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Thread: Dark Matter

  1. #1
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    Dark Matter

    Hello everyone,

    Wish you all a lovely day.

    I recently read that there might be more than 3 times more stars than we have previously predicted to be in the universe and those new starts turned out to be Red Dwarf. I was wondering, in what way this could affect our previous estimate of available dark matter around our galaxy and in the universe too. Since Red Dwarfs are related to MACHOs and that MACHOs are candidates for dark matter.

    Hope it wasn't a lame question

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    Quote Originally Posted by brook View Post
    ... I was wondering, in what way this could affect our previous estimate of available dark matter around our galaxy and in the universe too. Since Red Dwarfs are related to MACHOs and that MACHOs are candidates for dark matter...
    Interesting question. Here's a hand-waving answer. Our Sun is about ten or twenty times the mass of the low-end red dwarfs, so even on the face of it, we're not talking about a substantial amount of the missing mass.
    Going a little further, there are studies (e.g. MACHO and OGLE) which have been searching for MACHOs using strong gravitational lensing events. Because of these studies we have a very good idea about the number of MACHOs in the vicinity of the Milky Way. The number is small.
    One more thing, the WMAP studies predict a certain amount of non-Baryonic dark matter was required in the early days, and that is where we get the most precise percentages of Baryonic vs. Cold Dark Matter vs. Hot Dark Matter vs. Dark Energy.
    Forming opinions as we speak

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    Quote Originally Posted by brook View Post
    Hello everyone,

    Wish you all a lovely day.

    I recently read that there might be more than 3 times more stars than we have previously predicted to be in the universe and those new starts turned out to be Red Dwarf. I was wondering, in what way this could affect our previous estimate of available dark matter around our galaxy and in the universe too. Since Red Dwarfs are related to MACHOs and that MACHOs are candidates for dark matter.
    Hope it wasn't a lame question
    I think it is a good question. Dark matter was first proposed to explain rotation velocities of disc stars in spiral galaxies. This hypothesized dark matter only makes sense if the majority of it, maybe 90%, surrounds the galaxy rather than being within it. This large additional amount of red dwarfs could not effect rotation rates of the galaxy unless they were unevenly distributed toward the peripheral parts of the galaxy. Although their numbers may be great such stars would still be a minority of the mass within the galaxy. For the proposed dark matter idea, its supposed mass is roughly 10 times greater than the visible galaxy itself, primarily surrounding the galaxy to explain these rotation velocities. Bottom line, it would seem, is that red dwarfs cannot be the missing "dark matter" to explain orbital velocities.
    Last edited by forrest noble; 2010-Dec-13 at 11:23 PM.

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    Quote Originally Posted by forrest noble View Post
    I think it is a good question. Dark matter was first proposed to explain rotation velocities of disc stars in spiral galaxies. This hypothesized dark matter only makes sense if the majority of it, maybe 90%, surrounds the galaxy rather than being within it. This large additional amount of red dwarfs could not effect rotation rates of the galaxy unless they were unevenly distributed toward the peripheral parts of the galaxy.
    It is a good question. Welcome, Brook. But, forrest, your response is misleading and incorrect. Both dark and regular matter that is exterior to the luminous part of the galaxy - the disk - is not going to be gravitationally affecting the orbits of the stars (and dust and gas) in the disk1. It is the dark matter that is within the disk that causes the disk stars to orbit faster than one would expect, judging from the amount of matter in the galaxy that we can detect (composed of stars, gas, and dust). And it causes the orbits to be non-keplerian, that is, stars out at the edge of the disk are orbiting just as fast as stars closer to the center (with the exception of stars very near the center).

    So I think your supposition is correct, brook. But "three times as many stars" does not mean three times the mass. As antoniseb pointed out, red dwarfs are not very massive, so the "mass of the luminous matter" that we can detect has not been increased three times - probably much less than that. As you suppose, that will have some effect, but it's not nearly enough to explain the "dark matter effect," which makes it appear that there must be within the galaxy something in the order of 10+ times more mass than we can detect.

    _________________________
    1 Technically, there is a small effect since the disk is not spherical, in which case there would be no effect, as Newton proved geometrically in his Principia.
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by Cougar View Post
    Both dark and regular matter that is exterior to the luminous part of the galaxy - the disk - is not going to be gravitationally affecting the orbits of the stars (and dust and gas) in the disk.
    This is because the exterior matter on one side of the galaxy is counteracted - canceled - by the exterior matter on the other side. An "edge star" on one side will be closer to the exterior matter on that side, increasing the gravitational effect, but then there's a lot more matter "behind" that edge star, on the opposite side (since it's far from the center), which, oddly enough, very nearly cancels the near-side gravitational effect. As mentioned, with a uniform sphere, the cancellation is exact.
    Everyone is entitled to his own opinion, but not his own facts.

