Explanation for Galaxy Rotation Curves

[sha1000]

TL;DR Summary

Op wonders about the possibility that larger-than-expected black holes at galaxy centers could explain the rotational speeds of stars

The existence of dark matter was initially proposed to address discrepancies between observed galaxy rotation curves and the expected behavior dictated by our current understanding of gravity. Typically, it's argued that stars at the edges of galaxies rotate faster than expected, leading to theories about the presence of dark matter to explain the gravitational forces at play.

However, has there been significant research into the possibility of significantly larger black holes at the centers of galaxies?
Could the high mass of these hypothetical super-black-holes be actually consistent with the rotational speeds of outer stars. And maybe the real problem comes from the velocity of inner stars, which are too slow due to some unknown mechanisms.

Is there any fundamental reason why this hypothesis can not even be considered? Or maybe you are aware of some papers which explored this kind of scenarios. I would appreciate if you could share them.

 

[sha1000]

To make it as simple as possible. I am wondering: Why when we look at the galaxy rotational curves, our first reaction is to say: "oh, the outer stars are way too fast" and not "oh, the inner stars are way too slow"..

 

[Orodruin]

The inner stars match the orbital speed expected from visible matter.

 

[Orodruin]

In addition, the issue is that orbital speed flattens out rather than decreasing at large distances, where there is essentially very little visible matter. This would not be the case with more mass in the center.

 

[Vanadium 50]

Since this is A-level, if the mass density with radius is higher than expected at large radius, how can adding mass at small radius fix things?

 

[berkeman]

Since this is A-level, if the mass density with radius is higher than expected at large radius, how can adding mass at small radius fix things?

Yeah, I'm pretty sure that the OP doesn't really want replies at the graduate school level. I've bumped the thread prefix down to "I" (undergraduate) just now.

 

[PeroK]

To make it as simple as possible. I am wondering: Why when we look at the galaxy rotational curves, our first reaction is to say: "oh, the outer stars are way too fast" and not "oh, the inner stars are way too slow"..

Maybe that's not just a first reaction. Maybe some people have spent years of their lives studying the data? Not everyone is an armchair physicist!

 

[sha1000]

The inner stars match the orbital speed expected from visible matter.

Thanks for the response.
To explain the velocities of outer stars, scientists propose the existence of dark matter, uniformly distributed throughout galaxies. However, why not attribute all this mass to a supermassive black hole at the center of the galaxy instead? This could also potentially account for the fast-moving outer stars.

But in this case. If we consider this additonal mass at the galaxy's center, the question would be: why then do outer stars move slower than expected? Could it be that gravitational forces behave differently near such massive objects?

Modified Newtonian Dynamics (MOND) suggests adjustments to gravitational laws for outer stars. Could we alternatively consider adjusting the gravitational laws for inner stars, assuming that the mass attributed to dark matter is actually concentrated in the galaxy's center?

Is there a specific reason this possibility has been ruled out?

 

[sha1000]

Maybe that's not just a first reaction. Maybe some people have spent years of their lives studying the data? Not everyone is an armchair physicist!

I'm not trying to be arrogant.

This is exactly why I'm asking why such possibilities have not been considered. What could be the fundamental reason to dismiss this idea without consideration?

Is it simply because the visible mass is sufficient to account for the velocities of outer stars, so why complicate things further?

 

[Orodruin]

However, why not attribute all this mass to a supermassive black hole at the center of the galaxy instead?

I already told you:

In addition, the issue is that orbital speed flattens out rather than decreasing at large distances, where there is essentially very little visible matter. This would not be the case with more mass in the center.

Please read the answers provided.

What could be the fundamental reason to dismiss this idea without consideration?

Simple: It doesn't match observations. It therefore merits no further consideration.

 

[Ibix]

This is exactly why I'm asking why such possibilities have not been considered. What could be the fundamental reason to dismiss this idea without consideration?

To put it slightly differently from Orodruin, it has been considered. The problem with a "visible matter only" model of a galaxy is not just that it doesn't spin fast enough, but also that the orbital velocity drops off too rapidly with distance from the center compared to reality. Adding more mass to the center of this model makes the drop off steeper, not flatter, so it makes the model less like observation, not more.

 

[Vanadium 50]

I'm not trying to be arrogant.

I already told you:

Perhaps you see the problem.

You asked a question, got an answer, and then asked the original question again. You might reread the answers and then if you have questions, ask questions about the answers you got, rather than just repeating them. Otherwise, its unlikely this excgange will get you where you want to go.

 

[PeterDonis]

I'm not trying to be arrogant.

Maybe you don't think you are, but when you repeat a question that has already been answered, without any indication that you've actually read and understood the answers, that's what it looks like.

This is exactly why I'm asking why such possibilities have not been considered. What could be the fundamental reason to dismiss this idea without consideration?

You were already told why the idea was not dismissed "without consideration".

Your question has been answered and this thread is now closed.