Dark matter, why?

[headscratcher]

So the orbital speed around the galaxy of stars in the outskirts of the galaxy orbit faster than they should.

First how could astronomers go so long without noticing?

Second why "dark matter" to solve the problem? We don't expect the speed to be a certain amount based on the mass we can see. We can't see all the mass. We estimate the mass based on the orbital velocities.


Seems to me that the problem is easily solved by the fact that the galaxy is not a single mass with stars orbiting. It is a disperse mass of stars, gas and dust that the stars orbit within. Inner stars orbit a mass inside their orbit and outer stars orbit the greater mass of all the inner stars. More mass inside the orbit means faster orbit. We should expect them to orbit faster than inner stars as they are orbiting larger mass.

So it shouldn't be a problem of invisible matter or dark matter. Even visible more matter doesn't solve the problem. Examining it as disperse matter solves the problem as that is what the galaxy is.

[Chris Peterson]

So the orbital speed around the galaxy of stars in the outskirts of the galaxy orbit faster than they should.
First how could astronomers go so long without noticing?

It didn't go that long. It was observed in the 1930s that galaxies in clusters were moving at the wrong orbital velocities, and in 1959 that individual galaxies were rotating in an unexpected way. These aren't such easy things to observe- you have to aim a pretty big telescope at a galaxy, and then look at the spectrum across it, so that the Doppler shift can provided velocity.

Second why "dark matter" to solve the problem? We don't expect the speed to be a certain amount based on the mass we can see. We can't see all the mass. We estimate the mass based on the orbital velocities.

The problem isn't the absolute velocities, but the fact that all the stars have similar velocities, regardless of radial distance. The mass density can be estimated by luminosity. The only reasonable way to explain the observed rotation is to assume there is a lot of invisible mass well outside the visible part of the galaxy.

Seems to me that the problem is easily solved by the fact that the galaxy is not a single mass with stars orbiting. It is a disperse mass of stars, gas and dust that the stars orbit within. Inner stars orbit a mass inside their orbit and outer stars orbit the greater mass of all the inner stars. More mass inside the orbit means faster orbit. We should expect them to orbit faster than inner stars as they are orbiting larger mass.

Doesn't work. The calculations consider the variable mass density of the galaxy.

So it shouldn't be a problem of invisible matter or dark matter. Even visible more matter doesn't solve the problem. Examining it as disperse matter solves the problem as that is what the galaxy is.

Keep in mind that the rotation curve of galaxies is only one piece of evidence for dark matter. Its effects are also seen in the dynamics of galaxy clusters, in the way images are formed by gravitational lenses, in the structure of the CMB, and in other independent observations, as well. It is also essential to the mainstream Big Bang model (lambda-CDM), which is itself supported by multiple lines of evidence.

[headscratcher]

Thanks for the answer. I'm not buying it. It doesn't help that you misquoted me and answered your misquote not my question.

The misquotes;

I asked about dark matter not invisible mass. Of course most of the mass of the galaxy is invisible from Earth. That's no reason to posture it as "dark matter". And, as I wrote, dark matter, invisible matter or brilliantly lighted matter doesn't solve the problem.

I didn't point out that the galaxy's mass is "variable". I wrote was that it is disperse.

If what you meant to write when you wrote "The calculations consider the variable mass density of the galaxy" was really "The calculations consider the disperse mass density of the galaxy" I'd have to ask why isn't this disperse mass density "calculation" mentioned when the rotational anomaly is discussed either in the media or anywhere the rotational anomaly is discussed? Particularly when the orbital velocity curve of stars in the galaxy is discussed in comparison to the orbital velocity curve of planets in the solar system. The only "calculation" I ever see mentioned is the standard Newtonian, as if the stars are orbiting a single central mass.

[Chris Peterson]

I asked about dark matter not invisible mass. Of course most of the mass of the galaxy is invisible from Earth. That's no reason to posture it as "dark matter". And, as I wrote, dark matter, invisible matter or brilliantly lighted matter doesn't solve the problem.

It was initially treated as missing mass. An early suggestion was that it was ordinary matter that was simply invisible because it was non-luminous. That is now recognized as incorrect, because non-luminous, ordinary matter would be apparent from its infrared signature, and would show scattering artifacts that our instruments are now good enough to detect. That is why nearly everybody now considers the invisible mass to be dark matter- a form of matter that does not interact with EM, or only interacts very weakly.

