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B What's the deal with dark matter

  1. Nov 14, 2017 #21
    Nothing that we know of, that is why it is called 'dark'.
  2. Nov 15, 2017 #22
    I think the short answer is no, within current accepted theory. The most concise representation of General Relativity is,
    Gμν = - κ Tμν
    where Tμν is the energy-momentum tensor and by implication since mass is energy, it accounts for mass as well. Gμν is the Einstein curvature term and κ is Einstein's constant of gravity which is directly related to Newton's constant G. So curvature of space is directly linked the amount of mass and energy in a locality. Because of the relation E = mc^2, mass is by far the largest contributor to the curvature of space.

    However, the point you make at the start of your post, 'We observe that the visible matter of galaxies is moving in ways that cannot be attributed to only the gravitational forces associated with this visible matter', is not backed up by all observational data!
  3. Nov 15, 2017 #23

    stefan r

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    Are you sure? Light going into a black hole increases the gravity of the hole and therefore "space time warping" [link is not peer reviewed, arXiv.org]. Photons of light do not have mass.

    This is not useful information for dark matter. Dark matter is clearly not made of photons.
  4. Nov 15, 2017 #24
    Everyone is going to laugh about my opinion in that if we could calculate the frame drag of the galaxy using earths frame drag as a representation and apply that figure with regards to the amount of visible matter contained within our galaxy would we get a figure that is close to what we have calculated for the amount of dark matter needed? Maybe I’m in left field on this or not I don’t know.
  5. Nov 15, 2017 #25


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    Frame dragging is just too tiny an effect compared to what would be needed.
  6. Nov 15, 2017 #26


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    This is not actually true.

    In fact, all straightforward applications of the dark matter hypothesis, which looked promising at first, have pretty much been ruled out by observational evidence. The plain vanilla model in which there is a single type of thermal dark matter with a mass O(1-100) GeV has been ruled out by observation for almost a decade. And, there are, in fact, modified gravity theories (although not the most well known example of the genre called MOND) that do fit the data better than any of the extant dark matter theories (see, e.g., Moffat's MOG theory), and do so with fewer free parameters in their models, although few modified gravity theories have been tested as rigorously and by as large a group of investigators as the leading dark matter theories have been.

    This is not to say that an entirely satisfactory and well tested solution on any front exists. But, a lot of the data points which folk wisdom assumes destroyed modified gravity theories (e.g. the Bullet Cluster) do no such thing. Indeed, data points like the Bullet Cluster actually do more harm to dark matter particle theories than to modified gravity theories (many of which can accommodate this observation).

    Also, just to be clear, there is really no reasonable doubt that phenomena usually attributed to dark matter, that can not be explained with GR (at least as currently interpreted and applied*) and ordinary matter, exist and are pervasive. The phenomena attributed to dark matter can only be explained with some sort of new physics that either involves beyond the Standard Model particles, or forces that have effects different from GR as currently interpreted and applied, or both. These phenomena are by far the most compelling direct observational evidence that the "Core Theory" of GR plus the Standard Model is not complete and that New Physics are necessary to explain what is observed. (In contrast, "dark energy" phenomena can be completely explained to the limits of experimental observation with the cosmological constant of conventional GR.)

    * There are a couple of promising gravitation based theories that claim that they do not actually modify GR but involve a means of applying GR-like concepts different than the way that the vast majority of researchers in the field apply GR to the analysis of complex systems operationally.
  7. Nov 15, 2017 #27
    I'm still not convinced that MACHOs are ruled out.
    Just recently we observe a compact object from deep space flying past the solar system.
  8. Nov 16, 2017 #28
    Why not calculate the frame drag associated with the black hole (or holes) in the center of a galaxy based on its mass and revolution speed (earths frame drag has been observed). Then determine both the value and relative position of the wake created by the frame drag/revolutions and plug that into a computer model to see if that can fill the void that is suggesting dark matter is needed. I believe some kind of estimate can be derived using the dynamics associated with a so called perfect liquid (or what I would call cold plasma)?
  9. Nov 16, 2017 #29


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    Doing so for the black hole of a similar mass to the one at the center our galaxy and figuring the framing dragging effect at, say, 50,000 ly from the center, it works out to being the equivalent of an additional 7.4e-54 km/sec. Besides, frame dragging falls off with distance from the mass, so any effect it would have would be stronger near the BH than it is further, But stellar speeds nearer the center of the galaxies aren't the problem, it's the ones on the outskirts.
  10. Nov 16, 2017 #30

    stefan r

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    Does anyone claim MACHOs do not exist? Someone could claim MACHOs are 80% of the Milky Ways mass, or 1%, or 0.001%. If it is 0.001% as comets/asteroids that would be 109 solar mass. Something like 1026 comets. Finding one of them will not prove much.

    1/'Oumuamua was not a halo object.
  11. Nov 17, 2017 #31


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    Allow me to remind you of some of the pertinent evidence which rules out MACHOs.

    As noted by stefan r above, there is no doubt that some MACHO candidates exist (although primordial black holes have not yet been observed and there is good reason to doubt that they exist), but there simply are enough of them and they aren't in the right places to account for a meaningful share of dark matter phenomena. The smallest primordial black holes are impossible:

    The constraints on non-primordial black hole (PBH) MACHOs (terrestrial planets, baby or ordinary gas giants, neutron stars, cannibalized white dwarfs that become helium or diamond planets) are severe and these candidates have been ruled out for many years. Indeed, massive compact halo objects (MACHOs) are pretty much ruled out, in general, as dark matter candidates (citations in the original omitted and paragraph breaks added in the quotation below):

    There have been arguments that PBHs are not subject to some of the exclusions above applicable to other kinds of MACHOs. But, a the MACHO exclusion chart below (which is a few years out of date (with the left side showing the ratio of MACHOs to dark matter and the horizontal axis showing MACHO mass) is as follows:

    : DM_amounts.png

    In terms of MACHOs are a dark matter candidate, only the very top part matters, since 10-2 means just 1% of dark matter can be accounted for by MACHOs of that type. You really need to be above 10-1 (i.e. 10%) to be considered as a significant source of dark matter phenomena.