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    Cougar,

    But, forrest, your response is misleading and incorrect. Both dark and regular matter that is exterior to the luminous part of the galaxy - the disk - is not going to be gravitationally affecting the orbits of the stars (and dust and gas) in the disk.
    http://en.wikipedia.org/wiki/Dark_matter_halo

    This is the general hypothesis that I am referring too. I think this is the presently preferred dark matter model, isn't it.

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    Quote Originally Posted by forrest noble View Post
    Cougar,


    http://en.wikipedia.org/wiki/Dark_matter_halo

    This is the general hypothesis that I am referring too. I think this is the presently preferred dark matter model, isn't it.
    The important point, though, is that it is only the dark matter within the radius of the visible galaxy that has any (significant) effect on rotation dynamics.

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    Quote Originally Posted by forrest noble View Post
    Cougar,


    http://en.wikipedia.org/wiki/Dark_matter_halo

    This is the general hypothesis that I am referring too. I think this is the presently preferred dark matter model, isn't it.
    The article you linked to fully supports what Cougar said, and rather contradicts what you stated. You would benefit from reading the very sources you cite.

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    MY understanding based upon this quote and many others, as well as reading the link, is that the vast majority of dark matter must lie beyond the observable galaxy in a halo to account for the observed rotation rates for disc stars.

    The presence of dark matter in the halo is demonstrated by its gravitational effect on a spiral galaxy's rotation curve. Without large amounts of mass in the extended halo, the rotational velocity of the galaxy should decrease at large distance from the galactic core.
    http://en.wikipedia.org/wiki/Dark_matter_halo

    If further argument is desired please provide links that either conflict with the statements in the link or my own statements.
    Last edited by forrest noble; 2010-Dec-16 at 06:42 AM.

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    Quote Originally Posted by forrest noble View Post
    MY understanding based upon this quote and many others, as well as reading the link, is that the vast majority of dark matter must lie beyond the observable galaxy in a halo to account for the observed rotation rates observed for disc stars.



    http://en.wikipedia.org/wiki/Dark_matter_halo

    If further argument is desired please provide links that conflict with the statements in this link or my own statements.
    No argument -- you're just not integrating Cougar's statement (reread Post #5 carefully) with what the Wiki article says. It's just a question of reading comprehension.

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    Quote Originally Posted by Geo Kaplan View Post
    It's just a question of reading comprehension.
    Well, I have to add that it is also a matter of the horrendously awful use of the word "halo" that has come to be standard usage by the astronomical community. If one takes the typical meaning of that word, i.e.,

    "a geometric shape, usually in the form of a disk, circle, ring, or rayed structure, traditionally representing a radiant light around or above the head of a divine or sacred personage, or as to Meteorology, any of a variety of bright circles or arcs centered on the sun or moon, caused by the refraction or reflection of light by ice crystals, or an undesirable bright or dark ring surrounding an image on the fluorescent screen of a television tube,"

    then it is understandable that the astronomical meaning can become easily misunderstood. Even Dictionary.com doesn't define the astronomical meaning very well: "a spherical cloud of gas clusters and stars that form part of a spiral galaxy." In fact, that's horrible! At least they got the spherical right. The dark matter halo is spherical in shape, typically centered on the galaxy center (galaxy collisions can alter this). Forrest has apparently forgotten Grey's excellent account of the dark matter distribution within that generally spherical shape, which is slightly tricky because it appears to interact only gravitationally, but otherwise dark matter "particles," whatever they are, do not interact with normal matter, and they don't even interact among themselves.
    Everyone is entitled to his own opinion, but not his own facts.

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    I should add that the dark matter [strikethru] halo [/strikethru] sphere does extend well beyond the galaxy disk, so as the wiki article says, much of the galactic dark matter is going to be beyond the disk. But as Grey says, its distribution falls off as the inverse square of the distance from the galaxy center, so it will be more dense toward that center. I'm not sure of wiki's claimed percentage interior vs. exterior to the disk of the galaxy. A little math problem there if you know how far it extends beyond the disk (which I know is pretty far)....
    Everyone is entitled to his own opinion, but not his own facts.

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    Cougar,

    You are right, I do remember Grey's model whereby I discovered that his model was the presently excepted model as apposed to the older model that I had thought was still valid. The point for this thread however is that most hypothetical dark matter must lie outside the boundary of the observable galaxy (for any model) in what has been called a rapidly orbiting halo, for the disc star velocities to be explainable by dark matter. This I think is the latest and "greatest" version.
    Last edited by forrest noble; 2010-Dec-16 at 06:44 AM.