If what you meant to write when you wrote "The calculations consider the variable mass density of the galaxy" was really "The calculations consider the disperse mass density of the galaxy" I'd have to ask why isn't this disperse mass density "calculation" mentioned when the rotational anomaly is discussed either in the media or anywhere the rotational anomaly is discussed? Particularly when the orbital velocity curve of stars in the galaxy is discussed in comparison to the orbital velocity curve of planets in the solar system. The only "calculation" I ever see mentioned is the standard Newtonian, as if the stars are orbiting a single central mass.

I've never seen a simple Newtonian (or Keplerian) calculation applied to a galaxy. If you assume a mass distribution derived from luminosity, the rotation curve is wrong (this assumes, of course, a distributed mass). The best solution that matches the observed rotation curves is a spherical mass distribution, with a high density in the center and decreasing density with radius, extending beyond the luminous galaxy and containing many times the mass of the luminous galaxy. Again, this is strong evidence for dark matter.

There are many other lines of observational evidence supporting the existence of dark matter (as non-baryonic matter making up most of the mass of the Universe).

[headscratcher]

You've never seen any calculation of a galaxy's rotation according to Newton? You seem unfamiliar with the fundamentals involved in this topic.

If you're interested here is the wiki on the subject http://en.wikipedia.org/wiki/Galactic_rotation_curve

Note from the wiki "(stars) revolve much faster than would be expected if they were in a free Newtonian potential.

Generally speaking, Kepler related by geometric proportion the orbital period of planets (their velocity) around the sun to their distance (semi major axis) from it. Newton generalized this for any body being orbited to the mass of the body being orbited. Therefore the mass of the body being orbited is related to the period and distance of orbiting bodies.

Therefore the mass of the body being orbited can be calculated (not estimated, even if unseen) to high precision by relating the distance of the orbiting body and its period.

However Newtons law states (in part) "Every point mass attracts every single other point mass". Therefore treating the galaxy as a point mass for bodies orbiting inside it is a mistake. If the body is outside of the galaxy the galaxy can be treated as a point mass.

This being the case the anomaly can be solved.

Observing the period of stars near the center of the galaxy gives the mass of stars in the area inside the orbit. Therefore it gives a statement of the density of mass per unit area of the galaxy inside the orbit. Observing stars farther out also gives the density of mass per unit area of the galaxy inside their orbits.

Treating the galaxy as observed, ie, more or less uniformly distributed stars in a flat disk tapering to thin at the edges we should expect to be able to calculate the mass per unit area as more or less uniformly distributed stars in a flat disk (a more or less flat line of mass distribution) tapering to thin at the edges. This if the mass of the galaxy is as observed. In which case there is no anomaly. If the mass distribution as calculated above shows a strong curve then the shape of the mass distribution might be interpreted as an unseen spherical or oblate spheroid "halo". But still no exotic "dark matter" needed.

By the way the topic is not about general reasons postulated for the existance of dark matter to explain the greater universe. It's just about the so called "galaxy rotation problem" with "dark matter" as the explanation.

To put a fine point on it. If the sun were made of "dark matter" but massed one solar mass it would not affect the orbits of the planets one iota. Asserting dark matter as a solution to the "galactic orbital problem" and as therefore support for the existance of dark matter is circular reasoning and erroneous.

The unseen matter of the galaxy may be "dark matter". Or it may be ineffective instruments or interpretation. Given that the calculation above is never mentioned in the subject discussions I suspect the latter.

[Chris Peterson]

The unseen matter of the galaxy may be "dark matter". Or it may be ineffective instruments or interpretation. Given that the calculation above is never mentioned in the subject discussions I suspect the latter.

If you are so comfortable with your own calculations, and sure that everyone else has overlooked something obvious to you, then I'd suggest you write it up in a paper and publish it in a peer reviewed journal.

[headscratcher]

The unseen matter of the galaxy may be "dark matter". Or it may be ineffective instruments or interpretation. Given that the calculation above is never mentioned in the subject discussions I suspect the latter.

