluster posted here not long ago, or the latest Planck results on intrinsic dipole anisotropy at very large scale. And these have been refuted. Just repeating the questionable turnover assumptions over and over, as many LCDM proponents do, will not make those assumptions true. True. Neither does repeating over and over that there is no turnover make that claim true. Citing only evidence that suggests a cutoff at a few hundred Mpc, and ignoring the published evidence for structure on far larger scales, is not the way to proceed scientifically. Citing only evidence (even if it has been refuted) which suggests no cutoff, and ignoring the published (and not refuted) evidence for lack of structure on far larger scales, is not the way to proceed scientifically. If anyone wants to see published papers on Gpc scale structure, simply do an arxiv.org search on "very large scale structure" or "cosmological inhomogeneity" or "cosmological anisotropy". One can find papers on many things. If they have been refuted, and the original authors have not refuted the refutation, then for me it is refuted, even if the authors don't want to admit it. If anyone wants to subsequently claim that I cannot back up what I say with published research, THEN I will be happy to provide specific documentation to show that the claim is quite incorrect. But first give me the scientific freedom of speech to make my general argument, and then let's see who can counter it scientifically. [Mod. note: reformatted. Scientific practice, which _all_ participants in the group are encouraged to follow, is to provide references to support your claim at the time you make it -- mjh] Indeed. YOU cite a SPECIFIC paper, and we can discuss whether its claims are justified. But in turn, you have to examine the refutations. |
#24
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Advantage Inhomogeneity
On Saturday, February 20, 2016 at 2:57:10 AM UTC-5, Phillip Helbig (undress to reply) wrote:
In article , "Robert L. Oldershaw" writes: Indeed, it could very much "go either way" on either cosmic or microscopic scales, or both. So why do people make the thoroughly unscientific assertion that nature's hierarchy MUST have cutoffs at both "ends"? Who makes such claims? Most people say "let's see what we find". It seems that YOU make the claim that there MUST NOT be any cutoff, e.g. fractal cosmology. This is based not on observation, but on your theory. Jos stated the "either way" idea. My claim, with emphasis so you cannot ignore it, is: I CLAIM THERE IS NO REASON TO ASSUME THERE MUST BE CUTOFFS. Do you hear me now? Can you make the distinction between what I actually laim and what you falsely say I claim? A turnover at a few hundred Mpc is not at all a matter of fact. Plenty of authors have published papers claiming structure on the Gpc scale [see my post on the vast quasar cluster posted here not long ago, or the latest Planck results on intrinsic dipole anisotropy at very large scale. And these have been refuted. You have cited no evidence for what is only your opinion. Just repeating the questionable turnover assumptions over and over, as many LCDM proponents do, will not make those assumptions true. True. Neither does repeating over and over that there is no turnover make that claim true. I only claim that the widespread and long-hyped assumption of a "turnover" is not a certain fact of nature, i.e., there is no guarantee of a final "turnover". Citing only evidence that suggests a cutoff at a few hundred Mpc, and ignoring the published evidence for structure on far larger scales, is not the way to proceed scientifically. Citing only evidence (even if it has been refuted) which suggests no cutoff, and ignoring the published (and not refuted) evidence for lack of structure on far larger scales, is not the way to proceed scientifically. Again you cite no references to support your pronouncements, nor the published papers denying your opinions. If anyone wants to see published papers on Gpc scale structure, simply do an arxiv.org search on "very large scale structure" or "cosmological inhomogeneity" or "cosmological anisotropy". One can find papers on many things. If they have been refuted, and the original authors have not refuted the refutation, then for me it is refuted, even if the authors don't want to admit it. Again, you offer no citations. Just your personal opinions. If anyone wants to subsequently claim that I cannot back up what I say with published research, THEN I will be happy to provide specific documentation to show that the claim is quite incorrect. But first give me the scientific freedom of speech to make my general argument, and then let's see who can counter it scientifically. [Mod. note: reformatted. Scientific practice, which _all_ participants in the group are encouraged to follow, is to provide references to support your claim at the time you make it -- mjh] Indeed. YOU cite a SPECIFIC paper, and we can discuss whether its claims are justified. But in turn, you have to examine the refutations. Fine! T. Buchert et al, Class. Quantum Grav., 32, 215021, 2015 Steinhardt et al, arXiv:1506.01377v1 Pawlowski et al, http://arxiv.org/abs/1510.08060 Kroupa et al, http://arxiv.org/abs/1301.3907 Would you like 10 or 20 more references to published papers citing problems with the LCDM toy cosmological model? [Mod. note: quoted text trimmed, reformatted -- mjh] Robert L. Oldershaw http://www3.amherst.edu/~rloldershaw Discrete Scale Relativi |
#25
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Advantage Inhomogeneity
In article , "Robert L.
