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#11
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Advantage Inhomogeneity
In article ,
"Phillip Helbig (undress to reply)" writes: As for homogeneity, this was more an assumption. Obviously stars indicate a very inhomogeneous distribution of mass (no dark matter back then); If the question is history, inhomogeneity was the dominant assumption starting with the solar system itself. At the beginning of the twentieth century, dark patches in the Milky Way were thought to be regions lacking stars. A good history is at http://adsabs.harvard.edu/abs/1940PASP...52...80S (I confess the early date at which dust was recognized surprised me.) Of course the distribution of the nebulae was obviously inhomogeneous, even a century ago. If the question is present understanding: the idea (then an assumption, now something for which there is much observational evidence) is that homogeneity nevertheless exists on large enough scales. I think that's a little too simplistic. Everyone recognizes inhomogeneity on small scales, and the microwave background is very nearly -- but not quite -- homogeneous. Current work is a matter of _quantifying_ the amount of inhomogeneity on different scales, the jargon for this being "cosmic variance." Both observations and simulations are relevant to the problem. There is even a web-based cosmic variance calculator at http://casa.colorado.edu/~trenti/CosmicVariance.html (though its defaults may not be quite up to date, and I wouldn't be surprised if other such calculators exist). As I understand it, the definition of a fractal is that the amount of inhomogeneity is independent of scale. That does not describe the universe we live in. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#12
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Advantage Inhomogeneity
On 2/5/2016 6:30 AM, Phillip Helbig (undress to reply) wrote:
In article , .. larger scales may begin to accrue once again. You are speculating about what happens beyond the horizon. Observations show that the universe is homogeneous on scales larger than, at most, a few hundred Mpc, But don't we have the hierarchical sequence: galaxies - clusters - superclusters - filaments and voids, all up to the scale you mention? much smaller than the size of the observable universe (and very much smaller than the observable universe). So, your claim boils down to the universe becoming inhomogeneous beyond the horizon. Possible, but there is no evidence for it. So your argument is that above the scale of the filaments, and up to that of the observed part of the observable universe, there is a range where no additional structuring has been seen, so we could have the situation that no new inhomogeneities will exist at any larger scale. (Also possible, but couldn't there still be percolating bubbles in some inflation schemes?) .. This has happened in the past several times where one started with an assumption of statistical homogeneity, only to later learn that this assumption was quite incorrect and had to be rejected for a more natural inhomogeneous model. Yes, but that doesn't mean it will always happen. So both possibilities exist, but I wonder what is the favored view by current theories? A few hundred years ago, many new islands and even continents were discovered, but that process stopped. One cannot just extrapolate forever. I expect that better telescopes will one day discover lots of new continents! (But we will need quite good telescopes..) -- Jos |
#13
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Advantage Inhomogeneity
On Friday, February 5, 2016 at 9:59:50 AM UTC-5, Martin Hardcastle wrote:
In article , Firstly Helbig cleverly changes my specific year of "1920", into the phrase "in the 1920s". Those who know their history of the crucial discovery that nature's hierarchy has an additional galactic scale level, also know that in 1920 that idea was rigorously doubted and by 1930 was it accepted by most astronomers. It was most assuredly NOT accepted in 1920. Secondly, and quite interestingly, I planned to argue in today's post that the fact that Obler's Paradox was a dominant enigma in 1920, I repeat 1920, is strong evidence for the case I am making, because Obler's Paradox was based on the assumption of an infinite cosmos made up of a statistically homogeneous distribution of stars. Additionally: JT, New Scientist, and space.com should also acknowledge that there are many different types of fractal models, and that ruling out simplistic continuous fractal models does NOT rule out all fractal models. Sigh, RLO http://www3.amherst.edu/~rloldershaw And FEEL THE BERN!!!! |
#14
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Advantage Inhomogeneity
On Friday, February 5, 2016 at 10:12:17 PM UTC-5, Jos Bergervoet wrote:
On 2/5/2016 6:30 AM, Phillip Helbig (undress to reply) wrote: In article , .. larger scales may begin to accrue once again. You are speculating about what happens beyond the horizon. Observations show that the universe is homogeneous on scales larger than, at most, a few hundred Mpc, But don't we have the hierarchical sequence: galaxies - clusters - superclusters - filaments and voids, all up to the scale you mention? Good point. Helbig and those who have a vested interested in preserving the assumption of "cosmological homogeneity" simply use coarse-graining to hand-wave away the well-known and well-observed large-scale structures you specifically mention in your post. RLO http://www3.amherst.edu/~rloldershaw [Question argument from authority and inadequately tested assumptions] |
#15
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Advantage Inhomogeneity
In article ,
