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I Questions regarding dark matter dynamics

  1. Apr 16, 2017 #1
    I recall reading somewhere (I can't remember where) that there is astronomical evidence that dark matter has different densities in different parts of the relatively near parts of the observable universe.
    Q1. Is this correct? If so, can someone please post a link to a relevant article?

    The following observational possibility come to mind.
    Consider the relationship R(r,v) between (1) the middle radius r of a cylindrical shell centered on the center of gravity a galaxy, and parallel to the axis of the total angular momentum of the stars in the galaxy, and (2) the average orbital velocity v of stars observed within that cylindrical shell. I understand that observers have noticed that this relationship R(r,v) is very much dissimilar from what would be expected if dark matter was not present.
    Q2. Can the density ρDM of a dark matter "cloud" surrounding a galaxy be estimated by the galaxy's relationship R(r,v)? Does anyone know whether this has been done? If "yes", does anyone know if the variability of such estimates show statistically significant differences (based on the observational error range of the individual ρDM estimates) between the individual values and their average?

    Q3. The same question as Q2 except applied to galaxy clusters and their galaxies, rather than galaxies and their stars.

    Any responding posts will be much appreciated, especially those with links to relevant articles.
  2. jcsd
  3. Apr 21, 2017 #2
    Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.
  4. Apr 23, 2017 #3

    stefan r

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    I thought they were measuring hydrogen ions. Emission frequency gets red shifted which tells us the ion's velocity. The interstellar gas that is orbiting far away from galaxy centers moving too fast relative to stars and ions in the galaxy center. This is the original reason to believe that dark matter exists. There has to either be mass we cannot see or our model of gravity is wrong.

    Here is a paper from 1966:
    And from 1978:

    I found them on wikipedia's references.

    The recent work on dark matter using gravitational lensing is not very local.
  5. May 6, 2017 #4
    I agree.

    There is a general consensus that Einstein rings show a large mass discrepancy between luminous and non-luminous mass compatible with the missing mass of dark matter. Looking at some recent papers, Magain, P. and Chantry, V. (2013) and Yong Tian1 and Chung-Ming Ko (2017) there appears only about as much missing mass as there is luminous mass. In the latter paper the researchers plot Mob / Mbar versus gbar (acceleration due to gravity from the baryonic mass), where Mob is the observed mass from the Einstein ring and Mbar is the baryonic (luminous) mass which gives an almost flat line between 1 and 2. Since Mob = Mbar + Mdm, Mob / Mbar = 1 + Mdm / Mbar: then if Mob / Mbar = 1, then Mdm = 0; and if Mob / Mbar = 2, then Mdm = Mbar. The conclusion is that the range of Mdm is; 0 < Mdm < Mbar. The first paper concludes: ‘Our results thus suggest that, if dark matter is present in early-type galaxies, its amount does not exceed the amount of luminous matter and its density follows that of luminous matter, in sharp contrast to what is found from rotation curves of spiral galaxies.’

    These two papers are in complete agreement. Similar conclusions were reached by McGaugh et at (2016) and Lelli (2016). This seriously questions the idea of extra-galactic dark matter of any density.
  6. May 7, 2017 #5

    stefan r

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    The conclusion that dark matter is not present in early galaxies is not the same as concluding that dark matter does not exist. The early galaxies are also missing iron for example.
  7. May 8, 2017 #6
    I would suggest that the formation of iron from precursor nuclei is well established. The same is not apparent for dark matter. Even if dark matter evolves from one type of dark matter particle to another there would have to be some form of dark matter in an early galaxy unless baryonic matter can transform to dark matter! Alternatively, the dark matter can coalesce around the galaxy later but I thought one of the drivers for dark matter was seeding of galaxies so one would think it would be there already at galaxy formation.
  8. May 8, 2017 #7
    Hi stefen and Adrian:

    There is strong evidence that the amount of non-baryonic (dark) matter is about five times as much as baryonic (ordinary) matter, and this ratio has been stable since the time of primordial nucleosynthesis which took place between about ten seconds and twenty minutes after the big bang.
    The discovery that only about 1/6 of matter was baryonic was known before the dark matter influence on star velocities within a galaxy was discovered. The following 1999 paper discusses the discovery based on Deuterium abundances, but I think this idea was around much earlier.