Oldershaw" writes: Indeed, it could very much "go either way" on either cosmic or microscopic scales, or both. So why do people make the thoroughly unscientific assertion that nature's hierarchy MUST have cutoffs at both "ends"? Who makes such claims? Most people say "let's see what we find". It seems that YOU make the claim that there MUST NOT be any cutoff, e.g. fractal cosmology. This is based not on observation, but on your theory. Jos stated the "either way" idea. It seems to me that "either way" is pretty open-minded. Where is the strict assumption? My claim, with emphasis so you cannot ignore it, is: I CLAIM THERE IS NO REASON TO ASSUME THERE MUST BE CUTOFFS. Do you hear me now? Can you make the distinction between what I actually laim and what you falsely say I claim? By definition, a fractal distribution is scale-invariant. I don't ASSUME that there must be cutoffs. I claim that the observational evidence supports a turnover to homogeneity at a few hundred Mpc, well below the scale of the observable universe. A turnover at a few hundred Mpc is not at all a matter of fact. Plenty of authors have published papers claiming structure on the Gpc scale [see my post on the vast quasar cluster posted here not long ago, or the latest Planck results on intrinsic dipole anisotropy at very large scale. And these have been refuted. You have cited no evidence for what is only your opinion. Again, you cite a paper and we can see if there is a refutation. You are asking me the equivalent of proving my innocence. I only claim that the widespread and long-hyped assumption of a "turnover" is not a certain fact of nature, i.e., there is no guarantee of a final "turnover". References, please. Again you cite no references to support your pronouncements, nor the published papers denying your opinions. Again, you started the claim. The ball is in your court. Steinhardt et al, arXiv:1506.01377v1 This is about galaxy formation. Of course, high-redshift galaxies are farther away than low-redshift ones, and due to the finite speed of light we are seeing them at an earlier time. The paper is about the latter, not the former, and has nothing to do with large-scale homogeneity or the lack thereof. Pawlowski et al, http://arxiv.org/abs/1510.08060 This is about satellites in the Local Group. That's a long way from several hundred Mpc. Kroupa et al, http://arxiv.org/abs/1301.3907 This is mainly about MOND. Note that even most MOND supporters agree that CDM works well on large scales; the problem is at small scales. So, we didn't even get as far as the refutation, since none of these papers is about the lack of a turnover to homogeneity on the scale of several hundred Mpcs. After three misses, I didn't bother to look at the first paper you cited (without an easy arXiv link---maybe it's on arXiv, maybe I can access it if it is not, but after three misses, I don't think I need to try). This is not the first time you have cited papers in support of your arguments where, in fact, they claim something entirely different. Last time, you even quote-mined it to make it appear that the authors claim the opposite of what they claim! You've had your chance. I don't see any point in discussing this further. |
#26
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Advantage Inhomogeneity
On 2/21/2016 4:10 PM, Robert L. Oldershaw wrote:
On Saturday, February 20, 2016 at 2:57:10 AM UTC-5, Phillip Helbig (undress to reply) wrote: In article , "Robert L. Oldershaw" writes: Indeed, it could very much "go either way" on either cosmic or microscopic scales, or both. So why do people make the thoroughly unscientific assertion that nature's hierarchy MUST have cutoffs at both "ends"? Who makes such claims? Most people say "let's see what we find". It seems that YOU make the claim that there MUST NOT be any cutoff, e.g. fractal cosmology. This is based not on observation, but on your theory. Jos stated the "either way" idea. There might be a cutoff point, or not! That was my either way claim, but in either case this doesn't tell us anything about fractal properties To be fractal means that the sequence should have a certain characteristic, described in some way by a (not necessarily interger) number that is independent of the scale, at least as far as the sequence goes. The claim that "the coast of Britain has a fractal dimension" does *not* imply that repetition of the structure goes on forever in both directions. At least clearly not in the large-scale direction (although they might in the future rebuild the British Empire to an infinite size, it has not yet happened). On the other hand it *does* require that the fractal dimension is (reasonably) constant over some significant scaling range. So we have two completely unrelated properties: 1) The presence of hierarchic sub-structuring over an infinite range of scales. 2) The substructures showing a (more or less) fixed fractal dimension over the existing range. We were discussing 1), which is neither necessary not sufficient for 2), i.e. fractal geometry has nothing to do with this discussion. -- Jos |
