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)
[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)