If you are so comfortable with your own calculations, and sure that everyone else has overlooked something obvious to you, then I'd suggest you write it up in a paper and publish it in a peer reviewed journal.

I would not be so fast to consider you representative of "everyone else" (by the way, that's a rather juvinile phrase considering that you are not "everyone else".). I would welcome anyone else (besides you since you seem unfamiliar with it) to point out where in published journals the mass of the galaxy is calculated from the orbital velocity curve of the stars in the galaxy calculated in consideration of the galaxy as a disperse mass as I point out. Absent that I would welcome a list of a line of stars from near the center of the galaxy to the edge with their orbital periods and distances from center listed.

If this is something that has been overlooked by "everyone else" then I may write it up and try to publish. It may be important since it is on this list

http://en.wikipedia.org/wiki/Unsolved_p ... in_physics

of unsolved problems in Astronomy and astrophysics
.

[bystander]

Mapping the Dark Matter
Discovery Blogs | Cosmic Variance | 13 Nov 2010

Have any friends or colleagues who don’t believe in dark matter? Showing them this should help.

Click to view full size image

That ghostly haze is dark matter — or at least, an impression of the gravitational field created by the dark matter. This is galaxy cluster Abell 1689, in the constellation Virgo. (We feel compelled to add that information, in case you’re going to go looking for it in the night sky tonight or something.) It’s easy to see that the images of many of the galaxies have been noticeably warped by passing through the gravitational field of the cluster, a phenomenon known as strong gravitational lensing. This cluster has been studied for a while using strong lensing. The idea is that the detailed distribution of dark matter affects the specific ways in which different background images are distorted (similar to what was used to analyze the Bullet Cluster). Astronomers use up massive amounts of computer time constructing different models and determining where the dark matter has to be to distort the galaxies in just the right way. Now Dan Coe and collaborators have made an unprecedentedly high-precision map of where the dark matter is (paper here).

This isn’t all about the pretty pictures. We have theoretical predictions about how dark matter should act, and it’s good to compare them to data. Interestingly, the fit to our favorite models is not perfect; this cluster, and a few others like it, are more dense in a central core region than simple theories predict. This is an opportunity to learn something — perhaps clusters started to form earlier in the history of the universe than we thought, or perhaps there’s something new in the physics of dark matter that we have to start taking into account.

http://asterisk.apod.com/vie ... 31&t=21990

But, seeing as you already have it all figured out, I'm sure none of this is of interest to you.

[bystander]

... I would welcome anyone ... to point out where in published journals the mass of the galaxy is calculated from the orbital velocity curve of the stars in the galaxy calculated in consideration of the galaxy as a disperse mass as I point out. Absent that I would welcome a list of a line of stars from near the center of the galaxy to the edge with their orbital periods and distances from center listed. ...

So you want us to do your research for you? I'll just point out that wikipedia (that you are so fond of quoting) is a good source of articles on the subject, if you care to read the footnotes.

I will also point out this is a mainstream forum, it is not the place for you to develop your alternative theory (read the rules). So, unless you have something other than supposition or speculation to present, or perhaps a genuine question to ask, consider this topic closed.

[Chris Peterson]

I would not be so fast to consider you representative of "everyone else" (by the way, that's a rather juvinile phrase considering that you are not "everyone else".)

I'm talking about the "everyone else" who have not published a theory of galactic rotation explained by a simple distribution of ordinary matter.

If you do a little research, you will learn that the problem was initially discovered by doing a simple calculation: the mass distribution of a galaxy was calculated using a standard mass-luminosity relationship, and then the rotation speed was measured very far from the center, where a simple Newtonian calculation was accurate (that is, you could assume that most of the mass was central to the rotating material). Once the problem became apparent, the analysis was turned around. People began constructing mass distribution models that could explain the observed rotations. All these models require far more mass outside the central part of the galaxy than can be observed. It could not be ordinary matter that is simply non-luminous (like gas or dust) because it would show up in parts of the spectrum we can now image.

Galaxy dynamics are now studied with very complex simulations, which perform n-body calculations on millions of points. These galaxy models fully account for the mass distribution of visible matter, and allow researchers to experiment with different distributions of invisible mass. These are not simple Keplerian approximations that assume a single central mass.

