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Advantage Inhomogeneity
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Advantage Inhomogeneity
In article ,
"Robert L. Oldershaw" writes: http://arxiv.org/abs/1505.07800 also https://telescoper.wordpress.com/201...oes-it-matter/ And? The blog post essentially points to the same paper, which is ONE SIDE in a current debate about whether backreaction can be neglected or not (i.e. how strong the effect is and to what extent, IF ANY, standard conclusions could be wrong by failing to take it properly into account). While some well known pundits are authors of the above paper, one of the main players on the other side is Bob Wald, who is in the same league. Yes, the universe is not completely homogeneous. This is obvious. The question is whether it matters. And, just to be clear, the above paper is not concerned with any sort of fractal stuff. Yes, fractals are inhomogeneous, but not all inhomogeneities are fractal. |
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Advantage Inhomogeneity
On Friday, January 22, 2016 at 10:55:32 PM UTC-5, Phillip Helbig (undress to reply) wrote:
Yes, the universe is not completely homogeneous. This is obvious. The question is whether it matters. And, just to be clear, the above paper is not concerned with any sort of fractal stuff. Yes, fractals are inhomogeneous, but not all inhomogeneities are fractal. --------------------------------------- Wow! Could we be in agreement? Almost. To me everything matters. |
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Advantage Inhomogeneity
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. 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]] |
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Advantage Inhomogeneity
In article , David Staup
writes: Wouldn't a better question be: why is the universe not completely homogeneous? I don't think so. First, if it were completely homogeneous, there could be no-one around to note that fact. Second, complete or even near homogeneity implies special initial conditions or some sort of interaction to smooth it out. In other words, the default expectation is that it is inhomogeneous. The problem is to try to explain the observed near homogeneity, in other words why it is not completely inhomogeneous. If you imagine the expansion of the universe running in reverse, you will find that parts of the universe which now are widely separated were not in causal contact in the early universe, so the question arises why they are so similar. This is the isotropy problem (sometimes called the horizon problem). Inflation provides an answer: the very early universe is not just a backwards extrapolation of the current expansion, but there was a short phase of exponential inflation, which allows the entire observable universe to have expanded from a patch so small that it was in equilibrium. This also provides an explanation for the origin of the fluctuations which we do see, namely quantum fluctuations. This sounds like a just-so story, but, even though there are many varieties of inflation, a fairly generic prediction was made regarding the perturbation spectrum, i.e. the relative strength of perturbations on different scales. This was made long before there was any observational evidence one way or the other, and the prediction has been confirmed. So, this lends some credence to the inflationary idea. |
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Advantage Inhomogeneity
On Wednesday, February 3, 2016 at 11:25:09 PM UTC-5, Phillip Helbig (undres=
s to reply) wrote: is that it is inhomogeneous. The problem is to try to explain the=20 observed near homogeneity, in other words why it is not completely=20 inhomogeneous. =20 ------------------------------- At the risk of repeating myself yet again on a message that never seems to register in certain places, consider this simple historical fact. In 1920 astronomers considered the most likely distribution of matter in the observable universe, and beyond, to be a statistically homogeneous distribution stars. That is the way the cosmos looked to most physicists and astronomers at the time. Then came the the discovery of galaxies at far greater distances than had previously been considered to be observable. We could be in a roughly analogous situation right now today. We look out and say, "Hmmm, it looks very homogeneous to us", but when our observational capacities improve enough to increase the observable part of the local universe, the evidence for strong inhomogeneity on larger scales may begin to accrue once again. 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. Sigh. Or should I say - Duh! RLO http://www3.amherst.edu/~rloldershaw [[Mod. note -- I don't think the statement "In 1920 astronomers considered the most likely distribution of matter in the observable universe, and beyond, to be a statistically homogeneous distribution stars." is true: Galileo's observations of the Milky Way resolved it into individual stars, which were manifestly distributed in a highly inhomogeneous manner (i.e., the brightness and number-of-stars-per-square-degree of the Milky Way vary a lot from one part of the sky to another). -- jt]] |
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Advantage Inhomogeneity
On Thursday, February 4, 2016 at 11:27:53 AM UTC-5, Robert L. Oldershaw wro=
