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#1




Questions on the enigmatic rotational curve of spiral galaxies
I just want to add a thought experiment to help us visualise what is being
suggested/asked. Consider a number of stars arrayed in a line across space, say a few light years in length. Assuming the initial condition is one of a stationary motion of each star relative to the other, what would we expect to occur next? I assume that the mutual attraction of the stars will cause the row of stars to shorten until they end up clumped together. So to make my 'barred' galaxy stable I rotate it fast enough so that the stars on the two ends don't proceed toward or away from the rotational centre. Will this work? I assume it will not work close to the rotational centre but will work further out. There is a greater gravitational pull on objects closer to the two ends because there is more mass between those objects and the centre, but the rotational speed is greater as well  nicely balanced Now we add more mass to the centre. Objects toward the ends of the arms are going to be drawn inwardly unless the arm describes an arc. Now the pull directly from the central mass can be added to the less effective pull of the curved arm and the galaxy is again stable. Why do I think this has been missed? Models consist of known and unknown quantities, such as the numbers of and masses of the stars that can be observed. But unobserved is the central mass which is estimated. The central mass, I assume, is estimated at far higher than it actually is. I don't think modellers have considered relatively tiny central masses, as in my barred galaxy model above. Thus a higher central mass then requires a greater mass beyond the galaxy for it to rotate as observed. I do not have the skills to test these ideas but I assume that either: It has already been considered and shown to be flawed/viable or members of this list can estimate the viability of the idea using a simplified model (as I have suggested above). Robert 
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#2




Questions on the enigmatic rotational curve of spiral galaxies
On 22 Feb, 12:11, "Robert Karl Stonjek"
wrote: I just want to add a thought experiment to help us visualise what is being suggested/asked. Consider a number of stars arrayed in a line across space, say a few light years in length. Assuming the initial condition is one of a stationary motion of each star relative to the other, what would we expect to occur next? I assume that the mutual attraction of the stars will cause the row of stars to shorten until they end up clumped together. So to make my 'barred' galaxy stable I rotate it fast enough so that the stars on the two ends don't proceed toward or away from the rotational centre. Will this work? *I assume it will not work close to the rotational centre but will work further out. There is a greater gravitational pull on objects closer to the two ends because there is more mass between those objects and the centre, but the rotational speed is greater as well  nicely balanced Now we add more mass to the centre. *Objects toward the ends of the arms are going to be drawn inwardly unless the arm describes an arc. *Now the pull directly from the central mass can be added to the less effective pull of the curved arm and the galaxy is again stable. Why do I think this has been missed? Models consist of known and unknown quantities, such as the numbers of and masses of the stars that can be observed. *But unobserved is the central mass which is estimated. *The central mass, I assume, is estimated at far higher than it actually is. *I don't think modellers have considered relatively tiny central masses, as in my barred galaxy model above. *Thus a higher central mass then requires a greater mass beyond the galaxy for it to rotate as observed. I do not have the skills to test these ideas but I assume that either: It has already been considered and shown to be flawed/viable or members of this list can estimate the viability of the idea using a simplified model (as I have suggested above). http://www.google.co.uk/search?hl=en...tation&me ta= All galaxies seem to contain a supermassive black hole. This seems to be required from the point of view of stability. A number of simulations have been performed. http://www.mpagarching.mpg.de/galform/ This is a website for one of the foremost groups.  Ian Parker 
#3