There are only two mainstream ideas in place now to explain galactic rotation curves: dark matter halos around galaxies, and modifications to the theory of gravity itself. The latter view is definitely a minority opinion, and doesn't actually eliminate the need for dark matter, which is seen in other areas than just galaxy rotation dynamics.

I would welcome anyone else (besides you since you seem unfamiliar with it) to point out where in published journals the mass of the galaxy is calculated from the orbital velocity curve of the stars in the galaxy calculated in consideration of the galaxy as a disperse mass as I point out.

Since you are apparently interested in a simple Newtonian method of looking at galaxies, let me point out that the mass distribution doesn't matter. You can calculate the entire mass of a galaxy inside a given radius using nothing more than M = v^2 * r / G, since the formula works for any symmetrical mass distribution, not just a central mass. It doesn't matter how much mass is outside the orbiting body. The velocity can be measured out to several times the luminous radius of galaxies, by looking at cool gas. Mass estimates made by looking at orbits are vastly larger than those made by looking at mass-luminosity relationships. This is a powerful argument for unseen mass. But the point is, there is no need to consider the mass distribution. It is basic Newtonian physics that the distribution of the mass in an orbiting system doesn't change the orbital characteristics.

If you simply use Google to look for "galaxy mass distribution" you'll find lots of papers that attempt to calculate mass distributions by looking at orbital velocities at different radii.

[headscratcher]

No answer for a "list of a line of stars from near the center of the galaxy to the edge with their orbital periods and distances from center listed".

Didn't think so.

Ignorant, dishonest longswords.

[rstevenson]

No answer for a "list of a line of stars from near the center of the galaxy to the edge with their orbital periods and distances from center listed".

Didn't think so.

Ignorant, dishonest Gothic minuscules.

There must be a forum for someone with that big a chip on the shoulder. Please seek it out immediately.

Rob

[bystander]

No answer for a "list of a line of stars from near the center of the galaxy to the edge with their orbital periods and distances from center listed".

Can't get anyone to do your homework?

[headscratcher]

I'm going to take one last stab at this.

I'm going to call the "Galactic Rotational Problem" a misnomer. It is miss named. It is and should be called a luminance problem. It is, simply put, based on the assumption that mass in the galaxy should be equaly luminant on a ton by ton basis regardless of whether it is part of the bulge or the disk.

The mass of the galactic bulge is more luminant on a ton by ton measure than an equal mass in the galactic disk. The two primary contender theories to explain this are 1, most widely accepted: "dark matter", which requires least modification of known physical law, and 2: modified Newtonian Dynamics (MOND) which is a which is a widely contentious modification of known physical law.

I would simply point out that the mass of the galactic bulge (the source of the "problem" to begin with) is known to be more densly packed than the mass distributed in the disk. As we know, the more densly packed mass is the more it is going to acrete into stars and undergo fusion.

Bottom line: mass in the bulge is undergoing more fusions on a ton by ton comparison to mass in the sparsly distributed mass of the disk. Therefore densly packed mass in the bulge undergoing more fusion is more luminant than an equal mass that is undergoing less fusion in the less dense disk. End of story.

[Chris Peterson]

Bottom line: mass in the bulge is undergoing more fusions on a ton by ton comparison to mass in the sparsly distributed mass of the disk. Therefore densly packed mass in the bulge undergoing more fusion is more luminant than an equal mass that is undergoing less fusion in the less dense disk. End of story.

There is no theory to support the notion that a given amount of mass in the galactic center is more luminous than the same amount of mass further out in the disc. The only thing that is producing light is stars, and individually, stars behave the same wherever they are, with a luminosity determined by their own mass and evolutionary history. The galactic center has a higher density of stars, and is therefore more luminous in any given volume than the outer area. That is the basis of luminosity-mass models, which are very well accepted.

As noted previously, you could completely throw away the observations of galaxy rotation curves and there are still many completely independent observations that are best explained by dark matter. If you have a problem with dark matter, you need to find better explanations for many other things than galaxy rotation.

[headscratcher]

There is no theory to support the notion that a given amount of mass in the galactic center is more luminous than the same amount of mass further out in the disc.

Not true. (And it's disk not disc. A disc is what you stick in the slot disc drive.)

