How much Dark Matter is needed and where is it located?

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dougettinger
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How much Dark Matter is needed and where is it located?

Post by dougettinger » Fri Apr 09, 2010 4:09 pm

For a typical spiral galaxy such as our Milky Way how much more mass other than observed stars and dust is needed to explain its orbital mechanics? Where is this mysterious mass located by theorists to explain the galaxy's observed orbital characteristics? At the center? Evenly distributed across the the plane of the disk?
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Re: How much Dark Matter is needed and where is it located?

Post by Chris Peterson » Fri Apr 09, 2010 4:24 pm

dougettinger wrote:For a typical spiral galaxy such as our Milky Way how much more mass other than observed stars and dust is needed to explain its orbital mechanics? Where is this mysterious mass located by theorists to explain the galaxy's observed orbital characteristics? At the center? Evenly distributed across the the plane of the disk?
The hidden mass typically constitutes two to three times the luminous mass, and appears to be distributed spherically around the disc of the galaxy.
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Re: How much Dark Matter is needed and where is it located?

Post by dougettinger » Fri Apr 09, 2010 6:18 pm

Just to clarify, the hidden mass you speak of is what is called Dark Matter? There is also the non-luminous mass of dust, gas, planets, black holes, brown dwarfs, etc. that is not part of Dark Matter.

If this Dark Matter is supposely distributed spherically, I am very puzzled. I thought Dark Matter was needed to create the gravity field required to produce the observed velocities of the stars traveling around a spiral galaxy. (?) Is the spherical distribution needed to explain the halo of globular clusters?
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Re: How much Dark Matter is needed and where is it located?

Post by BMAONE23 » Fri Apr 09, 2010 9:41 pm

Chris Peterson wrote:
dougettinger wrote:For a typical spiral galaxy such as our Milky Way how much more mass other than observed stars and dust is needed to explain its orbital mechanics? Where is this mysterious mass located by theorists to explain the galaxy's observed orbital characteristics? At the center? Evenly distributed across the the plane of the disk?
The hidden mass typically constitutes two to three times the luminous mass, and appears to be distributed spherically around the disc of the galaxy.
You are talking about the Dark Matter Halo's surrounding galaxies. I have a difficult time understanding the mechanism that causes external material to exert an internal force, e.g. external dark matter halo causing increased internal gravity influence.

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Re: How much Dark Matter is needed and where is it located?

Post by dougettinger » Mon Apr 12, 2010 4:34 pm

Yes, Bmaone23, I have difficulty understanding Chris' explanation, although I am not disputing it. I hope more Dark Matter specialists are in the audience that will speak up to defend Newton and Einstein concerning the characteristics of individual galaxies.

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Re: How much Dark Matter is needed and where is it located?

Post by Chris Peterson » Mon Apr 12, 2010 5:33 pm

dougettinger wrote:Just to clarify, the hidden mass you speak of is what is called Dark Matter? There is also the non-luminous mass of dust, gas, planets, black holes, brown dwarfs, etc. that is not part of Dark Matter.
Yes, I'm talking about dark matter. There is very little non-luminous matter other than DM. "Luminous" means it produces electromagnetic radiation, but not necessarily in the visible spectrum. In fact, all baryonic matter is luminous, and most of it luminous enough for us to detect (at least theoretically).
If this Dark Matter is supposely distributed spherically, I am very puzzled. I thought Dark Matter was needed to create the gravity field required to produce the observed velocities of the stars traveling around a spiral galaxy. (?) Is the spherical distribution needed to explain the halo of globular clusters?
The apparent spherical distribution of dark matter does explain the nature of the rotation curve of spiral galaxies. I don't think it is a key component in explaining the distribution of globular clusters.
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Re: How much Dark Matter is needed and where is it located?

Post by dougettinger » Mon Apr 12, 2010 11:35 pm

Chris Peterson wrote:
dougettinger wrote:Just to clarify, the hidden mass you speak of is what is called Dark Matter? There is also the non-luminous mass of dust, gas, planets, black holes, brown dwarfs, etc. that is not part of Dark Matter.
Yes, I'm talking about dark matter. There is very little non-luminous matter other than DM. "Luminous" means it produces electromagnetic radiation, but not necessarily in the visible spectrum. In fact, all baryonic matter is luminous, and most of it luminous enough for us to detect (at least theoretically).
I did not know that the word "luminous" had that distinction. So astronomers believe that they can detect most of the galaxial material via electromagnetic radiation.
If this Dark Matter is supposely distributed spherically, I am very puzzled. I thought Dark Matter was needed to create the gravity field required to produce the observed velocities of the stars traveling around a spiral galaxy. (?) Is the spherical distribution needed to explain the halo of globular clusters?
The apparent spherical distribution of dark matter does explain the nature of the rotation curve of spiral galaxies. I don't think it is a key component in explaining the distribution of globular clusters.
Is the rotation curve the observed spirals? And does this dark matter distribution explain the puzzling velocity distribution of higher outer velocities?

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Re: How much Dark Matter is needed and where is it located?

