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Old 04-18-2012, 08:16 PM
 
Location: Texas
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The most accurate study so far of the motions of stars in the Milky Way has found no evidence for dark matter in a large volume around the Sun. According to widely accepted theories, the solar neighbourhood was expected to be filled with dark matter, a mysterious invisible substance that can only be detected indirectly by the gravitational force it exerts. But a new study by a team of astronomers in Chile has found that these theories just do not fit the observational facts. This may mean that attempts to directly detect dark matter particles on Earth are unlikely to be successful.
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Old 04-18-2012, 08:54 PM
 
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It goes to show there are a lot of things about the universe we don't know about. Maybe Dark Matter doesn't occupy the interior of galaxies around stars, but rather reside just outside of galaxies and larger structures such as filaments?
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Old 04-19-2012, 12:34 PM
 
Location: Texas
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Maybe the filaments are defining the boundaries of clumps of Dark Matter?
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Old 04-19-2012, 04:23 PM
 
Location: Brooklyn, New York
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The thread title is misleading, since they are talking about no dark matter around the Sun, not in the Milky Way galaxy as a whole.
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Old 04-19-2012, 09:36 PM
 
Location: Texas
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Yeah, so maybe there is something going on.
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Old 04-20-2012, 02:48 AM
 
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Quote:
Originally Posted by Gantz View Post
The thread title is misleading, since they are talking about no dark matter around the Sun, not in the Milky Way galaxy as a whole.
The article wasn't talking about "no dark matter around the Sun" either. It was about mapping the motion of more than 400 red giant stars located up to 13,000 light-years in the region around the Sun, especially those located at differing heights above the plane of the Milky Way and in the direction toward the south galactic pole. The purpose of mapping such motion was to determine and confirm how much dark matter is involved with the motion of the stars in the study.

It's not that there isn't any dark matter in or around the region of the galaxy studied, but rather that there's a lot less of it than had been expected. The article stated "Theories predict that the average amount of dark matter in the Sun’s part of the galaxy should be in the range 0.4-1.0 kilograms of dark matter in a volume the size of the Earth. The new measurements find 0.00±0.07 kilograms of dark matter in a volume the size of the Earth."

It also states, "The new results also mean that attempts to detect dark matter on Earth by trying to spot the rare interactions between dark matter particles and “normal” matter are unlikely to be successful."

It goes on to say, "Despite the new results, the Milky Way certainly rotates much faster than the visible matter alone can account for. So, if dark matter is not present where we expected it, a new solution for the missing mass problem must be found. Our results contradict the currently accepted models. The mystery of dark matter has just become even more mysterious. Future surveys, such as the ESA Gaia mission, will be crucial to move beyond this point." (Bold type added for emphasis)

The title of the article itself asks the question, "Serious Blow to Dark Matter?" The results of the mapping showed that current models and theories about dark matter are not as had been expected for the region (the stellar neighborhood which includes the Sun) that was studied. Although the study obviously doesn't cover the entire galaxy, it may be applicable to our better understanding the galaxy.

While the galaxy is thought to be surrounded by a halo of dark matter, we don't really know the shape of it. The study suggests that if the halo is highly elongated, that could explain the lack of dark matter. The problem with dark matter is that it can't be directly seen and has never been detected in labs on Earth. It is compellingly inferred to exist by gravitational effects. It could be that dark matter is more abundant in other similar regions around the galaxy than it is in the region of the study. More study and better equipment will be needed to continue beyond the finding.

Is the lack of dark matter typical in other regions around the galaxy, or is it unique to the region studied? What would it mean if other regions around the galaxy show a similar lack of dark matter? The finding raises a number of unexpected questions. It's findings like this that provide plenty of surprises and make such studies interesting.
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Old 04-20-2012, 12:58 PM
 
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Quote:
Originally Posted by Brian.Pearson View Post
Maybe the filaments are defining the boundaries of clumps of Dark Matter?
I think it may be the other way around. I would think the filaments are defined by Dark Matter. The filaments we see today are greatly stretched an elongated because of the continued expansion of the universe, but Dark Matter may have played a role in holding them together because of gravitational attraction. In the more distant future, the filaments could become so greatly stretched and thinned that they 'break' apart and dissipate, leaving Dark Matter to occupy voids. That doesn't resolve the question as to why there's virtually no Dark Matter in our region of the galaxy though. I think I'd still stand (at the moment) with the idea that Dark Matter might be rather clumpy and uneven in its distribution. Think in terms of the clumpiness in the CMB.

Here's another article on the subject. It'll be interesting to see what future studies will come up with, especially a survey involving millions of stars.
If Not Dark Matter, then What? | Dark Matter Alternatives | Cosmology | Space.com


Below is an animation showing the idea of Dark Matter surrounding the galaxy.


Dark Matter Missing From Milky Way Galaxy | Video - YouTube
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Old 04-20-2012, 06:55 PM
 
Location: Texas
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There might be something going on with Andromeda and the Milky Way, since the two galaxies have been crossing paths. Wonder if they've taken that into account.
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Old 04-21-2012, 12:40 AM
 
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Originally Posted by Brian.Pearson View Post
There might be something going on with Andromeda and the Milky Way, since the two galaxies have been crossing paths. Wonder if they've taken that into account.
How would that explain the lack of Dark Matter?
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Old 04-21-2012, 12:35 PM
 
Location: Texas
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Originally Posted by NightBazaar View Post
How would that explain the lack of Dark Matter?
I just hypothesized that some could've shifted from one galaxy to another during the last collision. Same thing could've happened the previous time.
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