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Supermassive black holes and dark matter

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  • Supermassive black holes and dark matter

    So, I've been listening to course lectures on the modern state of astronomy and cosmology. I can keep with most if it (helps that it's intro level), but there are some things the professor skims over that I wish would be more fully fleshed out.

    As I understand it, scientists have concluded that dark matter must exist since the observed rotational velocities don't match the estimated mass of a given galaxy. This seems to have been concluded prior to the observation (and measurement) of supermassive black holes, but I might have some of the dates wrong. From what I understand, the mass of supermassive black holes is estimated in part by these same observed rotational velocities. However, as best I can tell the mass of these supermassive black holes are rough estimates, in some cases with a margin of error of +/- 15 billion solar masses. That seems like a pretty huge range.

    What I don't understand, then, is why the presence of these supermassive black holes and their estimated mass doesn't account for the supposed discrepancies that gave rise to the suggestion of dark matter. Things like gravitational lensing and faster-than-expected rotational velocities would, to me, indicate that perhaps our estimate of supermassive black hole masses are instead quite low. I'm going to provide as a bit of vindication this particular article:

    "The calculations of the mass (weight) of the supermassive black holes at the heart of galaxies depends on two main factors: the rotational speed of the stars in the galaxy and how far it is from the black hole to the stars," he said. "The rotational speed can be observed, and the distance from the black hole out to the rotating disc of stars can now be calculated precisely using the new method."

    Initial indications suggest that supermassive black hole masses have been underestimated by perhaps 40 percent. Researchers hope to extend their measurements to other active galaxies; the technique could eventually help astronomers better understand the rate at which the universe is expanding, study team members said.


    So...what am I missing? Is the actual mass of these supermassive black holes not enough to account for the missing matter? Are existing methods for measuring these masses accurate enough to suggest there is still missing matter to be found? As far as I can tell, we've spent 40+ years looking for dark matter without much success, and I'm definitely not sure I understand why we're still convinced it's a real thing. FWIW, I don't actually mind if it's a thing or not. I'm just trying to understand why we think it is, especially in the face of 'new' discoveries and information about things like supermassive black holes.
    I'm not here anymore.

  • #2
    Originally posted by Carrikature View Post
    What I don't understand, then, is why the presence of these supermassive black holes and their estimated mass doesn't account for the supposed discrepancies that gave rise to the suggestion of dark matter.
    The total mass of a galaxy is much more than the mass of the central black hole. The dark matter of a galaxy is in the form of a halo out around the galaxy, extending out beyond visible stars. (Either that; or we have our laws of gravity all wrong.) The super massive black hole in the center has a strong gravitational influence on nearby stars in the same central region; but for the majority of stars, the major gravitational forces come from the rest of the galaxy; not the super massive black hole.

    The dark matter halo is inferred by the "rotation curve"; the relationship between velocities of stars, and their distance from the center of the galaxy. These curves are much "flatter" than would be expected from the apparent mass of visible stars. Stars a long way out continue to orbit at high velocities; indicating that larger and larger circles around the galaxy are enclosing more and more matter; that is, the mass is distributed as a halo of spread out matter.

    To weigh the super massive black hole in the center, you have to measure velocities and distances for stars right in close to the center; where the black hole dominates. That is; dark matter from the entire galaxy's rotation curve; black hole from the motions of individual stars in close to the center.

    There's something wrong with your numbers, I think; super massive black holes weigh in at several billion solar masses. Heaviest seems to be about 6 billion solar masses. So the margin of error can't be 15 billion. But in any case, a big galaxy will weigh in at several trillion solar masses.

    Cheers -- sylas

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    • #3

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      • #4
        Originally posted by sylas View Post
        There's something wrong with your numbers, I think; super massive black holes weigh in at several billion solar masses. Heaviest seems to be about 6 billion solar masses. So the margin of error can't be 15 billion. But in any case, a big galaxy will weigh in at several trillion solar masses.

        Cheers -- sylas
        Thanks, sylas. I'll come back to this soon, but I wanted to answer your question about numbers. I had found a list of black hole masses on Wikipedia, but following the source takes me here:

        Although no single model is consistent with all of the observed kinematic features in NGC 4889, we can define a confidence interval forMBH by considering the most extreme confidence limits from the cumulative set of models. The corresponding 68% confidence interval is (0.6–3.7)x10^10 [solar masses]. We adopt a blackhole mass of 2.1x10^10 [solar masses], corresponding to the midpoint of this interval.
        Last edited by Carrikature; 01-27-2015, 02:09 PM.
        I'm not here anymore.

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        • #5
          Originally posted by Carrikature View Post
          As I understand it, scientists have concluded that dark matter must exist since the observed rotational velocities don't match the estimated mass of a given galaxy.
          As I understand it, scientists have concluded that dark matter must exist because we're standing on it.