Post by Chris Peterson » Tue Apr 13, 2010 1:06 am

dougettinger wrote:So astronomers believe that they can detect most of the galaxial material via electromagnetic radiation.
I believe that's the case. Early after dark matter was hypothesized, people looked at the possibility that it was simply ordinary matter too cool to detect. But that idea has been largely discarded based on what we are actually able to see with instruments. It does seem likely that only a relatively small percentage of ordinary matter is not being seen.
Is the rotation curve the observed spirals? And does this dark matter distribution explain the puzzling velocity distribution of higher outer velocities?
The rotation curve of spirals is superficially anomalous because the outer velocities are too high. This can be explained by having a large mass distributed around a galaxy. While there are presumably an infinite number of possible mass distributions, it happens that a spherical distribution works to describe the actual rotation curve. This works well with the theories of galaxy formation, and it is consistent with the observed apparent spherical distribution of dark matter around galaxy clusters.
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Re: How much Dark Matter is needed and where is it located?

Post by dougettinger » Wed Apr 14, 2010 3:47 pm

Chris Peterson wrote:
dougettinger wrote:So astronomers believe that they can detect most of the galaxial material via electromagnetic radiation.
I believe that's the case. Early after dark matter was hypothesized, people looked at the possibility that it was simply ordinary matter too cool to detect. But that idea has been largely discarded based on what we are actually able to see with instruments. It does seem likely that only a relatively small percentage of ordinary matter is not being seen.
Is the rotation curve the observed spirals? And does this dark matter distribution explain the puzzling velocity distribution of higher outer velocities?
The rotation curve of spirals is superficially anomalous because the outer velocities are too high. This can be explained by having a large mass distributed around a galaxy. While there are presumably an infinite number of possible mass distributions, it happens that a spherical distribution works to describe the actual rotation curve. This works well with the theories of galaxy formation, and it is consistent with the observed apparent spherical distribution of dark matter around galaxy clusters.
I am puzzled by your explanations:

1) Why are the spirals superficially anomalous? Because they should not exist when using normal gravity equations? or because of the unexpected velocity distribution?
2) In still am not sure in which way a galaxy rotates ? In the direction of the spirals ? or opposite the direction of the spirals ?
3) You mentioned that dark matter has an observed apparent distribution around galaxy clusters. I thought dark matter could not be observed ? only postulated ? Does this mean scientists need dark matter to explain galaxy clusters, too?

Current explanations in mainstream science for dark matter are not well constructed. I am definitely looking forward to having some of my questions answered. Thanks.

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Re: How much Dark Matter is needed and where is it located?

Post by Chris Peterson » Wed Apr 14, 2010 4:29 pm

dougettinger wrote:1) Why are the spirals superficially anomalous? Because they should not exist when using normal gravity equations? or because of the unexpected velocity distribution?
The unexpected velocity profile is the anomaly. It is superficial because it can be readily explained by unseen mass. There is no problem with the underlying theories of gravity- they are used, unmodified, to explain what we see.
2) In still am not sure in which way a galaxy rotates ? In the direction of the spirals ? or opposite the direction of the spirals?
It rotates the way you would expect, with the spirals. Keep in mind that the galaxy rotates faster than the spirals, however. That is, stars in their orbits approach spirals from behind, pass through them, and continue on to the next spiral. You can talk about a galaxy rotating, and define a direction, but the galaxy itself doesn't rotate at a fixed speed. As you'd expect, it rotates faster near its center than it does at the periphery. You can imagine it sort of winding itself up- indeed, until the dynamics of spiral arms were better understood, it was an open question why the spirals didn't get wound up over time.
3) You mentioned that dark matter has an observed apparent distribution around galaxy clusters. I thought dark matter could not be observed ? only postulated ? Does this mean scientists need dark matter to explain galaxy clusters, too?
Of course dark matter is observed. Otherwise, we wouldn't be developing theories around it. How is observing in the "light" of gravity different than observing in the light of EM? Many things in the Universe are observed indirectly.

The structure of galaxy clusters is consistent with a large amount of dark matter around them. Observations of the movement of galaxies in clusters, as well as the nature of gravitational lensing caused by galaxies and galaxy clusters require dark matter (or something that behaves like dark matter) in order to be explained.
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Re: How much Dark Matter is needed and where is it located?

Post by dougettinger » Wed Apr 14, 2010 4:50 pm

I am getting closer to the truth. Gravity explains the distribution of "luminous" mass in our solar system and other star systems. Gravity can explain the distribution of "luminous" masss in galaxies and in galaxy clusters by adding dark matter to the "luminous" mass in these systems. But in the scheme of particle theory scientists do not know what dark matter is. Did I summarize these statements about dark matter correctly ?

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Re: How much Dark Matter is needed and where is it located?

Post by Chris Peterson » Wed Apr 14, 2010 4:58 pm

dougettinger wrote:I am getting closer to the truth. Gravity explains the distribution of "luminous" mass in our solar system and other star systems. Gravity can explain the distribution of "luminous" masss in galaxies and in galaxy clusters by adding dark matter to the "luminous" mass in these systems. But in the scheme of particle theory scientists do not know what dark matter is. Did I summarize these statements about dark matter correctly ?
Yes. And not just the distribution, but the motion as well. There are extensions to the standard model of particle physics that include candidate dark matter, so this is really an area where the theories are still being developed. Fortunately, these ideas are generally testable, which means it's likely that knowledge will advance at a reasonable pace.
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Re: How much Dark Matter is needed and where is it located?

Post by dougettinger » Wed Apr 14, 2010 5:29 pm

Chris, thanks for your patience. I now know where dark matter is located in the minds of the theorists.
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