Questions on the enigmatic rotational curve of spiral galaxies
"Ian Parker" wrote in message ... On 22 Feb, 12:11, "Robert Karl Stonjek" wrote: Snip I do not have the skills to test these ideas but I assume that either: It has already been considered and shown to be flawed/viable or members of this list can estimate the viability of the idea using a simplified model (as I have suggested above). http://www.google.co.uk/search?hl=en...tation&me ta= All galaxies seem to contain a supermassive black hole. This seems to be required from the point of view of stability. A number of simulations have been performed. http://www.mpagarching.mpg.de/galform/ This is a website for one of the foremost groups.  Ian Parker Thanks, Ian, The simulations are a little to big but the paper search is more interesting. For instance in The method of Galactic Rotation http://adsabs.harvard.edu/abs/1996A&AS..118...59J the authors assume an instant propagation of gravity and the gravitational pull of the galactic arm itself appears to be left out altogether. My quick skim of several results leads me to conclude that the Kepler model is the first approximation and this fits well with the inner region of a galaxy eg 1pc for the Milky Way (the supermassive black hole accounts for the motion of the inner objects). But the bar model may be a better approximation for the outer region ie starting with evenly spaced massive objects across, say 20kpc (comparable to the Milky Way) and then consider the initial conditions of a static and rotating bar and see what happens. The simplified model is the bread and butter of physics modelling, but the sphere or disc may not be the best simplified model in this case. The bar may be more instructive for the outer region. Here is my objection clearly outlined in the abstract of another paper: "Galaxies as rotating systems involve a balance of gravitational attraction and centripetal force. When the central mass is dominant the dynamics, referred to as Keplerian, are that the orbital velocities are proportional to the inverse square root of the path radius. A plot of velocity vs. path radius is a Rotation Curve. For galaxies viewed as the disk edge we see one end moving toward us relative to the center and the other moving away. The rotational velocities are measured from the variation in redshift along the galactic diameter. Rotation curves so obtained are not the expected Keplerian inverse square root; rather, [beyond the galactic core] they are flat. In a solid sphere all parts move at rotational velocities directly proportional to radius. Since the observed flat curves lie between the Keplerian inverse square root of radius and the solid's direct proportion to radius, a mass discrepancy is inferred  that unseen matter is dispersed throughout the galaxy, a halo of "dark matter" that causes the curve form exhibited  thus the "dark matter" hypothesis. The mass discrepancy only appears where the acceleration is less than 108 cm/sec2. Modeling gives an alternative hypothesis: Modification of Newtonian Dynamics or MOND, that gravity and or inertia are modified when g or a is less than 108 cm/sec2. No justification has been developed except correlation with the mass discrepancies. An alternative explanation is presented  the general exponential decay of the overall universe, which has been analyzed and developed in several papers. The universal decay accounts for the mass discrepancies because the effect of the decay is to make the rotation curves appear to deviate from the form expected in a Keplerian galactic disk although the actual rotational behavior does not." http://www.citeulike.org/group/48/article/70704 Referring back to my illustration at the root of this thread we note that force '3' has been ignored. I can't tell if the models on the page you point to consider those forces or not. Robert 
#4




Questions on the enigmatic rotational curve of spiral galaxies
On Feb 22, 7:40 am, "Robert Karl Stonjek"
wrote: "Ian Parker" wrote in message ... On 22 Feb, 12:11, "Robert Karl Stonjek" wrote: Snip I do not have the skills to test these ideas but I assume that either: It has already been considered and shown to be flawed/viable or members of this list can estimate the viability of the idea using a simplified model (as I have suggested above). http://www.google.co.uk/search?hl=en...alactic+rotati... All galaxies seem to contain a supermassive black hole. This seems to be required from the point of view of stability. A number of simulations have been performed. http://www.mpagarching.mpg.de/galform/ This is a website for one of the foremost groups.  Ian Parker Thanks, Ian, The simulations are a little to big but the paper search is more interesting. For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J the authors assume an instant propagation of gravity and the gravitational pull of the galactic arm itself appears to be left out altogether. My quick skim of several results leads me to conclude that the Kepler model is the first approximation and this fits well with the inner region of a galaxy eg 1pc for the Milky Way (the supermassive black hole accounts for the motion of the inner objects). But the bar model may be a better approximation for the outer region ie starting with evenly spaced massive objects across, say 20kpc (comparable to the Milky Way) and then consider the initial conditions of a static and rotating bar and see what happens. The simplified model is the bread and butter of physics modelling, but the sphere or disc may not be the best simplified model in this case. The bar may be more instructive for the outer region. Here is my objection clearly outlined in the abstract of another paper: "Galaxies as rotating systems involve a balance of gravitational attraction and centripetal force. When the central mass is dominant the dynamics, referred to as Keplerian, are that the orbital velocities are proportional to the inverse square root of the path radius. A plot of velocity vs. path radius is a Rotation Curve. For galaxies viewed as the disk edge we see one end moving toward us relative to the center and the other moving away. The rotational velocities are measured from the variation in redshift along the galactic diameter. Rotation curves so obtained are not the expected Keplerian inverse square root; rather, [beyond the galactic core] they are flat. In a solid sphere all parts move at rotational velocities directly proportional to radius. Since the observed flat curves lie between the Keplerian inverse square root of radius and the solid's direct proportion to radius, a mass discrepancy is inferred  that unseen matter is dispersed throughout the galaxy, a halo of "dark matter" that causes the curve form exhibited  thus the "dark matter" hypothesis. The mass discrepancy only appears where the acceleration is less than 108 cm/sec2. Modeling gives an alternative hypothesis: Modification of Newtonian Dynamics or MOND, that gravity and or inertia are modified when g or a is less than 108 cm/sec2. No justification has been developed except correlation with the mass discrepancies. An alternative explanation is presented  the general exponential decay of the overall universe, which has been analyzed and developed in several papers. The universal decay accounts for the mass discrepancies because the effect of the decay is to make the rotation curves appear to deviate from the form expected in a Keplerian galactic disk although the actual rotational behavior does not."http://www.citeulike.org/group/48/article/70704 Referring back to my illustration at the root of this thread we note that force '3' has been ignored. I can't tell if the models on the page you point to consider those forces or not. Robert May I add a thought? The Milky Way is about a hundred thousand light years across. Let's take this as about average. Now, if I am looking at a similar galaxy out in space, won't I be seeing the front edge as it was positioned at Time X in the same image as the back edge as it was positioned at Time X minus 100,000 years? So why are galaxies all so symmetrical? Shouldn't they all be skewed in a way that reflects their movement and our position and the difference between when light arrives to us from their closest and farthest parts relative to us? john 
#5