  9. May 19, 2017 #8
    I thought nucleosynthesis was about the production of nuclei from baryons, so I can't see how this has any relation to dark matter. The second paper you gave a link to is quite old (1996) and comments that 40% dark matter is in 'compact nucleonic objects'. Now these would be labelled as MACHOs but the current standard model is that dark matter is non-baryonic.
  10. May 19, 2017 #9
    Hi Adrian:
    It is known what the density of matter is (to reasonable precision), and also what it was was during nucleosynthesis. The reasoning that a large fraction was not protons and neutrons at the time is that if it had been, then the amount of deuterium that would have been made and not consumed in the making of helium would be different than the observed deuterium density.

    Regarding the wrong guesses during the 1990's about what dark matter might be made of, scientists of several persuasions are still making wrong guesses, so that hasn't changed much.

  11. May 21, 2017 #10
    I think it's important to be extremely cautious about claiming to hold knowledge about the existence of exotic forms of matter based on astronomical studies rather than actual lab results in controlled experimentation. The existence of "dark matter" is strongly based upon the validity of the "estimates" of baryonic mass that we've been using. I can think of at *least* 5-6 different flaws in the baryonic mass estimates that were used in the famous Bullet Cluster study of 2006. The recent revelations of two different types of 'halos' of both million degree plasma, and of hydrogen gas surrounding our own galaxy might also go a long way to explaining galaxy rotation patterns that we observe.

    About all you can tell from the lensing data specifically is that there's a serious discrepancy between our baryonic mass estimates techniques based upon light, and our mass estimates based on lensing. The revelations of the last decade would suggest that the lensing data is probably accurate, whereas the baryonic mass estimate techniques of galaxies that we've been using are considerably inaccurate:

    The existence of exotic forms of matter are dependent upon the *assumption* that the baryonic galaxy mass estimation techniques were accurate in 2006, and therefore any "missing mass' was necessarily found in a *non baryonic* form of matter.


    Since 2006 however, there have been five major revelations of a systematic problem with our flawed calculation of stellar masses that are present in various galaxies and galaxy clusters:

    1) Two years later in 2008, we "discovered" that we've been underestimating the amount of scattering taking place in the IGM, and the universe is actually at least *twice as bright* as *assumed*, leading to an *underestimation* of stellar mass:


    Keep in mind that the entire basis for the baryonic mass calculation of stellar masses relates back to galaxy brightness. They underestimated the brightness aspect by a factor of two in that 2006 lensing study.

    2) We also "discovered" a year later that we've been using a *flawed* method of 'guestimating" the number of smaller stars that cannot be directly observed at a distance, compared to the larger mass stars that we actually can observe at a distance. We underestimated stellar counts of stars the size of our sun by a factor of 4. and all of it was quite ordinary baryonic material.


    3) The following year in 2010, we 'discovered" that we've been underestimating the most *common* sized star (dwarf stars) in various galaxies by a *whopping* factor of between 3 and 20 depending on the galaxy type. Again, we grossly underestimated the *normal baryonic material* that is present in galaxies.


    4) Two years after that, in 2012, we 'discovered' more ordinary baryonic matter *surrounding* every galaxy that exist inside of the stars themselves. In fact they discovered more ordinary baryonic matter in 2012 than had been ''discovered' since the dawn of human history.


    5) In 2014 we also "discovered" that we have underestimated the number of stars *between galaxies*, particularly galaxies undergoing a collision process like that Bullet Cluster study:


    There's been at least *five* revelations of *serious* baryonic mass underestimation problems used in that 2006 lensing study that claimed to find 'proof' of exotic forms of matter, and all of those error are *in addition to* the missing baryon problem that was resolved recently when we found that our galaxy is embedded in a halo of neutral hydrogen gas.


    There is far more evidence to suggest that the baryonic mass estimates which were used in that Bullet Cluster lensing study were flawed than there is evidence to suggest that any of the "missing mass" was necessarily related to 'exotic' forms of matter. The lab results over the past 10 years would also tend to support that interpretation, including the most recent results from Xenon-1T.

    Last edited: May 21, 2017
  12. May 21, 2017 #11
    I randomly picked one:

    that's absolutely NOT what the paper itself says. It says (https://arxiv.org/pdf/1205.5037.pdf abstract):

    "Comparable" here means ~2% since Milky Way mass is variously estimated to be 500-800 billion solar masses.
  13. May 21, 2017 #12
    Hi Michael:

    I much appreciate your post and all of the links to evidence about underestimating the amount of baryonic matter.

    I hope you will comment on the following.
    I confess this article is a bit more technical than I can feel comfortable that I have understood it. What I think I understand from this (especially section 2.2 starting on page 7) and other readings from the past I can't site from memory, is that the amount of Deuterium in the observable part of our universe gives us eveidence that a large fraction of matter cannot be baryonic.