#27
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Advantage Inhomogeneity
On 2/10/16 9:29 AM, Nicolaas Vroom wrote:
IMO it is rather "simple" to simulate local inhomogeneities, but that does not say "anything" about our entire universe at present. Is there a "simple" logic to explain the consistent sizes of: 1. Stellar planetary systems 2. Galaxies throughout the visible universe? Richard D Saam |
#28
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Advantage Inhomogeneity
On Sunday, February 21, 2016 at 3:39:03 PM UTC-5, Phillip Helbig (undress to reply) wrote:
It seems to me that "either way" is pretty open-minded. Where is the strict assumption? Not on my part, as I made you aware in capital letters in my 2/21 post. By definition, a fractal distribution is scale-invariant. Look, I am sorry to bring you bad news but your definition is wrong on 2 separate grounds. Let's make this crystal clear. 1. I have an essay on my website that lists about 80 examples of fractal structures observed (*by others, not me*) in the real physical world of nature. Virtually all of them are *not* continuously scale invariant (i.e., completely scale free). If your view of natural fractals involves complete, exact scale invariance , then I regret to inform you that your definition is both incorrect and discouragingly naive. 2. If you have ever read Mandelbrot's book, The Fractal Geometry Of Nature, W.H. Freeman, New York, 1983, then you must have at least been exposed to the fact that all of the archetypal fractal models (mathematical in these cases) involve discrete scaling. This is built into the discrete iteration method of constructing these fractals. Please commit this factual information to your memory so I do not have to repeat it every few months. Bottom Line: There are many different types of fractal models and modeling principles. If you claim there is only one with total scale invariance, then you are wrong - plain and simple. [Mod. note: further discussion of fractals should be directly related to astrophysics or should be taken elsewhere -- mjh] I don't ASSUME that there must be cutoffs. I claim that the observational evidence supports a turnover to homogeneity at a few hundred Mpc, well below the scale of the observable universe. You mean to say *statistical* homogeneity, and to make such a claim you must be ignoring the inconvenient and highly fractal cosmic web via coarse-graining. This is not the first time you have cited papers in support of your arguments where, in fact, they claim something entirely different. Last time, you even quote-mined it to make it appear that the authors claim the opposite of what they claim! Firstly, I am discussing problems with the LCDM toy model, of which the inhomogeneity problem is just one of many. I note you totally ignore the Buchert et al article. Tell us why. You dismiss Kroupa's identification of many serious LCDM toy model problems solely on the basis that he favors a MOND model. I hate to break the bad news to you but the problems that Kroupa has identified exist whether or not MOND models are valid and useful. If you think that is not true, prove it by addressing his points one-by-one and showing references indicating flaws in each specific problem. You've had your chance. I don't see any point in discussing this further. A wise choice on your part, in my opinion. However, it will not be long before new empirical evidence will help us to move toward a scientific answer to the inhomogeneity problem (see Buchert et al, previously cited), and the other problems with LDCM models/assumptions as noted and published in the 4 papers I cited previously. Please note that I would be happy to furnish you with more relevant information should you need and/or want it. Robert L. Oldershaw http://www3.amherst.edu/~rloldershaw Discrete Scale Relativity |
#29
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Advantage Inhomogeneity
In article , "Richard D. Saam"
writes: Is there a "simple" logic to explain the consistent sizes of: 1. Stellar planetary systems 2. Galaxies throughout the visible universe? Depends on the definition of "simple". Galaxies range over several orders of magnitude, so "consistent size" is a non-starter here. Larger galaxies haven't had time to form, smaller ones have been eaten by larger ones. More or less. I don't think this is an outstanding puzzle. As for planetary sytems, we have much less data. As far as our own Solar System goes, if the Oort cloud were much farther away, it probably wouldn't be stable due to perturbations from other stars. So, again, I don't see any mystery here. |
#30
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Advantage Inhomogeneity
In article , "Robert L.
