I Vera Rubin's research

[tedj121]

TL;DR Summary

Why stars at the periphery of galaxies orbit faster than they apparently should.

Is it correct to say that the reason why stars at the periphery of galaxies are observed to orbit faster than can be accounted for by Newtonian physics is because they are gravitationally bound to relatively high density distributed matter also present at the periphery that must be attributed to the presence of dark matter. And that those peripheral stars are bound to that peripheral matter more so than to their supermassive black hole. But the peripheral matter IS bound to the central black hole. The situation being much as the moon is bound to the earth. And the earth is bound to the sun. And the moon moves more or less with the earth.

 

[Bandersnatch]

No, that's not the right way to see it. The way the hypothesised dark matter needs to be distributed to account for the rotation curves requires it to be less dense towards the periphery. There's even less dark matter per unit volume out there than in our vicinity. There's nothing to be locally bound to.
The dark matter mass that accounts for the rotation curves is the entire mass inside the radius of the orbit of a given star.

 

[Orodruin]

In the same lind as in post #2, it is worth pointing out that stars are generally not bound to single objects like the supermassive black hole. They are bound to the galaxy as a whole of which the sbh at the center is a minor contribution to the mass.

 

[tedj121]

So the peripheral stars studied by Dr Rubin were bound to the galaxy according to Newtonian physics, alright. There's just a lot more mass than is observable, including (but decreasing towards) the periphery, as one would also expect from Newtonian physics, in order for increasingly less dense and more distant matter regions to still be bound to their parent galaxy. Unobserved mass also accounting for the larger orbital velocities than expected. I've been watching "NOVA: Decoding the Universe: Cosmos" and portions of it seem rather inadequately worded/presented. The dark matter could include naked atomic nuclei stripped of their electrons (and thus unable to emit thermal radiation), neutrinos and other exotic non-atomic matter.

 

[renormalize]

The dark matter could include naked atomic nuclei stripped of their electrons (and thus unable to emit thermal radiation), neutrinos and other exotic non-atomic matter.

How would this work? A large collection of completely ionized nuclei produces a tremendous electrostatic force-field in the absence of a similar number of balancing electrons to make the whole collection electrically neutral. Since Coulomb repulsion between like charges is much stronger than gravitational attraction, why wouldn't the positively charged nuclei repel each other right out of the galaxy?

 

[Orodruin]

The dark matter could include naked atomic nuclei stripped of their electrons (and thus unable to emit thermal radiation), neutrinos and other exotic non-atomic matter.

No it could not. First of all, a plasma certainly is able to emit thermal radiation. Look at the Sun! (Actually, don't look at the Sun - it emits a lot of thermal radiation!)

Neutrinos, while technically dark matter, do not work very well as a dark matter candidate for several reasons. Mainly it is what would be called hot dark matter, which would wipe out a lot of large scale structure in the early Universe. It could not account for more than a tiny fraction of all of the dark matter.

A lot of different dark matter candidates have been - and continue to be - considered. What we really know is that it essentially has to be cold dark matter and that it cannot interact electromagnetically (it could have extremely tiny charges, but not of the same order as the electron charge).

I've been watching "NOVA: Decoding the Universe: Cosmos" and portions of it seem rather inadequately worded/presented.

That's because it is popular science. Its intention is to tell you a story and captivate you, not to teach you actual physics. Popular science is notoriously effective at making people they think they understand things, when in reality they do not.

 

[tedj121]

Fascinating. I stand thoroughly corrected, caught in a physics cross-fire. But tell me what facts rule out non-luminous normal matter, if you would be so kind and complete. What ARE the current leading candidates?

 

[Vanadium 50]

But tell me what facts rule out non-luminous normal matter

Like gas clouds? We don't observe them.

 

[Ibix]

But tell me what facts rule out non-luminous normal matter,

In the quantities we need for the gravitational effect of dark matter it would absorb and scatter star light and we'd see it. More fundamentally, electromagnetic interactions are how matter collides and slows down and clumps and eventually forms stars and galaxies. To have the "galactic halo" distribution it does dark matter has to be very nearly collisionless, unlike normal matter.

What ARE the current leading candidates?

An unknown particle was favourite, but detectors have been built and haven't seen it. A lot of small-ish black holes might do the job, but in such large numbers that we should have seen gravitational lensing from one by now and we haven't.