          Roy
          Jorge: Functional Complex Information is INFORMATION that is complex and functional.

          mikewhitney: What if the speed of light changed when light is passing through water? ... I have 3 semesters of college Physics.

          Mountain Man: First of all, the Bible is a fixed document.
          Mountain Man: this is how liberals argue these days, with labels instead of ideas.

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          • #6
            There are multiple indications of dark matter, including things like gravitational lensing where there are no galaxies present, indications of its presence in the cosmic microwave background, etc. You also can't get a universe that looks like our present one without its influence; critically, some of this influence occurs before stars even formed, which is why it can't possibly be something familiar from the visible universe.

            So, dark matter solves multiple problems in cosmology. Even if you were to catch an issue with just one of these (such as galaxy rotation), you'd need a dark matter replacement that handles the rest of the problems.
            "Any sufficiently advanced stupidity is indistinguishable from trolling."

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            • #7
              Thanks Carrikature; I did not know that super massive holes had been found that big! I was thinking 6 or 7 billion solar masses as the biggest.

              Cheers -- sylas

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              • #8
                Ok, before I begin, please understand that my intent here is not to show that dark matter fails as an explanation (just in case my posts may come across that way). I haven't the knowledge to do that. What I am trying to do is get a better grasp on the reasoning behind it. Most of what I've come across ends up being a "we need more mass" statement and not much else. Bear with me if I cut up posts in an attempt to isolate those things I want to pursue further.


                Originally posted by sylas View Post
                The total mass of a galaxy is much more than the mass of the central black hole. The dark matter of a galaxy is in the form of a halo out around the galaxy, extending out beyond visible stars. (Either that; or we have our laws of gravity all wrong.) The super massive black hole in the center has a strong gravitational influence on nearby stars in the same central region; but for the majority of stars, the major gravitational forces come from the rest of the galaxy; not the super massive black hole.
                I do know that the mass of a galaxy is much more than the central black hole. The total mass would obviously have to include all the stars/planets/gas/dust/etc that makes up the galaxy. From what I've seen, though, it's been postulated in a few different ways that our laws of gravity are at least flawed, if not completely wrong. I'm given to understand that our studies have eliminated the Electromagnetic and Strong forces as options for how dark matter would interact with baryonic matter (am I using that right?). The common candidate now seems to be left to WIMPs, but that seems to partly be based on the fact that we don't have any quantum theory of gravity. I haven't found anything that points to what those particles might actually be, though. It looks to me like our explanatory options are some unknown (and unhinted at) WIMP or the big gap that is our theory of gravity. I don't understand why the former is preferred over the latter unless it's just because we have some concept of how to go about detecting the former. Not to say that the two things are mutually exclusive, but it seems like we should be focusing more of our efforts on figuring out how gravity actually works.


                Originally posted by sylas View Post
                The dark matter halo is inferred by the "rotation curve"; the relationship between velocities of stars, and their distance from the center of the galaxy. These curves are much "flatter" than would be expected from the apparent mass of visible stars. Stars a long way out continue to orbit at high velocities; indicating that larger and larger circles around the galaxy are enclosing more and more matter; that is, the mass is distributed as a halo of spread out matter.

                To weigh the super massive black hole in the center, you have to measure velocities and distances for stars right in close to the center; where the black hole dominates. That is; dark matter from the entire galaxy's rotation curve; black hole from the motions of individual stars in close to the center.
                Alright, so I have a couple more questions here. In our solar system, the rotational velocities of the orbits are dominated by the mass of the sun. Jupiter plays a part, but it's still dwarfed by the sun's influence. In a galaxy, though, we're looking at billions of solar masses in the black hole and trillions of them in the galaxy as a whole. More importantly, the density of these masses are much higher in the center than in the fringes. I understand that the high velocity orbits of the fringe stars point to these halos, but are these halos not accounted for by the presence of all the other solar masses within? Just how far off are the observed rotational velocities from what we would expect to see?


                Originally posted by Roy View Post
                As I understand it, scientists have concluded that dark matter must exist because we're standing on it.

                Roy
                I'm afraid you've lost me with this comment. Can you elaborate?


                Originally posted by TheLurch View Post
                There are multiple indications of dark matter, including things like gravitational lensing where there are no galaxies present, indications of its presence in the cosmic microwave background, etc. You also can't get a universe that looks like our present one without its influence; critically, some of this influence occurs before stars even formed, which is why it can't possibly be something familiar from the visible universe.

                So, dark matter solves multiple problems in cosmology. Even if you were to catch an issue with just one of these (such as galaxy rotation), you'd need a dark matter replacement that handles the rest of the problems.
                I'm not really trying to catch an issue with dark matter so much as understand the reasoning behind it. Most of the rest of this is not anything I had heard. Do you know of some good resources for someone like me to read up on them?