te: =20 In 1920 astronomers considered the most likely distribution of matter in the observable universe, and beyond, to be a statistically homogeneous distribution stars. That is the way the cosmos looked to most physicists and astronomers at the time. =20 =20 RLO http://www3.amherst.edu/~rloldershaw =20 [[Mod. note -- I don't think the statement "In 1920 astronomers considere= d the most likely distribution of matter in the observable universe, and beyond, to be a statistically homogeneous distribution stars." is true: Galileo's observations of the Milky Way resolved it into individual stars= , which were manifestly distributed in a highly inhomogeneous manner (i.e., the brightness and number-of-stars-per-square-degree of the Milky Way var= y a lot from one part of the sky to another). -- jt]] -------------------------------------------------- Well, that [a statistically homogeneous distribution of stars] explicitly was Einstein's opinion in 1920 and early evidence for galactic scale structure was vigorously resisted by the conservative wing of the physics/astronomy community of the time - not the least bit surprisingly. This is a matter of record. [[Mod. note -- It's important not to confuse the hypotheses (1) Stars are uniformly distributed throughout the universe at any (or some particular) time. (2) Galaxies are uniformly distributed throughout the universe at any (or some particular) time. (3) Averaged over a sufficiently large volume, galaxies are uniformly distributed throughout the universe at any (or some particular) time. (4) Averaged over a sufficiently large volume, mass(-energy) is uniformly distributed throughout the universe at any (or some particular) time. As I noted above, (1) is manifestly false, and its falsity has been known for a long time. (2) is also false, though this fact wasn't well-known until the advent of large-scale galaxy redshift surveys and the discovery of the "great wall" in the early 1980s. (3) and (4) are assumed to be true by almost all cosmological models (including Einstein's). A fractal cosmology necessarily implies that (3) and (4) are false. Large-scale galaxy redshift surveys argue that (3) is true, and observations of the cosmic microwave background argue that (4) is true. -- jt]] |
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Advantage Inhomogeneity
In article ,
"Robert L. Oldershaw" writes: On Wednesday, February 3, 2016 at 11:25:09 PM UTC-5, Phillip Helbig (undress to reply) wrote: is that it is inhomogeneous. The problem is to try to explain the observed near homogeneity, in other words why it is not completely inhomogeneous. ------------------------------- At the risk of repeating myself yet again on a message that never seems to register in certain places, consider this simple historical fact. In 1920 astronomers considered the most likely distribution of matter in the observable universe, and beyond, to be a statistically homogeneous distribution stars. That is the way the cosmos looked to most physicists and astronomers at the time. This was never a viable model, and never was a model at all, at least in the last few hundred years in Europe. In the 1920s, it was determined that many nebulae are extragalactic systems, i.e. other galaxies. Up until then, the "island universe" model was common, i.e. a finite assembly of stars, surrounded by the void. Then came the the discovery of galaxies at far greater distances than had previously been considered to be observable. We could be in a roughly analogous situation right now today. We could, but we are not. We look out and say, "Hmmm, it looks very homogeneous to us", but when our observational capacities improve enough to increase the observable part of the local universe, the evidence for strong inhomogeneity on 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, 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. 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. A few hundred years ago, many new islands and even continents were discovered, but that process stopped. One cannot just extrapolate forever. |
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Advantage Inhomogeneity
In article ,
"Robert L. Oldershaw" writes: In 1920 astronomers considered the most likely distribution of matter in the observable universe, and beyond, to be a statistically homogeneous distribution stars. That is the way the cosmos looked to most physicists and astronomers at the time. Well, that [a statistically homogeneous distribution of stars] explicitly was Einstein's opinion in 1920 Yes, but Einstein was not an astronomer. Just yesterday, I read "astronomers, as opposed to cosmologists". Yes, there is some overlap, but not always. The early days of cosmology occurred at the same time as, but largely in isolation from, the debate (including the Curtis-Shapley "Great Debate") about the size of the Galaxy, whether the nebulae were extragalactic, etc. Einstein's main initial assumption was a STATIC universe. As for homogeneity, this was more an assumption. Obviously stars indicate a very inhomogeneous distribution of mass (no dark matter back then); the idea (then an assumption, now something for which there is much observational evidence) is that homogeneity nevertheless exists on large enough scales. Einstein's main mistake was assuming that the universe is static. |
#10
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Advantage Inhomogeneity
In article ,
Phillip Helbig (undress to reply) wrote: This was never a viable model, and never was a model at all, at least in the last few hundred years in Europe. (For those who don't know why, look up 'Olbers' paradox'.) Martin -- Martin Hardcastle School of Physics, Astronomy and Mathematics, University of Hertfordshire, UK Please replace the xxx.xxx.xxx in the header with herts.ac.uk to mail me |
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