  14. May 21, 2017 #13

    stefan r

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    Errors in observed mass can work both ways. Suppose the center of galaxies have more mass than we thought. That makes the rotational velocity more deviant. You would need even more dark matter to explain it.
  15. May 21, 2017 #14
    I'm pretty sure that both the article and paper are discussing the comparison to the *known baryonic* (stellar) component of the galaxy, and not the virial mass *including* dark matter, but I may have been a bit overzealous with my use of the term "more". It's still a huge amount of mass compared to the known baryonic mass that's been identified to date. Note we've also discovered a whole host of new satellite galaxies around our galaxy since 2006.

    Page 7-8:

    Keep in mind however that this mass is *in addition to* all the other 5 papers relate to galaxy mass estimation problems that were used in 2006.
  16. May 21, 2017 #15
    You're right that the density and the mass layout of the halo makes a difference with respect to galaxy rotation patterns, although it all matters in lensing studies. I would be inclined to believe that the plasma mass would be concentrated closer to the core, and would thin out with distance from the core as "dark matter" halo models predict. The same is true of the mass layout of the non-ionized hydrogen halo, or at least I would presume so.
  17. May 21, 2017 #16
    I'll try to read through that paper this evening and I'll comment more once I've read it, but.....

    I'm frankly a little apprehensive about "assuming" the existence of exotic forms of matter only to make sure that LCDM mathematically "fits' some other completely different observation. I'd probably feel differently were it not for all the negative results from the various dark matter "tests" at LHC, LUX, PandaX, Xenon100,and even recent Xenon-1T results. As it stands, the nucleosynthesis argument seems more like a case of special pleading, only so that LCDM can be considered exempt by falsification by the non-existence of exotic forms of matter. If exotic forms of matter do not exist (and no lab evidence suggests it does exist), the lack of LCDM to be able to "fit" the elemental composition observations of the universe without exotic matter should be used a means to falsify the LCDM model, not used as a method to try to overlook all the baryonic mass estimates that we've been making since the dawn of time.

    *If* we can demonstrate the existence of exotic forms of matter via experiments on Earth, then the nucleosynthesis argument becomes valid argument IMO, but not before that point. It's an interesting "requirement" of LCDM, but nothing more IMO.
  18. May 22, 2017 #17
    Hi Michael:

    I am not sure I understand this. What does "plasma mass" mean here? Is it the mass of baryonic matter in a plasma state prior to the time when the universe has cooled and the state changes from plasma to gas? If so, what does "core mean? I am confused since I understood that "dark matter" halo models were about the formation of galaxies (or perhaps galactic clusters) at a time long after the plasma state of the universe.

  19. May 22, 2017 #18
    Hi Michael:

    I looked at your first link to the 2008 paper and wondered whether that would fit with the paper from Magain, P. and Chantry, V. (2013), which I mentioned earlier in this thread, looking at Einstein rings from early galaxies and finding an error of factor of 1.8 between observed mass by luminosity and lensing mass. I thought both methods were robust and the difference due to non-radiating baryonic matter.

  20. May 22, 2017 #19
    Hi Michael:

    The following is another, and a more clearly written, discussion of the relationship between Deuterium and dark matter.
    Here is a pertinent quote:
    During the formation of helium nuclei, perhaps only one in 10,000 deuterons remained unpaired. An even smaller fraction fused into nuclei heavier than helium, such as lithium. (All the other familiar elements, such as carbon and oxygen, were produced much later inside stars.) The exact percentages of helium, deuterium and lithium depend on only one parameter: the ratio of protons and neutrons--particles jointly categorized as baryons--to photons. The value of this ratio, known as n (the Greek letter eta), remains essentially constant as the universe expands; because we can measure the number of photons, knowing n tells us how much matter there is. This number is important for understanding the later evolution of the universe, because it can be compared with the actual amount of matter seen in stars and gas in galaxies, as well as the larger amount of unseen dark matter.​

    For the big bang to make the observed mix of light elements, n must be very small. The universe contains fewer than one baryon per billion photons. The temperature of the cosmic background radiation tells us directly the number of photons left over from the big bang; at present, there are about 411 photons per cubic centimeter of space. Hence, baryons should occur at a density of somewhat less than 0.4 per cubic meter. Although cosmologists know that n is small, estimates of its exact value currently vary by a factor of almost 10. The most precise and reliable indicators of n are the concentrations of primordial light elements, in particular deuterium. A fivefold increase in n, for example, would lead to a telltale 13-fold decrease in the amount of deuterium created. ​

  21. May 22, 2017 #20

    stefan r

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    Does that help? An order of magnitude range sounds like "dark matter may or may not be there".
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