Oldershaw" writes: You mean to say *statistical* homogeneity, That should go without saying. and to make such a claim you must be ignoring the inconvenient and highly fractal cosmic web via coarse-graining. The term "coarse-graining" has a very specific meaning in physics. Why not say "averaging"? The consensus is that the cosmic web is not highly fractal by any useful definition of the term. In particular, there is a scale above which no further structure seems to exist. Firstly, I am discussing problems with the LCDM toy model, of which the inhomogeneity problem is just one of many. The term "toy model" has a very specific meaning, which does not apply to LCDM and doesn't mean "any model which is not completely exact". Please use the terms as they are normally understood. I note you totally ignore the Buchert et al article. Tell us why. I already did. The other three contained nothing about the lack of large-scale homogeneity. You dismiss Kroupa's identification of many serious LCDM toy model problems solely on the basis that he favors a MOND model. Says who? Actually, I am not unsympathetic to MOND. I do think there are better spokesmen than Kroupa, though. I hate to break the bad news to you but the problems that Kroupa has identified exist whether or not MOND models are valid and useful. If you think that is not true, prove it by addressing his points one-by-one and showing references indicating flaws in each specific problem. Just this morning on the arXiv: Title: Reconciling dwarf galaxies with LCDM cosmology: Simulating a realistic population of satellites around a Milky Way-mass galaxy Authors: Andrew R. Wetzel, Philip F. Hopkins, Ji-hoon Kim, Claude-Andre Faucher-Giguere, Dusan Keres, Eliot Quataert Categories: astro-ph.GA Comments: 6 pages, 5 figures. Submitted to ApJ Letters Abstract is below. For a while, people thought that there was a problem because the Hubble constant implied a universe which was younger than other estimates of its age. But this assumed a wrong cosmological model. For decades, it was known that other models----without any new physics or additional assumptions---exist which don't have an "age problem". Some people tried to lower the value of the Hubble constant, some tried to lower other age estimates. It turned out that both were more or less right, and the problem was that one had assumed that we don't live in a low-density universe with a positive cosmological constant. There was always evidence for the former, and the has been evidence for the latter for some time now. So, these days writing a paper on the "age problem" is a non-starter, because it has been solved. The problems of LCDM are not surprising considering that they essentially stem from dark-matter-only simulations, which of course cannot be the last word. Nevertheless, some have seen a crisis in LCDM, some thought that observations were incomplete. But maybe the LCDM scenario AND the observations are essentially correct, if one uses a more realistic model. This is what the paper mentioned above (abstract below) addresses. Please tell us, specifically, what is wrong: quote some text (without leaving anything out within the quotation), tell us why it is wrong, and provide a reference. This is not the first paper to make such claims. It just turns out that "gastrophysics" is not easy to do, and predictions which can be compared to observations might need millions of hours of CPU time. You often mention arXiv papers here. In fairness, shouldn't you have mentioned this one? It explicitly addresses things you are interested in. ----------8-------------------------------------------------------------------- Low-mass "dwarf" galaxies represent the most significant challenges to the cold dark matter (CDM) model of cosmological structure formation. Because these faint galaxies are (best) observed within the Local Group of the Milky Way (MW) and Andromeda (M31), understanding their formation in such an environment is critical. We present the first results from the Latte Project: the Milky Way on FIRE (Feedback in Realistic Environments). This simulation models the formation of a MW-mass galaxy to z = 0 within LCDM cosmology, including dark matter, gas, and stars at unprecedented resolution: baryon mass of 7070 M_sun at spatial resolution down to 1 pc. Latte was simulated using the GIZMO code with a mesh-free method for accurate hydrodynamics and the FIRE model for star formation and explicit feedback within a multi-phase interstellar medium. For the first time, Latte self-consistently resolves the internal structure of dwarf galaxies that form around a MW-mass host down to M_star 10^5 M_ sun. Latte's population of dwarf galaxies agrees well with those observed in the Local Group across a broad range of properties: (1) distributions of stellar masses and stellar velocity dispersions (dynamical masses), including their joint relation, (2) the mass-metallicity relation, and (3) a diverse range of star-formation histories, including their mass dependence. Thus, Latte produces a realistic population of dwarf galaxies at M_star 10^5 M_sun that does not suffer from the "missing satellites" or "too big to fail" problems of small-scale structure formation. We conclude that baryonic physics can reconcile observed dwarf galaxies with standard LCDM cosmology. |
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