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accdd
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Is it possible that galaxies are spinning faster than expected due to a particular configuration of the moving mass and not due to dark matter?
Rotation of Galaxies: Mass vs Dark Matter?
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In summary: I don't think we can assume that an effect we can't see is caused by Dark Matter without more evidence.
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Nugatory
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No.accdd said:
If that were possible then we wouldn't have an unsolved problem, we'd have observational data telling us how matter is distributed within galaxies.
(Unless some other unknown physics is at work, and in the absence of any plausible candidate theory that's just idle speculation).
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Vanadium 50
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Well, you could put rocket engines on all the stars.Nugatory said:
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Ibix
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Arguably, Deur's idea that we've failed to properly account for the GR effects of rotating matter fits the question.
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PeterDonis
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I thought Deur's idea was about GR effects of non-spherical configurations of matter, not about GR effects of rotating matter. His basic idea, as I understand it, is that the GR effects of a flat disk are significantly different from those of a sphere, and the standard method of calculating galaxy rotation curves is basically assuming that they're not, and using the simpler calculations for a sphere.Ibix said:
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Ibix
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You're more likely to be right than I am. Either way, he's explaining galaxy rotation curves using the visible matter and vanilla GR so he doesn't need dark matter. (Or so he says, anyway.)PeterDonis said:
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PeterDonis
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Yes, that's correct. But since the topic of this thread is particularly the effects of rotation, I don't think Deur's proposal is relevant to this particular thread.Ibix said:
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The initial post is too vague to really discuss. "A particular configuration of moving mass" that we cannot see? How does that differ from Dark Matter?
I am not an expert, but the problem I have with Deur-like theories is that the effect we are ascribing to Dark Matter is comparable to (actually, larger than) the Newtonian components. GR effects are usually smaller - the one exception I am immediately aware of goes as β, which is tiny for galactic rotation curves.
I am not an expert, but the problem I have with Deur-like theories is that the effect we are ascribing to Dark Matter is comparable to (actually, larger than) the Newtonian components. GR effects are usually smaller - the one exception I am immediately aware of goes as β, which is tiny for galactic rotation curves.
FAQ: Rotation of Galaxies: Mass vs Dark Matter?
What is the rotation curve of a galaxy and how does it relate to mass and dark matter?
The rotation curve of a galaxy is a plot that shows the orbital velocity of stars or gas clouds as a function of their distance from the center of the galaxy. In a typical galaxy, the orbital velocity increases as you move away from the center and then levels off. This curve is directly related to the distribution of mass within the galaxy, with the majority of the mass being located in the center. However, the observed rotation curves of galaxies do not match what is expected based on the visible mass alone, leading to the hypothesis of dark matter.
What evidence supports the existence of dark matter in galaxies?
The primary evidence for dark matter in galaxies comes from the discrepancy between the observed rotation curves and the predicted rotation curves based on the visible mass. This discrepancy is also seen in other observations, such as gravitational lensing and the motion of galaxies within galaxy clusters. Additionally, simulations of galaxy formation and evolution support the need for dark matter to explain the observed structures of galaxies.
How does the amount of dark matter in a galaxy vary?
The amount of dark matter in a galaxy is believed to vary depending on the size and type of galaxy. Elliptical galaxies, which are larger and more massive, are thought to have a higher proportion of dark matter compared to spiral galaxies. Additionally, the distribution of dark matter within a galaxy is not uniform, with the inner regions having a higher concentration than the outer regions.
Can the rotation of galaxies be explained without the need for dark matter?
There have been attempts to explain the observed rotation curves of galaxies without the need for dark matter, such as modifying the laws of gravity. However, these theories have not been able to fully explain the observations and do not have as much supporting evidence as the dark matter hypothesis. Additionally, the existence of dark matter is supported by a wide range of observations beyond just the rotation of galaxies.
Is there a limit to how much dark matter a galaxy can have?
There is currently no known limit to how much dark matter a galaxy can have. The amount of dark matter in a galaxy is believed to be directly related to the total mass of the galaxy, so larger galaxies are expected to have more dark matter. However, there is still much to be learned about dark matter and its properties, so it is possible that there could be other factors that influence the amount of dark matter in a galaxy.