                ETA:
                I forgot to ask something. From what I've seen, we've known about dark matter since the late 30s (though it wasn't called that at the time). I realize that technology and general knowledge has advanced significantly in the last century, but I'm not sure I understand where the major difficulties lie in discovering either the nature of gravity or dark matter. What are the major setbacks we're facing in terms of answering these sorts of questions?
                Last edited by Carrikature; 01-28-2015, 01:11 PM.
                I'm not here anymore.

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                • #9
                  Originally posted by sylas View Post
                  Thanks Carrikature; I did not know that super massive holes had been found that big! I was thinking 6 or 7 billion solar masses as the biggest.

                  Cheers -- sylas
                  No problem. At some point, I think my sense of scale implodes and the numbers end up just being numbers. I can't really grasp the idea of a single solar mass let alone billions or trillions of them.
                  Last edited by Carrikature; 01-28-2015, 01:13 PM.
                  I'm not here anymore.

                  Comment


                  • #10
                    Originally posted by Carrikature View Post
                    I'm afraid you've lost me with this comment. Can you elaborate?
                    Early ideas on the composition of dark matter included large numbers of normal objects between and around stars. Under this definition, the Earth would qualify as dark matter.

                    Roy
                    Jorge: Functional Complex Information is INFORMATION that is complex and functional.

                    mikewhitney: What if the speed of light changed when light is passing through water? ... I have 3 semesters of college Physics.

                    Mountain Man: First of all, the Bible is a fixed document.
                    Mountain Man: this is how liberals argue these days, with labels instead of ideas.

                    Comment


                    • #11
                      Originally posted by Roy View Post
                      Early ideas on the composition of dark matter included large numbers of normal objects between and around stars. Under this definition, the Earth would qualify as dark matter.

                      Roy
                      Oh.
                      I'm not here anymore.

                      Comment


                      • #12
                        Originally posted by Carrikature View Post
                        I'm not really trying to catch an issue with dark matter so much as understand the reasoning behind it. Most of the rest of this is not anything I had heard. Do you know of some good resources for someone like me to read up on them?
                        A couple of reasonable explanations are available:

                        CMB and dark matter:
                        http://physics.stackexchange.com/que...ter-and-dark-e

                        Dark matter and gravitational lensing:
                        http://scienceblogs.com/startswithab...-lensing-show/

                        Dark matter and simulations of the universe:
                        http://www.newscientist.com/blogs/ns...rk-matter.html

                        And a follow up to that last one - observations of one of the predictions derived from these simulations:
                        http://www.nature.com/nature/journal...ture11224.html
                        "Any sufficiently advanced stupidity is indistinguishable from trolling."

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                        • #13
                          Originally posted by Carrikature View Post
                          ... I understand that the high velocity orbits of the fringe stars point to these halos, but are these halos not accounted for by the presence of all the other solar masses within? Just how far off are the observed rotational velocities from what we would expect to see?
                          The observed velocities are a LONG way off from what we would expect if the mass of the galaxy was mostly made up of the stuff than shines (stars or glowing gases). The inference is that most of the mass of a galaxy is "dark"; it can't be seen, and it also doesn't impede visibility of the light from stars.

                          Basically, the velocities of most stars in a galaxy (apart from right in the middle) seem to be roughly constant, even as you move well out from the central region. This occurs (in normal gravity) when the matter is proportional to distance from the center.

                          If nearly all the matter is concentrated at the center, then orbital velocities should decline with distance, so that V2R remains a constant. This is the case for planets in the solar system, for example. The two stylized curves below (and one measured example of a real galaxy) show the difference.

                          RotationCurves.png

                          Image adapted from the page Flat Rotation Curves produced for an astronomy course at Cornell.

                          Cheers -- sylas

                          Comment


                          • #14
                            Please bear with me as I continue to wrap my head around this. Early last month, I saw a couple of articles regarding a correction of the Raychaudhuri equation. The summary of the paper (Cosmology from quantum potential) is as follows:

                            In summary, we have shown here that as for the QRE, the second order Friedmann equation derived from the QRE also contains two quantum correction terms. These terms are generic and unavoidable and follow naturally in a quantum mechanical description of our universe. Of these, the first can be interpreted as cosmological constant or dark energy of the correct (observed) magnitude and a small mass of the graviton (or axion). The second quantum correction term pushes back the time singularity indefinitely, and predicts an everlasting universe.


                            I realize this correction will need further verification, but doesn't this address dark matter and/or energy in a way that effectively eliminates them in favor of a better understanding of gravity?
                            I'm not here anymore.

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