"Robert L. Oldershaw" writes: .. larger scales may begin to accrue once again. You are speculating about what happens beyond the horizon. Observations show that the universe is homogeneous on scales larger than, at most, a few hundred Mpc, But don't we have the hierarchical sequence: galaxies - clusters - superclusters - filaments and voids, all up to the scale you mention? Good point. Helbig and those who have a vested interested in Why do you think that I have a vested interest? preserving the assumption It's not an assumption. It follows from the observed isotropy unless we are in a special position. In some cases (i.e. inferring the CMB temperature at high redshift from excitation levels of atoms and so on) it can even be observed. of "cosmological homogeneity" What is the difference with and without the scare quotes? simply use coarse-graining Coarse-graining has a very specific meaning in physics which is unrelated to what you are discussing here. to hand-wave Hand-waving is an argument which has no substance; you merely claim that other opinions are hand-waving and have not demonstrated it. away the well-known and well-observed large-scale structures you specifically mention in your post. No-one doubts that there are large-scale structures. The point is that the largest structures are significantly smaller than the size of the observable universe and that there is no evidence of a fractal distribution (at least in the way that term is normally understood) even at smaller scales. |
#16
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Advantage Inhomogeneity
On 2/10/2016 1:37 AM, Robert L. Oldershaw wrote:
On Friday, February 5, 2016 at 10:12:17 PM UTC-5, Jos Bergervoet wrote: On 2/5/2016 6:30 AM, Phillip Helbig (undress to reply) wrote: In article , .. larger scales may begin to accrue once again. You are speculating about what happens beyond the horizon. Observations show that the universe is homogeneous on scales larger than, at most, a few hundred Mpc, But don't we have the hierarchical sequence: galaxies - clusters - superclusters - filaments and voids, all up to the scale you mention? Good point. Helbig and those who have a vested interested in preserving the assumption of "cosmological homogeneity" simply use coarse-graining to hand-wave away the well-known and well-observed large-scale structures you specifically mention in your post. 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. For the particles (or more correctly for the quantum fields we use to describe them) we have the fact that the standard model survived now for several decades without a discovery of further compositeness. But that is not usually taken as an indication that it doesn't exist. -- Jos |
#17
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Advantage Inhomogeneity
Op dinsdag 2 februari 2016 16:02:44 UTC+1 schreef David Staup:
On 1/22/2016 9:55 PM, Phillip Helbig (undress to reply) wrote: Yes, the universe is not completely homogeneous. This is obvious. The question is whether it matters. Wouldn't a better question be: why is the universe not completely homogeneous? [[Mod. note -- A region of the universe which is slightly denser than the average density tends to contract due to its self-gravitation, amplifying the inhomogeneity. (This is basically just the Jeans instability.) Numerical simulations of this process produce inhomogeneities which are pretty similar to those we see in the universe today. IMO it is rather "simple" to simulate local inhomogeneities, but that does not say "anything" about our entire universe at present. Of course, we still have to figure out where the initial (small) inhomogeneities came from. Here we start getting into the realm of inflation, quantum fluctuations in the big bang, etc. -- jt]] IMO a better question is can we humans decide if our universe at present (now) is either homogeneous or inhomogeneous. This ofcourse depents about the definition. I specific make here a difference between our universe versus the (entire) universe. Our universe was created "after" the Big Bang. The universe is something larger. If it applies. What ever the answer does it matter? It matter in the sense that assuming our universe is inhomogeneous you can raise the following question: how inhomogeneous is our universe? A related question is: Was our universe always as inhomogeneous? When you study the book: "The Big Bang" Joseph Silk at page 72 maskes a distinction between 13 Era. IMO the most reasonable assumption is that not all these Era or changes in (average) constitution happened every where throughout our entire Universe at the same time. If that assumption is correct than the previous assumption that our universe is inhomogeneous becomes more reasonable. IMO the most logical physical situation even is an increase. To keep our Universe homogeneous (uniform) requires "communication". What makes for me this whole issue so complicated is, that the accepted view is that our universe has no center and no rim, while at the same time the our universe expands (at an accelerated rate?) and becomes larger and emptier i.e. less dense? This is in conflict with the idea that our universe resembles the surface of a balloon. At the same time there exist no observational evidence for this because all what we can observe of the present is locally. To get a picture of what we can observe study this: http://users.telenet.be/nicvroom/friedmann's%20equation.htm Nicolaas Vroom [[Mod. note -- A few comments: 1. The phrases "the universe" and "our universe" have fairly well-established meanings in cosmology https://en.wikipedia.org/wiki/Universe which differ somewhat from the meanings you describe. 2. Silk's book "The Big Bang" is a fine book. However, so far as I can determine from some quick web searching, its newest edition is the 3rd edition, published in Dec 2000. So... it's 16 years old, or older still if you're referencing a prior edition. -- jt]] |
#18
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Advantage Inhomogeneity
In article , Jos Bergervoet
