Missing Matter Problem and Galactic Flows

[PeterDonis]

I want to know where the data is to show whether we're falling in, falling out or in a stable orbit.

If your definition of "stable" is "unchanging", that seems like a much too restrictive definition. The Earth's orbit about the Sun is stable--it's lasted for 4.6 billion years--but it is not unchanging, as you pointed out. Similarly, even if the solar system is "falling in" towards the center of the galaxy at 10 km/s, that does not mean the solar system's orbit is not stable, only that it's not unchanging. (Actually it doesn't necessarily even mean that; an unchanging elliptical orbit has nonzero radial velocity everywhere except at the points of minimum and maximum distance from the central object.) Expecting unchanging orbits for anything in the real universe where there are always perturbations from other bodies is unrealistic, so defining "stable" as "unchanging" basically means no orbit is ever stable.

 

[Michael Faraday]

If your definition of "stable" is "unchanging", that seems like a much too restrictive definition. The Earth's orbit about the Sun is stable--it's lasted for 4.6 billion years--but it is not unchanging, as you pointed out. Similarly, even if the solar system is "falling in" towards the center of the galaxy at 10 km/s, that does not mean the solar system's orbit is not stable, only that it's not unchanging. (Actually it doesn't necessarily even mean that; an unchanging elliptical orbit has nonzero radial velocity everywhere except at the points of minimum and maximum distance from the central object.) Expecting unchanging orbits for anything in the real universe where there are always perturbations from other bodies is unrealistic, so defining "stable" as "unchanging" basically means no orbit is ever stable.

I'm not the one who came up with these laws. The assumption of missing mass is based on Kepler's 3rd law which requires a closed orbit (and a constant amount of mass inside the closed orbit). If the orbit isn't closed and/or the mass isn't constant, then the assumption of Kepler's 3rd law needs to be examined. That's what I'm doing with these questions.

 

[Drakkith]

It turns out we're falling in by 10 km/s according to one study, but I had to go to Stack Exchange to find the answer.

That's because you never asked whether we had a radial velocity. Your question has been about whether or not we are spiraling in or out. All the answers have addressed this question, and the answer is that we are not spiraling in or out, despite the fact that we have a radial component to our velocity. As PeterDonis said, having a radial velocity of -10 km/s does not mean that we are spiraling inwards towards the center of the galaxy, it just means our orbit is not perfectly circular.

I'm not the one who came up with these laws. The assumption of missing mass is based on Kepler's 3rd law which requires a closed orbit (and a constant amount of mass inside the closed orbit). If the orbit isn't closed and/or the mass isn't constant, then the assumption of Kepler's 3rd law needs to be examined. That's what I'm doing with these questions.

Going by this and your original post, you seem to think that the discrepancy in the orbital velocity of stars seen in the outer areas of a galaxy can be explained by mass flow? Is that right?

 

[Michael Faraday]

As PeterDonis said, having a radial velocity of -10 km/s does not mean that we are spiraling inwards towards the center of the galaxy, it just means our orbit is not perfectly circular.

The figure quoted in the article, 10 km/s, if true and an average mean, means that in 1 billion years ago we were 10 kpc. further out than we are now. In about 850 million years we will have fallen into the black hole in the center of the galaxy.

That word, spiral, I don't think it means what you think it means.

 

[Bandersnatch]

-10 km/s radial velocity in the galactocentric coordinates is not the same as falling in or spiralling in. Every elliptical orbit will have a radial component everywhere apart from peri- and apoapsis.
Furthermore, it tells you nothing about the kinematics of the galaxy as a whole - you need to compare it with velocities of other stars. It's the bulk motion that defines the galactic structure. The Sun has got -10 km/s radial velocity, other stars have different velocities, including in the opposite direction.

 

[Michael Faraday]

-10 km/s radial velocity in the galactocentric coordinates is not the same as falling in or spiralling in. Every elliptical orbit will have a radial component everywhere apart from peri- and apoapsis.
Furthermore, it tells you nothing about the kinematics of the galaxy as a whole - you need to compare it with velocities of other stars. It's the bulk motion that defines the galactic structure. The Sun has got -10 km/s radial velocity, other stars have different velocities, including in the opposite direction.

Please tell me how you know it's an elliptical orbit and not a spiral. This has been my question from the start.

 

[PeterDonis]

The assumption of missing mass is based on Kepler's 3rd law which requires a closed orbit (and a constant amount of mass inside the closed orbit).

Yes, but a closed elliptical orbit can still have a nonzero radial velocity.

Please tell me how you know it's an elliptical orbit and not a spiral.

You ought to be able to answer this yourself using the numbers in the abstract of the paper you linked to. Just check to see if those numbers are consistent with a closed elliptical orbit.

 

[Drakkith]

The figure quoted in the article, 10 km/s, if true and an average mean, means that in 1 billion years ago we were 10 kpc. further out than we are now. In about 850 million years we will have fallen into the black hole in the center of the galaxy.

Where did you get this idea?

That word, spiral, I don't think it means what you think it means.

Let's see...

Spiral: a curve which emanates from a central point, getting progressively farther away as it revolves around the point.

Yeah, that's exactly how I'm using it.

 

[Michael Faraday]

You ought to be able to answer this yourself using the numbers in the abstract of the paper you linked to. Just check to see if those numbers are consistent with a closed elliptical orbit.

No, you can't. I can't believe I have to explain this. The report I references is a kinematic study. It makes no assumptions about the past or future. It simply says, as of right now, this is the measured flow of the Local Standard of Rest (LSR). So, again, I ask you, how do you know that our sun is in an elliptical orbit?

 

[Michael Faraday]

Spiral: a curve which emanates from a central point, getting progressively farther away as it revolves around the point.

Winding in a continuous and gradually widening (or tightening) curve, either around a central point on a flat plane or about an axis so as to form a cone.

Yeah, this is getting nowhere. If you can't see that a radial velocity of -10 km/s results in the complete decomposition of the sun's orbit in 850 million years then you are using either a different language or different math than I'm using, so there's no point in continuing this conversation.

 

[Drakkith]

No, you can't. I can't believe I have to explain this. The report I references is a kinematic study. It makes no assumptions about the past or future. It simply says, as of right now, this is the measured flow of the Local Standard of Rest (LSR). So, again, I ask you, how do you know that our sun is in an elliptical orbit?

We apply known physical laws regarding gravitation to a model and see what the results are. So far, those results have supported that that Sun is in an elliptical orbit around the center of the galaxy. As I already explained, kinematic data by itself is of limited value without an appropriate model.

If you can't see that a radial velocity of -10 km/s results in the complete decomposition of the sun's orbit in 850 million years then you are using either a different language or different math than I'm using, so there's no point in continuing this conversation.

Except that you're wrong. That's not how orbital mechanics works. Any object in a non-circular orbit will have nonzero radial velocity throughout most or all of the orbit, as has been explained already.

You are right about one thing. This is getting nowhere. You've asked us questions and then when our answers don't make sense to you, you berate us for not giving you the answers you wanted when you haven't made an actual effort to understand what we've told you. You should consider that the answers given may not have been the answers you wanted, but the answers you needed. Thread locked.