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    Quote Originally Posted by Cougar View Post
    I should add that the dark matter [strikethru] halo [/strikethru] sphere does extend well beyond the galaxy disk, so as the wiki article says, much of the galactic dark matter is going to be beyond the disk. But as Grey says, its distribution falls off as the inverse square of the distance from the galaxy center, so it will be more dense toward that center. I'm not sure of wiki's claimed percentage interior vs. exterior to the disk of the galaxy. A little math problem there if you know how far it extends beyond the disk (which I know is pretty far)....
    The integral you allude to is over volume elements in spherical coordinates, which scale like r2dr, so if the density scales with r-2, then the enclosed mass must scales proportionally with the radius of the distribution. So if the distribution extends to very large distances, most of the mass is well beyond the disk. However, as you say, that extra mass doesn't affect the rotation curve much-- it is of more importance to the pulling in of the gas that created the galaxy in the first place. Halo is indeed a horrendous term, not only because it suggests a ring instead of a sphere, but also because we have a considerably smaller halo of stars and globular clusters than we do a halo of dark matter. But all the language must get pretty nebulous out in the regimes between galaxies (no pun intended), as galaxies don't come with boundaries the way countries do!

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    Quote Originally Posted by forrest noble View Post
    Cougar,

    You are right, I do remember Grey's model whereby I discovered that his model was the presently excepted model as apposed to the older model that I had thought was still valid. The point for this thread however is that most hypothetical dark matter must lie outside the boundary of the observable galaxy (for any model) in what has been called an orbiting halo, for the disc star velocities to be explainable by dark matter. This I think is the latest and "greatest" version.
    that most hypothetical dark matter must lie outside the boundary of the observable galaxy ... for the disc star velocities to be explainable by dark matter

    As has been repeatedly pointed out in this (and the linked) thread, this is wrong. Yes, dark matter appears to spread far beyond the visible galaxy. But this "external" dark matter has no (significant) influence on the velocities of stars inside.

    OK. I will concede that very indirectly the large extent of dark matter is relevant in that the required distribution can only be achieved if the the dark matter halo is mich larger than the galaxy. But that doesn't seem to be what you were saying.

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    Quote Originally Posted by Strange View Post
    that most hypothetical dark matter must lie outside the boundary of the observable galaxy ... for the disc star velocities to be explainable by dark matter

    As has been repeatedly pointed out in this (and the linked) thread, this is wrong. Yes, dark matter appears to spread far beyond the visible galaxy. But this "external" dark matter has no (significant) influence on the velocities of stars inside.

    OK. I will concede that very indirectly the large extent of dark matter is relevant in that the required distribution can only be achieved if the the dark matter halo is much larger than the galaxy. But that doesn't seem to be what you were saying.
    From posting #3, here is what I said:
    This hypothesized dark matter only makes sense if the majority of it, maybe 90%, surrounds the galaxy rather than being within it.
    To dispute something that has been been presented for evidence concerning my link you have to provide your own link and evidence or explain how such link has been misinterpreted by the presenter (me), otherwise the initial interpretation will remain valid. Also I think this is not a minor interpretation concerning dark matter as a whole but it is of minor consequence concerning this OP proposal. I think the guide should be to never argue against evidence presented unless you have "evidence" of your own to present to the contrary.

    Here is another quote;

    The dark matter halo is the single largest part of the Galaxy as it covers the space between 100,000 light-years to 300,000 light-years from the galactic center. It is also the most mysterious part of the Galaxy. It is now believed that about 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy's matter and energy in any way except through gravity. The dark matter halo is the location of nearly all of the Galaxy's dark matter, which is more than ten times as much mass as all of the visible stars, gas, and dust in the rest of the Galaxy. The luminous matter makes up approximately 90,000,000,000 (9 x 1010) solar masses. The dark matter halo is likely to include around 600,000,000,000 (6 x 1011) to 3,000,000,000,000 (3 x 1012) solar masses of dark matter.
    (bold added)

    http://en.wikipedia.org/wiki/Dark_matter_halo

    Also realize that the original purpose concerning the proposal of dark matter was to explain orbital velocities of disc stars in spiral galaxies.

    Remember we are not discussing what has validity concerning dark matter (whether it is a valid concept or not), only what is the mainstream theory/hypothesis concerning how it might relate to the OP question.
    Last edited by forrest noble; 2010-Dec-16 at 02:25 AM.

