Perspective from the spaceship Earth

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ErnieM
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Perspective from the spaceship Earth

Post by ErnieM » Sun Nov 20, 2011 3:24 pm

The large potion of our view of the whole universe along the plane of the Milky Way is obstructed by the huge bulge and the dust in the galactic arms. Hence our observation is only partial and limited towards the directions vertical to the galactic plane that are free of any obstructions. If there is no dark matter in these areas, does this mean dark matter only "lives" along the galactic plane? If present, then light must be passing through dark matter. Is it not possible to see through the dust lanes using radio, x-ray and infrared telescopes? How about that part of the universe behind the bulge of the Milky Way? Is our Solar system permanently positioned in the same spot or does it periodically straddle in and out of the spur? If it does, is our perspective also changed in the same way when we switch between lanes on the freeway?

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Chris Peterson
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Re: Perspective from the spaceship Earth

Post by Chris Peterson » Sun Nov 20, 2011 3:47 pm

ErnieM wrote:The large potion of our view of the whole universe along the plane of the Milky Way is obstructed by the huge bulge and the dust in the galactic arms.
Actually, this isn't the "large portion", but only a fairly small one. Just go outside when the Milky Way is high, and you'll see that it only "occludes" a relatively small percentage of the entire sky.
Hence our observation is only partial and limited towards the directions vertical to the galactic plane that are free of any obstructions. If there is no dark matter in these areas, does this mean dark matter only "lives" along the galactic plane?
No... quite the opposite. Because dark matter doesn't interact with electromagnetic radiation, it is not subject to the same momentum transfer mechanisms of ordinary baryonic matter, and therefore doesn't end up spinning in discs. Instead, it clusters in (approximately) spherical halos. So our galaxy is surrounded by dark matter in all directions, and we should see about the same density no matter what direction we look (which is borne out by observations of its gravitational effects on our galaxy and on other galaxies).
If present, then light must be passing through dark matter.
Yes, but it doesn't affect the passage of the light. That's why it is called dark matter.
Is it not possible to see through the dust lanes using radio, x-ray and infrared telescopes? How about that part of the universe behind the bulge of the Milky Way? Is our Solar system permanently positioned in the same spot or does it periodically straddle in and out of the spur? If it does, is our perspective also changed in the same way when we switch between lanes on the freeway?
We can and do see through the dustier parts of our own galaxy using other wavelengths. Because we are orbiting inside the Milky Way, our viewpoint of the rest of the Universe changes... but only on a timescale of millions of years. Our position with respect to the galactic plane doesn't change enough with time to have much impact on our view, however.
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Re: Perspective from the spaceship Earth

Post by Guest » Mon Nov 21, 2011 1:21 am

Chris wrote:
No... quite the opposite. Because dark matter doesn't interact with electromagnetic radiation, it is not subject to the same momentum transfer mechanisms of ordinary baryonic matter, and therefore doesn't end up spinning in discs. Instead, it clusters in (approximately) spherical halos. So our galaxy is surrounded by dark matter in all directions, and we should see about the same density no matter what direction we look (which is borne out by observations of its gravitational effects on our galaxy and on other galaxies).
If no instruments see the dark matter, how can anyone ascertain its shape and structure? A ring surrounding the galaxy around its plane or a massive bubble enclosing the galaxy can be just as valid to explain its gravitational effects of the unaccounted galaxy mass. It is the mass not the shape and structure that determines the amount of gravitational force, is it not?

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Ann
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Re: Perspective from the spaceship Earth

Post by Ann » Mon Nov 21, 2011 2:40 am

Image
This image shows two colliding clusters of galaxies. The collision separates the ordinary matter from the dark matter. The ordinary matter is seen as red here, and it can easily be detected because of the emission and energy that it gives off. The dark matter, which is seen as blue in this picture, can't be detected by any other means that gravitational lensing. The way background galaxies are lensed, bent and magnified tells astronomers about the amount and distribution of the mass that is acting as a lens. Because of the lensing that is seen of background galaxies, astronomers have concluded that there must be a significant amount of dark matter in the blue areas in this image, even though there is little ordinary matter here.

http://www.msnbc.msn.com/id/14453775/ wrote:
Here's what the image reveals:

The hot gas — normal matter — was slowed by a drag force described as the cosmic equivalent of air resistance. But the dark matter was not slowed by this effect, presumably because it does not interact with normal matter, as theory had predicted.

So the normal matter and dark matter became separated.

"This proves in a simple and direct way that dark matter exists." Markevitch said during the teleconference.
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Chris Peterson
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Re: Perspective from the spaceship Earth

Post by Chris Peterson » Mon Nov 21, 2011 6:00 am

Guest wrote:If no instruments see the dark matter, how can anyone ascertain its shape and structure? A ring surrounding the galaxy around its plane or a massive bubble enclosing the galaxy can be just as valid to explain its gravitational effects of the unaccounted galaxy mass. It is the mass not the shape and structure that determines the amount of gravitational force, is it not?
Where do you get the idea that no instruments see dark matter? I think people are so centered on vision that they lose faith in other ways of seeing things. Do you believe in electrons? Because nobody has ever seen one in the light of EM. Likewise for neutrinos, and many other particles. Most of what we see, we see indirectly, by how other things are influenced.

A ring or disc of dark matter around a galaxy does not work to explain the observed rotation curves. A spherical halo does. It is not just the mass, but the mass distribution (shape) that produces the motions we observe- in galaxy rotation, in the motion of galaxies in clusters, and in the nature of gravitational lensing.
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ErnieM
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Re: Perspective from the spaceship Earth

Post by ErnieM » Mon Nov 21, 2011 3:04 pm

Ann wrote:
Here's what the image reveals:

The hot gas — normal matter — was slowed by a drag force described as the cosmic equivalent of air resistance. But the dark matter was not slowed by this effect, presumably because it does not interact with normal matter, as theory had predicted.

So the normal matter and dark matter became separated.
If the dark matter has separated, this must have greatly altered the motion of galaxies in the clusters. While separated, are the changes in motion of galaxies and their component stars evident? Is this separation of dark matter temporary or permanent? If permanent as space is expanding, there must island universe of dark matter floating around nearby merged galaxy clusters. Since it is claimed that dark matter does not behave the same way as ordinary matter, (ie. dark matter does not have rotational velocity?) how would (what is the process?) the two come together again?