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Missing matter found in the cosmic web
The article "Missing matter found in the cosmic web" in Nature of 21
June 2018 (See https://www.nature.com/articles/d41586-018-05432-2) Starts with the following sentence: "We live in a dark Universe: just 5% of it consists of ordinary matter such as that found in atoms, whereas the rest is `dark' matter and energy that cannot currently be detected directly" The word dark is written within '' indicating doubt. Accordingly to https://en.wikipedia.org/wiki/Dark_matter: "In the standard Lambda-CDM model of cosmology, the total mass-energy of the universe contains 4.9% ordinary matter and energy, 26.8% dark matter and 68.3% of an unknown form of energy known as dark energy. Thus, dark matter constitutes 84.5% of total mass, while dark energy plus dark matter constitute 95.1% of total mass-energy content." Next we read in the nature article: "However, observations of the nearby Universe suggest that up to 40% of this ordinary matter---which is made up primarily of particles known as baryons---is missing" This is a strange twist. What we observe/measure are 1) galaxy rotation curves and 2) an expanding universe. What we also observe is 3) stars and baryonic matter throughout the universe. However the amount found as #3 is not enough to explain #1 and #2. To solve this issue we introduced the concepts of dark (missing) matter and dark energy. And this missing matter is supposed to be nonbaryonic. However accoringly to Wikipedia there is also a Missing baryon problem. See: https://en.wikipedia.org/wiki/Missing_baryon_problem. That means there are two problems: 1) A dark matter problem and 2) a Missing baryon problem. (In reality there are more issues: CMBR and BB nucleosynthesis) What this article indicates is that there is much more baryonic matter in the cosmic web (Universe) as original thought. To me this seems logical because more and more ordinary matter becomes visible because technology improves. My question is why is newly found matter 'clasified' as a solution for problem #2 (and not #1) Different question: Why are there two problems in the first place? Maybe Fig 4 at page 408 shows the answer. They mention the word Local Universe which makes everything much more complex. Nicolaas Vroom http://users.pandora.be/nicvroom/ |
#2
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Missing matter found in the cosmic web
In article ,
Nicolaas Vroom writes: The article "Missing matter found in the cosmic web" in Nature of 21 June 2018 (See https://www.nature.com/articles/d41586-018-05432-2) Starts with the following sentence: "We live in a dark Universe: just 5% of it consists of ordinary matter such as that found in atoms, whereas the rest is `dark' matter and energy that cannot currently be detected directly" The word dark is written within '' indicating doubt. Not really doubt, but probably to indicate that it is not to be taken too literally. First, "transparent" might be a better term. Yes, it's dark, since it doesn't emit electromagnetic radiation, but neither does it interact with electromagnetic radiation at all. "Dark energy" is really a stupid term, modelled on "dark matter" (which does make at least some kind of sense). Substitute "cosmological constant" as there is no evidence at all against the idea, and much for it, that "dark energy" is just the good old cosmological constant. Accordingly to https://en.wikipedia.org/wiki/Dark_matter: "In the standard Lambda-CDM model of cosmology, the total mass-energy of the universe contains 4.9% ordinary matter and energy, 26.8% dark matter and 68.3% of an unknown form of energy known as dark energy. Thus, dark matter constitutes 84.5% of total mass, while dark energy plus dark matter constitute 95.1% of total mass-energy content." Right. Next we read in the nature article: "However, observations of the nearby Universe suggest that up to 40% of this ordinary matter---which is made up primarily of particles known as baryons---is missing" This is a strange twist. What we observe/measure are 1) galaxy rotation curves and 2) an expanding universe. What we also observe is 3) stars and baryonic matter throughout the universe. However the amount found as #3 is not enough to explain #1 and #2. To solve this issue we introduced the concepts of dark (missing) matter and dark energy. Right so far. And this missing matter is supposed to be nonbaryonic. Not all of it. Big-bang nucleosynthesis tells us rather precisely how many baryons there are. The difference between this and what is observed in baryons are the missing baryons. The rest of the missing matter is the dark matter, thus most of it is non-baryonic. Often "dark matter" is used as a synonym for "unknown non-baryonice matter". (Neutrinos, and electrons, for that matter, are known baryonic matter, but their contribution to the mass budget is much smaller than that of baryons.) Until recently, the uncertainty in the total mass density was greater than that of the mass of baryons, so, at least as far as the numbers go, saying that all dark matter is unknown non-baryonic matter, or vice versa, was an acceptable approximation. However accoringly to Wikipedia there is also a Missing baryon problem. See above. See: https://en.wikipedia.org/wiki/Missing_baryon_problem. That means there are two problems: 1) A dark matter problem and 2) a Missing baryon problem. Right. (In reality there are more issues: CMBR and BB nucleosynthesis) They aren't issues, but observations, and both tell us how many baryons there are. What this article indicates is