Talk:Galaxy rotation curve
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[edit] Refactored from Archived Discussion
[edit] Elliptical orbits
One thing that I have never seen is an explanation of the galactic rotation curve that also explicitly takes into account the theory that the spiral arms are not in fact coherent but are a construct of the elliptical orbits of the stars that make them up, as explained by Image:Spiral galaxy arms diagram.png. The implication of this is that a star in a spiral arm is near the aphelion end of the ellipse, and so is going more slowly than a star on a circular orbit at that distance would be. If the ellipses themselves are turning and giving the illusion of the spiral arms rotating evenly, then the discrepancy could disappear. It seems unlikely that this has been overlooked, but I'd be interested to see a discusson that includes this aspect. — PhilHibbs | talk 18:41, 23 Feb 2005 (UTC)
[edit] Velocity vs Speed
The vertical axis of the graph really should be "Speed". I know that "Velocity" sounds more scientific, but velocity is a vector and that's not being represented on the graph, but only the magnitude, and the magnitude component of velocity is "speed". -- Ch'marr 15:29, 10 October 2005 (UTC) (the pedant)
Actually it would be correct to call it angular velocity, since that's what it is. 132.229.87.144 12:51, 4 August 2007 (UTC)
- Not true. If it were angular velocity then it wouldn't be flat at large radii: v(r)~constant, but ω(r)=v(r)/r
1/r. And angular velocity is also a vector, although the distinction isn't important for a rotating disk. Cosmo0 —Preceding signed but undated comment was added at 18:41, 20 September 2007 (UTC)
[edit] Mysterious vs. Mundane Dark Matter
I've read in the past, either in "Light at the Edge of the Universe" or perhaps "The Whole Shebang" (I can't remember which) that the amount of matter required to explain the galaxy rotation curve effect was significantly less than the amount of matter required to explain the flat curvature of the universe in the FLRW metric. I should, I suppose, find the exact quote.
For the galaxy rotation curve, it was estimated that approximately 90% of the matter of the universe had to be "dark" which to me seems no great stretch of the imagination, just considering free hydrogen in a more-or-less smooth distribution. In the region of planets, of course, you wouldn't find it, because it's been swept away by the condensed masses. But the rings of Saturn (and the asteroid belt) are suggestive of a large amount of such sweeping. Anyway, a smooth region of gas would be consistent with the smoothness of the Cosmic Background. Consider that all the galaxies could be just condensations in a universe with an almost crystalline pattern of hydrogen gas, held apart by an almost perfectly symmetrical initial big bang. Galaxies could be whirlpools in a sea of hydrogen instead of a vacuum, as is usually assumed.
While nonbaryonic dark matter seems to exist, it seems to appear mainly in the course of violent explosions, with a very short half-life, and in insufficient quantities to create the Galaxy Rotation Curve result.
Corrections welcome. JDoolin 00:45, 22 August 2006 (UTC)
[edit] Reverted contributions
I reverted this contribution because it represents a rather extreme minority theory for an explanation of dark matter. If and when this idea receives more notice within the community we can begin to include it, but it will probably find itself at dark matter rather than here.
ScienceApologist (talk) 18:22, 11 January 2008 (UTC)
[edit] BEC or Scalar field dark matter model (SFDM) of halos is not an extrem minority theory
Dear ScienceApologist
I don't know how to contact you. So I wrote a mail here. Thank you for your efforts. But BEC or Scalar field dark matter model (SFDM) of halos is not an extremly minority theory. Actually, this model is a main alternative to standard cold dark matter model and the most successful model in explaing the rotation curves.It has many different names such as fuzzy, fluid, repulsive, scalar field, boson halo and so on.(Please see a review article in Science [1].)This model solves many problems of cold dark model (CDM) such as cuspy halo and missing satellite. There are already hundres of journal paper published about this idea and also many works saying the model predicts the observed rotation curves of galaxies and dwarf galaxies very well.(Please see review [2] page 21 and [3,4,5]) No other dark matter model have succeeded at this level in explaining the curves. Note that SFDM also behaves as CDM at the scale larger than galaxy, so it also explain large scale strures as much as CDM. Thus, I think, for balance, this model deserve space at least as much as the modified gravity theories which seems to be even not accepted by majority of physics community currently. As far as I understand, the philosophy of wikipedia is that users themselves make contents and the others review and edit the contents. Many experts in this field is already reading and editing this subject.(I am one of them) So, if my statements are wrong, missing something, or the model is an extremly minority which does not deserve publication, they will soon edit it. So please let the experts do it. [1] "New Light on Dark Matter" Jeremiah P. Ostriker and Paul Steinhardt Science 20 June 2003: Vol. 300. no. 5627, pp. 1909 - 1913 [2] "TOPICAL REVIEW: General relativistic boson stars" Franz E Schunck and Eckehard W Mielke arXiv:0801.0307v1 [astro-ph] [3] "Is dark matter a BEC or scalar field?"Jae-Weon Lee arXiv:0801.1442v1 [astro-ph] [4] "Mini-review on Scalar Field Dark Matter" L. Arturo Ure˜na–L´opez [5] "Scalar Field Dark Matter: head-on interaction between two structures" Phys.Rev. D74 (2006) 103002
Retrieved from "http://en.wikipedia.org/wiki/Talk:Galaxy_rotation_curve"
—Preceding unsigned comment added by Scikid (talk • contribs) 07:12, 12 January 2008
- Just to chime in with my view: it may not be an extreme minority theory but it is, at the moment, not well established as a serious alternative to CDM (despite your exaggerated account of it's successes). In any case, as ScienceApologist says, the article on dark matter is the correct place for outlining different theories on the nature of dark matter. From the point of view of galaxy rotation curves, the main debate is over whether dark matter is neccessary at all, although a single sentence pointing out that different kinds of dark matter predict slightly different rotation curves may be appropriate here. Cosmo0 (talk) 13:06, 12 January 2008 (UTC)