writes: .. larger scales may begin to accrue once again. You are speculating about what happens beyond the horizon. Observations show that the universe is homogeneous on scales larger than, at most, a few hundred Mpc, But don't we have the hierarchical sequence: galaxies - clusters - superclusters - filaments and voids, all up to the scale you mention? Good point. Helbig and those who have a vested interested in preserving the assumption of "cosmological homogeneity" simply use coarse-graining to hand-wave away the well-known and well-observed large-scale structures you specifically mention in your post. That was not my impression. I think we all agree that this sequence exists, Yes. but seems to stop a few hundred Mpc (i.e. from that point on no larger structures have been seen.) No; I don't think all agree on this. By definition, a fractal distribution doesn't "stop" at some scale... The only question is whether the sequence will *resume*, at some scale larger than what we can see today. ....and then resume again. We won't ever be able to see farther than we do today.* (Well, technically, the size of the observable universe is increasing, but negligibly within our lifetimes.) So, this is speculation about what is beyond the horizon. That doesn't put it out of the realm of science, but one would need a good reason to believe that the universe just on the other side of our horizon is substantially different. 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. There is no evidence for this. But this is really different, since we could in principle probe smaller scales than we do today. As Feynman said, there is a lot of room at the bottom. For the particles (or more correctly for the quantum fields we use to describe them) we have the fact that the standard model survived now for several decades without a discovery of further compositeness. But that is not usually taken as an indication that it doesn't exist. But there are several orders of magnitude here, and further elementary particles would not necessarily change the observed phenomena. On the other hand, it would be a coincidence---unless there were an explanation---why inhomogeneity should pick up again just beyond our horizon. (It might pick up again on much larger scales, but this would not be in accord with any fractal model.) ___ *When asked in an interview if he would have made the universe any differently, Maarten Schmidt, after overcoming his surprise at the question, said that he would have made it bigger, pointing out that with the help of pieces of glass which fit in a (big) room, one can see as far as it is possible to see. |
#19
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Advantage Inhomogeneity
In article ,
Nicolaas Vroom writes: IMO the most reasonable assumption is that not all these Era or changes in (average) constitution happened every where throughout our entire Universe at the same time. On the contrary, there is much evidence for this. If that assumption is correct than the previous assumption that our universe is inhomogeneous becomes more reasonable. IMO the most logical physical situation even is an increase. To keep our Universe homogeneous (uniform) requires "communication". Yes. This is known as the isotropy (or flatness) problem. Without inflation, or something similar, there is no solution. This is independent of the degree of inhomogeneity in large-scale structure and so on; you still have to explain the isotropy of the CMB. What makes for me this whole issue so complicated is, that the accepted view is that our universe has no center and no rim, while at the same time the our universe expands (at an accelerated rate?) and becomes larger and emptier i.e. less dense? Yes. This is in conflict with the idea that our universe resembles the surface of a balloon. Why? The VOLUME of the universe corresponds to the SURFACE of the balloon. At the same time there exist no observational evidence for this because all what we can observe of the present is locally. No. We can observe what is on our backward light cone (through astronomical observations) and, much more locally, what is inside it. We can infer stuff about the region near the light cone from measurements of the temperature of the CMB at higher redshift and so on. |
#20
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Advantage Inhomogeneity
On 2/10/2016 9:54 PM, Phillip Helbig (undress to reply) wrote:
In article , Jos Bergervoet .. .. By definition, a fractal distribution doesn't "stop" at some scale... The only question is whether the sequence will *resume*, at some scale larger than what we can see today. ...and then resume again. So a fractal distribution is ruled out (but I think it was ruled out already in the hierarchy of scales mentioned, unless you are very liberal about the definition). But new very large scale structures are not ruled out. We won't ever be able to see farther than we do today.* (Well, technically, the size of the observable universe is increasing, but negligibly within our lifetimes.) So, this is speculation about what is beyond the horizon. That doesn't put it out of the realm of science, but one would need a good reason to believe that the universe just on the other side of our horizon is substantially different. That would be a coincidence. But instead of looking just on the other side of the horizon, we could consider the scales incredibly far beyond the horizon, and then perhaps the opposite holds. It would be a coincidence to have a desert of perfect homogeneity all the way upwards. ... There is no evidence for this. And I wonder if there ever will be.. .. But this is really different, since we could in principle probe smaller scales than we do today. As Feynman said, there is a lot of room at the bottom. The Planck scale, and *much* further the Landau pole of QED, those are at least landmarks to look forward to! :-) But practically speaking they are just as unobservable now as the Teraparsec scale of the universe.. ... For the particles (or more correctly for the quantum fields we use to describe them) we have the fact that the standard model survived now for several decades without a discovery of further compositeness. But that is not usually taken as an indication that it doesn't exist. But there are several orders of magnitude here, and further elementary particles would not necessarily change the observed phenomena. On the other hand, it would be a coincidence---unless there were an explanation---why inhomogeneity should pick up again just beyond our horizon. But *never* to pick up any more is just as coincidental.. (It might pick up again on much larger scales, but this would not be in accord with any fractal model.) A fractal model is not needed. It would be a coincidence by itself! A completely irregular sequence of scales is the most unprejudiced assumption, I would think. .. *When asked in an interview if he would have made the universe any differently, Maarten Schmidt, after overcoming his surprise at the question, said that he would have made it bigger, pointing out that with the help of pieces of glass which fit in a (big) room, one can see as far as it is possible to see. He doesn't have enough patience. -- Jos |
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