that there is much more baryonic matter in the cosmic web (Universe) as original thought. To me this seems logical because more and more ordinary matter becomes visible because technology improves. Right; no surprise here. (While I am sympathetic to MOND, and think that most critics don't really understand it, one of my main objections to "MOND philosophy" is the assumption, explicitly stated or otherwise, that there is something strange about matter that we cannot see. This doesn't challenge standard physics any more than the discovery of gorillas challenged Linnaeus's binomial classification system.) My question is why is newly found matter 'clasified' as a solution for problem #2 (and not #1) Because there is much too little to solve problem #1. Different question: Why are there two problems in the first place? Problem 2: We don't see all the baryons (but might be seeing more now). No surprise there. Problem 1: the difference between what is deduced from BBN and CMB (which is more than directly observed---the difference is the missing baryons) and what we need to explain rotation curves of galaxies as well as cosmological observations. It is conceivable (in my view, even likely) that something like conventional dark matter is needed for the latter and something MOND-like for the former. Perhaps, as Khoury (who has done some of the most interesting work in astrophysics in the last few years) suggests, these are two sides of the same coin. |
#3
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Missing matter found in the cosmic web
In article ,
Nicolaas Vroom writes: The article "Missing matter found in the cosmic web" in Nature of 21 June 2018 (See https://www.nature.com/articles/d41586-018-05432-2) The above is a useful summary of the article. The actual paper is at http://www.nature.com/articles/s41586-018-0204-1 but behind a paywall. There is a free preprint at https://arxiv.org/abs/1806.08395 Accordingly to https://en.wikipedia.org/wiki/Dark_matter: "In the standard Lambda-CDM model of cosmology, the total mass-energy of the universe contains 4.9% ordinary matter and energy, 26.8% dark matter and 68.3% of an unknown form of energy known as dark energy. That looks like a fair summary. The evidence for this census is diverse and has been discussed here and elsewhere. I haven't read the Wikipedia article and can't vouch for it, but it describes the evidence. Next we read in the nature article: "However, observations of the nearby Universe suggest that up to 40% of this ordinary matter "Ordinary matter" refers to the 4.9% (which I'll round off to 5%). ---which is made up primarily of particles known as baryons---is missing" See Table 1 of the paper. There are large uncertainties, especially in the hot gas components. The "primarily" is because electrons count in this portion even though they aren't baryons, but they contribute a trivial amount of mass. What we observe/measure are 1) galaxy rotation curves and and many more things than that, all of which add up to about 3% of the total density, not 5% as they should. However accoringly to Wikipedia there is also a Missing baryon problem. See: https://en.wikipedia.org/wiki/Missing_baryon_problem. Which is what is described above: 3% 5%. there are two problems: 1) A dark matter problem and 2) a Missing baryon problem. I'm not sure what you mean by "problems," but missing baryons have nothing to do with non-baryonic matter. more and more ordinary matter becomes visible because technology improves. Indeed. The observations reported were from a heroic effort using a premier space observatory. My question is why is newly found matter 'clasified' as a solution for problem #2 (and not #1) What they have found is oxygen, which they extrapolate to give a mass of hydrogen associated with the oxygen. These elements are, of course, baryonic, and they add something like 2% to the 3% already known, potentially making up the 5% that baryons constitute. There are large uncertainties and possible systematic errors in the observations, and there have been other papers along these same lines. Many have been discussed in this newsgroup. The upshot is that the missing baryons are almost certainly hot gas, but the distribution of this hot gas is far from clear. Different question: Why are there two problems in the first place? I am not sure I understand the question. There are two forms of matter in the universe. Baryonic matter makes up 5% of the total density, but only 60% of this (3% of the total) has been accounted for. It would be nice to know what the rest is, and this paper provides evidence towards an answer. Non-baryonic matter makes up 27% of the total energy density, and we have little evidence of what it is. Some hypotheses are ruled out by existing observations, but others are still possible. Non-baryonic matter may be a mix of different things, and some or all may be something we haven't thought of yet. This has nothing to do with accounting for the baryons. Dark energy, the remaining 68%, is something different still. There is little evidence for what it is, but all the evidence I'm aware of is consistent with its being a cosmological constant. I personally have no problem with that. The cosmological constant has to have _some_ value, and there's no reason that value must be zero. Maybe Fig 4 at page 408 shows the answer. You mean Fig 4 of the article? That shows the new baryon census based on the results of the paper. It is far from the final word but is plausible. They mention the word Local Universe which makes everything much more complex. Why more complex? Measurements such as the one reported can only address the local universe. Presumably the census changes over time, for example as gas is converted to stars, but the baryon fraction should not change. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