The only thing that is producing light is stars, and individually, stars behave the same wherever they are, with a luminosity determined by their own mass and evolutionary history.

True as far as it goes. But you overlook the fact that mass doesn't behave the same wherever it is. Someplaces it fusions and some it doesn't.

The galactic center has a higher density of stars, and is therefore more luminous in any given volume than the outer area. That is the basis of luminosity-mass models, which are very well accepted.

Yes, the Tully-Fisher relation. which is purely phenomenological having no basis in natural physical law.

The galactic center has a higher density of stars, and is therefore more luminous in any given volume than the outer area.

Here the problem is terminalogical. The terms are brightness versus luminance.

If you have 100 lightbulbs spread out over a football field compared to 100 compacted into 10 square feet the compact area of bulbs is brighter by area than the football field but equaly luminous. But if only 75 of the 100 bulbs are burning in the football field it will be less bright by area and less luminous in total than the 100 bulbs burning in the concentrated area. You could invoke supernatural "dark matter" to call the non burning bulbs or you can call the non burning bulbs normal matter and simply note that more bulbs are burning in the concentrated area.

[Chris Peterson]

There is no theory to support the notion that a given amount of mass in the galactic center is more luminous than the same amount of mass further out in the disc.

Not true. (And it's disk not disc. A disc is what you stick in the slot disc drive.)

What theory supports your supposition? (Both "disk" and "disc" are equally acceptable; I prefer the latter.)

True as far as it goes. But you overlook the fact that mass doesn't behave the same wherever it is. Someplaces it fusions and some it doesn't.

It always fuses under the right set of conditions- conditions that don't depend on location.

Yes, the Tully-Fisher relation. which is purely phenomenological having no basis in natural physical law.

There are other relations, too. And these relations are observed in models that are entirely physical.

If you have 100 lightbulbs spread out over a football field compared to 100 compacted into 10 square feet the compact area of bulbs is brighter by area than the football field but equaly luminous. But if only 75 of the 100 bulbs are burning in the football field it will be less bright by area and less luminous in total than the 100 bulbs burning in the concentrated area.

Obviously. That's why I didn't refer to just luminosity, but luminosity for a given volume or luminosity for a given mass.

[headscratcher]

There is no theory to support the notion that a given amount of mass in the galactic center is more luminous than the same amount of mass further out in the disc.

Not true.

What theory supports your supposition?

Ignoring your characterization, are you asking because you don't know?

[Chris Peterson]

What theory supports your supposition?

Ignoring your characterization, are you asking because you don't know?

If I knew of a theory, I wouldn't have asked. My understanding is that what you describe is not supported by any accepted theory. You tell me that's not the case, so I'm asking you to describe this theory, and point me towards some reference where I can read about it in detail.

[headscratcher]

Stellar formation, a crucial function of which is density.

[Chris Peterson]

Stellar formation, a crucial function of which is density.

We're not discussing stellar formation. You have suggested that luminous matter in parts of the galaxies with higher star densities is somehow more luminous than matter with the same mass would be in less dense areas. That's what I'm saying isn't supported by any theory I'm aware of. If I'm misunderstanding your point, just let me know. Otherwise, there is no reason to think that the well accepted mass-luminosity models aren't substantially accurate.

[headscratcher]

Stellar formation, a crucial function of which is density.

We're not discussing stellar formation. You have suggested that luminous matter in parts of the galaxies with higher star densities is somehow more luminous than matter with the same mass would be in less dense areas. That's what I'm saying isn't supported by any theory I'm aware of. If I'm misunderstanding your point, just let me know. Otherwise, there is no reason to think that the well accepted mass-luminosity models aren't substantially accurate.

You are misunderstanding my point. You will never understand any answer I give or anything I write if you misread as evidenced by your misquotes.

Your misquote; "luminous matter in parts of the galaxies with higher star densities is somehow more luminous than matter with the same mass would be in less dense areas".

That's not's what I wrote. It's not even a paraphrase of what I wrote. In fact it loses the meaning of what I wrote.
If you will reform your question without the misquote?

I could do it for you but you need to evidence some effort to learn if to warrant my effort to teach you. You should also form your questions as questions rather than disputes if only for clarity.