    As this chart shows, there are pretty significant observational constraints on the size of primordial black hole dark matter, however (relying mostly on this source). The sweet spot is 10^22 kilograms, which is a bit less than the mass of the Moon (which is 7*10^22 kilograms), plus or minus, which would imply a typical primordial black hole with an event horizon radius of about 0.1 millimeters.

    Another paper on PBHs was even less bullish:

    Fabio Capela, et. al, "Constraints on primordial black holes as dark matter candidates from capture by neutron stars." Phys. Rev. D 87, 123524 (2013) (link is to open access pre-print version conformed to final print version).

    Since the chart above was made the observational constraints on PBHs have further tightened. For example:

    Daniele Gaggero, et al., "Searching for Primordial Black Holes in the radio and X-ray sky" (Pre-Print December 1, 2016).

    Another recent paper constraining MACHOs as cluster dark matter (keeping in mind that galaxy clusters are inferred to have much more dark matter proportionately than any kind of galaxy) finds:

    Theodorus Maria Nieuwenhuizen "Subjecting dark matter candidates to the cluster test" (October 3, 2017) (omitting from the abstract conclusions about sterile neutrino dark matter candidates).

    It also turns out that the model used in many PBH exclusion analyses is actually too lenient and that more realistic modeling makes the exclusions even tighter.
    Last edited: Nov 17, 2017
  12. Nov 19, 2017 #32
    Whether I have read this right but I was regarding 'math error' as using the wrong or incomplete math rather than an error like 2+2=5!
  13. Nov 19, 2017 #33
    I guess it's darker than light matter? Sounds like something out a sci-fi film.
  14. Nov 19, 2017 #34
    Hi ohwilleke:
    I confess that the links you provided are too technical for me to follow in detail, but I think I get the gist which I summarize as follows.
    PBHs can only make up a significant fraction of DM if they are very small, i.e., less massive than the moon.​
    From https://en.wikipedia.org/wiki/Hawking_radiation
    However, since the universe contains the cosmic microwave background radiation, in order for the black hole to dissipate, it must have a temperature greater than that of the present-day blackbody radiation of the universe of 2.7 K = 2.3×10−4 eV. This implies that M must be less than 0.8% of the mass of the Earth[21] – approximately the mass of the Moon.​
    If I am understanding all this correctly, if a sufficient number of moon size or smaller PBHs exist, they could not be detected by observing their Hawking radiation evaporation. I am not knowledgeable enough to read the diagram in #31 to estimate the number of possible PBHs that would be sufficiently larger than the moon to have their evaporation radiation detectable. Can you help me with that?

  15. Nov 21, 2017 #35


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    Nobody directly observes Hawking radiation. One uses its predicted evaporation rate and the age of the universe to determine how big PBHs created in the Big Bang era would be today. This sets a lower bound on the size of PBHs that still exist today.

    You put an upper bound on the size of PBHs mostly based upon the failure of microlensing observations to see light bending in ways consistent with masses of a certain size or larger. If PBHs were much larger than the Moon in mass, we would see microlensing caused by PBHs all over the place with telescopes.

    We know the total inferred amount of DM, for example, in the Milky Way, from star dynamics and gravitational lensing. And for any given PBH mass, you can determine the number of PBHs per unit volume that have to exist to produce that quantity of DM. You can then determine how common PBH scale gravitational lensing should be per area viewed with telescopes, and if the observed amount of lensing is much lower than it should be given this analysis, then you know that the PBH hypothesis to explain DM is wrong.

    In reality, of course, PBH DM would not all have exactly the same mass. And, if you make reasonable estimates of the distribution of PBH DM around a mean value as one of the links I reference does, it turns out that the exclusion of PBH DM is stronger than it would be if there was a uniform PBH mass, because the heavier than average PBHs would have a stronger signal which makes it easier to detect, than if every PBH were exactly the same mass, because it makes some lower resolution lensing measurements more useful.

    The radius of a PBH for a given mass is determined using GR.
  16. Dec 1, 2017 #36
    My question is whether the (too fast) speed of peripheral galactic stars is observed to vary by galactic longitude as well as distance from the galactic center? Is there any data on this?
  17. Dec 2, 2017 #37


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    What is "galactic longitude"?
  18. Dec 2, 2017 #38
  19. Dec 2, 2017 #39
    The existence of dark matter is not surprising if viewed as another indication that the universe that we find ourselves in is just one of an arbitrarily large number of universes formed from some pre-existing more basic "material". The properties of the parts of our universe that we interact with non-gravitationally are selected "anthropically". That is, they are consistent with the emergence of the objects and interactions that could produce the observed past, present, and possible futures of the universe that we find ourselves in. We do not propose that our own emergence was in some way the "goal" of this universe variant. Only that the negation of this supposition is not consistent with the facts of our current existence. One can suppose that the network of available basic "material" that is connected to the non-gravitational interactions that have and will occur in our universe during its existence only includes a small part of that pre-existing basic "material", with the unconnected part forming the "dark matter" that only interacts gravitationally with the rest. I have seen some articles here that disparage such anthropic explanations for things. That forces one to explain in other ways why the various values for material and interaction parameters have the specific values found. Other than future job security, it is hard to understand the justification for this bias.
  20. Dec 3, 2017 #40


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    There is quite a bit of data and I am pretty certain that the answer is no.
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