Questions on the enigmatic rotational curve of spiral galaxies
On 22 Feb, 13:40, "Robert Karl Stonjek"
wrote: Thanks, Ian, The simulations are a little to big but the paper search is more interesting. *For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J the authors assume an instant propagation of gravity and the gravitational pull of the galactic arm itself appears to be left out altogether. There is always the question about the validity of assumptions. Instantanious travekl for gravity can be defended on the basis that the errors in that approximation are of the order of rotation velocities relative to c. Now the Sun is travelling at 600km/s c = 300,000km/s. Hence the instantaneous travel of gravity can be justified. The velocities of the stars relative to each other are lower still. What is in fact more serious is assumptions about "dark matter".  Ian Parker 
#6




Questions on the enigmatic rotational curve of spiral galaxies
Sam Wormley wrote:
john190209 wrote: The Milky Way is about a hundred thousand light years across. Let's take this as about average. Now, if I am looking at a similar galaxy out in space, won't I be seeing the front edge as it was positioned at Time X in the same image as the back edge as it was positioned at Time X minus 100,000 years? So why are galaxies all so symmetrical? Shouldn't they all be skewed in a way that reflects their movement and our position and the difference between when light arrives to us from their closest and farthest parts relative to us? john I don't think M31 looks all that symmetrical. But getting to your argument. If you assume that the a given galaxy looks about the same as it did a quarter of a revolution earlier, why wouldn't you expect to observer relative symmetry even when looking back further in time for the backside? Also, if we assume a rotation rate for the galaxy consistent with the Solar System's orbit about the galactic center, that is, about 240 million years, then the 100,000 year light crossing time amounts to a rotational offset of only about 0.15 degrees from one side to the other. 
#7




Questions on the enigmatic rotational curve of spiral galaxies
On Feb 22, 7:27*am, Ian Parker wrote:
On 22 Feb, 13:40, "Robert Karl Stonjek" wrote: Thanks, Ian, The simulations are a little to big but the paper search is more interesting. *For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J the authors assume an instant propagation of gravity and the gravitational pull of the galactic arm itself appears to be left out altogether. There is always the question about the validity of assumptions. Instantanious travekl for gravity can be defended on the basis that the errors in that approximation are of the order of rotation velocities relative to c. Now the Sun is travelling at 600km/s c = 300,000km/s. Hence the instantaneous travel of gravity can be justified. The velocities of the stars relative to each other are lower still. Uh, no. Gravitational effects travel exclusively at c in GR, and this has been shown to be consistent with reality in observation. What is in fact more serious is assumptions about "dark matter". *  Ian Parker 
#8




Questions on the enigmatic rotational curve of spiral galaxies
On Feb 22, 7:11*am, "Robert Karl Stonjek"
wrote: I just want to add a thought experiment to help us visualise what is being suggested/asked. Consider a number of stars arrayed in a line across space, say a few light years in length. Assuming the initial condition is one of a stationary motion "stationary motion"? AHAHAHAHAHAHAHAHA Tom Davidson Richmond, VA 
#9




Questions on the enigmatic rotational curve of spiral galaxies
On 22 Feb, 19:52, Eric Gisse wrote:
On Feb 22, 7:27*am, Ian Parker wrote: On 22 Feb, 13:40, "Robert Karl Stonjek" wrote: Thanks, Ian, The simulations are a little to big but the paper search is more interesting. *For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J the authors assume an instant propagation of gravity and the gravitational pull of the galactic arm itself appears to be left out altogether. There is always the question about the validity of assumptions. Instantanious travekl for gravity can be defended on the basis that the errors in that approximation are of the order of rotation velocities relative to c. Now the Sun is travelling at 600km/s c = 300,000km/s. Hence the instantaneous travel of gravity can be justified. The velocities of the stars relative to each other are lower still. Uh, no. Gravitational effects travel exclusively at c in GR, and this has been shown to be consistent with reality in observation. I never said they did not. What I DID say was that the error involved is proportional to relative velocities/c. (assuming travel at c). If a lot of calculation is involved you may assume instantaneous travel for orbital velocities of 600km/s and relative velocities lower still.  Ian Parker 
#10




Questions on the enigmatic rotational curve of spiral galaxies
wrote in message ... On Feb 22, 7:11 am, "Robert Karl Stonjek" wrote: I just want to add a thought experiment to help us visualise what is being suggested/asked. Consider a number of stars arrayed in a line across space, say a few light years in length. Assuming the initial condition is one of a stationary motion "stationary motion"? AHAHAHAHAHAHAHAHA Tom Davidson Richmond, VA RKS: OK, I was thinking of writing either 'stationary' or 'no motion' and ended up with an insane hybrid. Thanks for pointing out the error (I think...) Robert 
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