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Missing matter found in the cosmic web
"Phillip Helbig (undress to reply)" wrote:
| "Dark energy" is really a stupid term, modelled on "dark matter" (which | does make at least some kind of sense). Substitute "cosmological | constant" as there is no evidence at all against the idea, and much for | it, that "dark energy" is just the good old cosmological constant. [...] Yes, but can't they simply call it "vacuum energy" as in "vacuum" displacement or polarisation when talking about eps_0 in EM? (I confess to be a little motivated by Carlo Ovelli "Reality Is Not What It Seems: The Journey to Quantum Gravity") -- ciao, Bruce drift wave turbulence: http://www.rzg.mpg.de/~bds/ [[Mod. note -- Calling it "vacuum energy" would be making an implicit statement that it has something to do with vacuum energy/polarization in the sense you're using it. I don't think we know that. (On the other hand... calling it "cosmological constant" is also making an implicit statement that it's trully *constant*, i.e., that it enters into the Einstein equations in a certain way, with NO terms involving the spacetime derivatives of the "cosmological constant". We don't know that, either. About all we know today is its average value over the past 10^10-or-so years. We probably won't know much about its time variation or lack thereof for another decade.) -- jt]] |
#5
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Missing matter found in the cosmic web
In article , "Bruce D. Scott"
writes: | "Dark energy" is really a stupid term, modelled on "dark matter" (which | does make at least some kind of sense). Substitute "cosmological | constant" as there is no evidence at all against the idea, and much for | it, that "dark energy" is just the good old cosmological constant. [...] Yes, but can't they simply call it "vacuum energy" as in "vacuum" displacement or polarisation when talking about eps_0 in EM? As Jonathan noted, this makes an assumption about its origin. (I confess to be a little motivated by Carlo Ovelli "Reality Is Not What It Seems: The Journey to Quantum Gravity") I'm reading that myself at the moment. :-) [[Mod. note -- To clarify, the following 7 quoted lines were written by me (Jonathan Thornburg), not Bruce D Scott. -- jt]] (On the other hand... calling it "cosmological constant" is also making an implicit statement that it's trully *constant*, i.e., that it enters into the Einstein equations in a certain way, with NO terms involving the spacetime derivatives of the "cosmological constant". We don't know that, either. About all we know today is its average value over the past 10^10-or-so years. We probably won't know much about its time variation or lack thereof for another decade.) True. On the other hand, there is no evidence that it is not constant, and people have looked for such a deviation. Obviously, one can put only upper limits on such deviations. The traditional cosmological constant was there long before observations made it clear that it or something like it actually exists. As long as a constant value fits the data, there is no reason to assume otherwise, unless someone has a really convincing theory (which should predict variation at some level which could, at least in principle, be confirmed). However, one should be open to a more complicated form, not repeating the mistakes of assuming it is zero until forced otherwise by the data, as happened 30 or so year ago. |
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Missing matter found in the cosmic web
On 2018-07-12, Steve Willner wrote:
Dark energy, the remaining 68%, is something different still. There is little evidence for what it is, but all the evidence I'm aware of is consistent with its being a cosmological constant. I personally have no problem with that. The cosmological constant has to have _some_ value, and there's no reason that value must be zero. Nice summary in general, just want to comment on this... you can take the position it should be zero unless you have a reason for it. Fitting the data is well enough (but I've seen that go wrong many times in plasma physics where the underlying asumption of the thing and its cause both being totally wrong and the community taking 20 years to wake up to it). I take the position that zero is a reasonable a priori assumption, but that if it has a value there should be a reason for it (ie, why is it not very large). It may be like the photon mass, so small as not to rock the boat with a theory in which it is zero and which is successful for anything else which is known (at least below whatever it is... 5 MeV or so for the nonlinearity in the electron scattering cross section). Do we have solid evidence that it is _different from zero_ and if so what does the curvature of the universe have to be? I guess if we say 68 percent of the curvature is due to the quoted dark energy fitting then this should be something. I think if we know enough it may be a property of space-time rather than a species of field/particle... but I guess this is the same thing as "cosmological constant". (does this follow from universe accelaration as per the supernovae observations from 20 years ago? but that's negative curvature isn't it) -- ciao, Bruce |
#7
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Missing matter found in the cosmic web