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    In the Galaxy, No evidence for dark matter

    1. Observation evidence for dark matter
    1) Galactic rotation curves
    2) Galaxy clusters and gravitational lensing
    3) WMAP
    4) Bullet cluster
    5) Structure formation

    2. In the Galaxy, No evidence for dark matter
    1) At the Earth : non-observation(Xenon100, CDMS-II...)

    2) At the solar system : non-observation

    3) At the center of galaxies : no evidence
    Greedy Supermassive Black Holes Dislike Dark Matter
    http://www.universetoday.com/13091
    Astronomers Find Black Holes Do Not Absorb Dark Matter
    http://www.universetoday.com/60422

    4) At the galactic plane : no evidence
    "No evidence for a dark matter disk within 4 kpc from the galactic plane"
    http://arxiv.org/abs/1011.1289v1.pdf

    5) At the galactic halo : no evidence
    Globular Cluster problem
    "Evidence Against Dark Matter Halos Surrounding the Globular Clusters MGC1 and NGC 2419"
    http://arxiv.org/abs/1010.5783


    Minor opinion(ATM opinion),
    “Centripetal force effect in the galaxy from dark matter halo out of the galaxy!”
    http://www.youtube.com/watch?v=ylEi2gpnD08

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    Quote Originally Posted by forrest noble View Post
    To dispute something that has been been presented for evidence concerning my link you have to provide your own link and evidence or explain how such link has been misinterpreted by the presenter (me), otherwise the initial interpretation will remain valid. Also I think this is not a minor interpretation concerning dark matter as a whole but it is of minor consequence concerning this OP proposal. I think the guide should be to never argue against evidence presented unless you have "evidence" of your own to present to the contrary.
    Maybe we are talking at cross purposes. This isn't a matter of evidence (assuming we both accept the observed rotation rates). The rotational speed of the visible matter in the galaxy can only be affected by the mass (visible and dark) inside the radius of that matter.

    Here is another quote;

    (bold added)

    http://en.wikipedia.org/wiki/Dark_matter_halo
    I don't see anything wrong with that, or that it contradicts what was said above. Although the first sentence is rather ambiguous. "It covers the space between 100,000 light-years to 300,000 light-years from the galactic center" doesn't mean that it only covers this space, but that it is the fact it (also) covers this space that makes if the largest part of the galaxy. It shouldn't be read to imply there is a 100,000 light-year "hole" in the dark matter halo.

    Also realize that the original purpose concerning the proposal of dark matter was to explain orbital velocities of disc stars in spiral galaxies.
    And this has since been extended to consider orbital velocities of material outside the visible disc of stars; which is one way we can deduce the characteristics of the larger dark matter halo.

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    Maybe we are talking at cross purposes. This isn't a matter of evidence (assuming we both accept the observed rotation rates). The rotational speed of the visible matter in the galaxy can only be affected by the mass (visible and dark) inside the radius of that matter.
    I understand your point but I disagree. I think the reason so much dark matter "is placed" outside the galaxy in a halo and given such a high orbital velocity is to enable the acceleration of stellar orbital velocities of peripheral stars in a domino effect inward, a kind of tail-wagging-the-dog effect.

    In the OP the query/proposal concerns whether there is enough brown dwarfs within the galaxy to explain the hypothetical proposal of dark matter. My point is that inside the galaxy the amount of matter is of a lesser consequence than placing 90-95% of the galaxies matter/ mass outside the galaxy and then giving it a very rapid orbital velocity. It is the orbital velocity of this hypothetical dark matter halo that is of primary importance. Just sticking 10 times more matter outward beyond the galaxy's visible matter in a sphere without great orbital velocity would instead slow down orbital stellar velocities within a galaxy. Brown dwarfs or any other kind of matter could not increase velocities within a galaxy because their orbital velocities would have to be the same as the adjacent stars within the galaxy. Please PM me if you disagree, or give further enlightenment to the OP proposal if you wish.

    best regards, Forrest
    Last edited by forrest noble; 2010-Dec-16 at 10:43 PM.

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    Quote Originally Posted by forrest noble View Post
    I think the reason so much dark matter "is placed" outside the galaxy in a halo and given such a high orbital velocity is to enable the acceleration of stellar orbital velocities of peripheral stars in a domino effect inward....
    Would you care to explain how "you think" you can so spectacularly violate physical law? Feel free to show your work.
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by Cougar View Post
    Would you care to explain how "you think" you can so spectacularly violate physical law? Feel free to show your work.
    Maybe dark matter is viscous (but only out beyond the visible galaxy).

    [Just kidding (before I get sent to ATM).]

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    Quote Originally Posted by Strange View Post
    [Just kidding (before I get sent to ATM).]
    You better be!
    Everyone is entitled to his own opinion, but not his own facts.

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