In article , Bruce Scott
writes: On 2018-07-12, Steve Willner wrote: Dark energy, the remaining 68%, is something different still. There is little evidence for what it is, but all the evidence I'm aware of is consistent with its being a cosmological constant. I personally have no problem with that. The cosmological constant has to have _some_ value, and there's no reason that value must be zero. Nice summary in general, just want to comment on this... you can take the position it should be zero unless you have a reason for it. Actually, it is usually the reverse in science. If nature has a degree of freedom, she uses it. If it is NOT observed, then there is a reason for it---a conservation law, for example. In particle physics, "if it can happen, it will" is the standard approach. If something doesn't happen, then it has to have a reason. The burden of proof is then on the person claiming that it is zero if this is just a theoretical, as opposed to observational, claim. Fitting the data is well enough (but I've seen that go wrong many times in plasma physics where the underlying asumption of the thing and its cause both being totally wrong and the community taking 20 years to wake up to it). Not really an issue here, as we are talking 1920s cosmology. If anything, the surprise is that despite the high quality and quantity of current data, all of it can be fit with ideas which have been around for decades. I take the position that zero is a reasonable a priori assumption, No; see above. but that if it has a value there should be a reason for it (ie, why is it not very large). Large compared to what? Some people complain it is way to small, compared to the expectation from quantum field theory. Others are surprised that the energy density in the cosmological constant is comparable to that in ordinary matter (with, for some, the additional puzzle that this is not always the case, but is now). It may be like the photon mass, so small as not to rock the boat with a theory in which it is zero and which is successful for anything else which is known (at least below whatever it is... 5 MeV or so for the nonlinearity in the electron scattering cross section). I'm sure that upper limits on the photon mass are much smaller than 5 MeV. Do we have solid evidence that it is _different from zero_ Yes. This is essentially what the Nobel Prize in physics for 2011 was awarded for. and if so what does the curvature of the universe have to be? The curvature of the universe depends on the sum of the cosmological constant and the density parameter. Observations indicate that the universe is close to being flat and perfect flatness is not yet ruled out. I guess if we say 68 percent of the curvature is due to the quoted dark energy fitting then this should be something. I think if we know enough it may be a property of space-time rather than a species of field/particle... but I guess this is the same thing as "cosmological constant". Right. This is essentially the question whether the cosmological constant is "geometric" and belongs on the left side of the Einstein equation, or is a source term with a certain equation of state and belongs on the right side. This goes back to a discussion between Einstein and Schr=F6dinger: E. Schr=F6dinger, _Physikalische Zeitschrift_, 19, 20, 1918. A. Einstein, _Physikalische Zeitschrift_, 19, 165, 1918. (does this follow from universe accelaration as per the supernovae observations from 20 years ago? but that's negative curvature isn't it) Yes, the supernova observations are an important reason for believing in a positive cosmological constant. But even without them, the data point to it. Not negative curvature, though; both matter and a (positive) cosmological constant make the curvature more positive. You might be thinking of negative pressure. Contrary to what one might think, positive pressure acts like normal matter: causes deceleration. Negative pressure thus causes acceleration. Since matter thins out as the universe expands and the cosmological constant doesn't (which is why it is called the cosmological CONSTANT), early on matter dominates, then with time (already in our past) the cosmological constant dominates. |
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Missing matter found in the cosmic web
In article ,
Bruce Scott writes: Do we have solid evidence that it is _different from zero_ If by "it" you mean dark energy, yes. There are three independent lines of evidence: the CMB fluctuations, the SN distances, and baryon acoustic oscillations. All three agree on the values within their respective uncertainties. and if so what does the curvature of the universe have to be? The universe is flat to within 0.5% or so. You can find a good summary in the _Planck_ 2015 paper at https://www.aanda.org/articles/aa/ab...a25830-15.html The paper is open access, and there are links to both html and pdf versions. Look in the various tables, but you will have to read the text for the symbol definitions. In particular, watch out for h = H_0/100 =~ 0.67, which is not very close to 1. I guess [dark energy] is the same thing as "cosmological constant". The term "dark energy" means something like a cosmological constant but allows for a more general case where the (negative) pressure varies with time. So far there is no evidence it does. -- Help keep our newsgroup healthy; please don't feed the trolls. Steve Willner Phone 617-495-7123 Cambridge, MA 02138 USA |
#9
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Missing matter found in the cosmic web
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