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Guru
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NASA to Announce Dark Matter on Monday

08/16/2006 8:41 AM

NASA is about to make an exciting announcement, apparently. On Monday 21st August there will be a press conference, where according to a "leak" on Slashdot, they're referring to Dark Matter (DM) in the Bullet Cluster. It's a merging system where a small cluster is passing through a large cluster leaving a shockwave that looks like a bullet's wake, hence the name.

DM is collisionless, i.e., the DM from the smaller system hasn't been slowed down by the collision and just zooms through. The gas is slowed down. So, the DM and gas are no longer in the same place. We can see the gas in an X-ray telescope (Chandra) and detect the DM by the gravitational lensing effect on the background galaxies.

I upheld, this basically disproves the entire category of theories like Modified Newton Dynamics (MOND), which says that DM is an illusion caused by us not understanding the action of gravity at long ranges.

Read the announcement of the press conference here: NASA. We will have to wait and see how close the 'leak' is to the truth!

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The Engineer
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#1

Dark Matter

08/16/2006 9:11 AM

In a previous discussion I thought you had mentioned that the gravitational lensing effects of dark matter had been spotted before. Is there something more in this announcement or is it just more evidence?

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Guru
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#2
In reply to #1

Re:Dark Matter

08/16/2006 9:26 AM

DM has been inferred before form lensing, but never before could such a strong case for it be made (apparently, I must say!) This is because CDM is postulated to be collisionless and this caused a separation of the ordinary matter and CDM. Ordinary matter could not move through the cluster like that and the lensing detects matter where none should be... But let's wait for the press release and (hopefully) better explanations!

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Guru
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#3

Collisionless

08/17/2006 2:55 PM

What does collisionless mean? I know auto insurance companies would love the stuff if it could be painted on the surface of our cars.

But what properties are we talking about? On one hand we have a gravitational lensing caused by dark matter, but it doesn't seem to be stopped when passing through an object.

So is the definition of collisionless more like a neutrino in that it is too small to catch, yet has mass (albeit infinitesimal) and therefore has gravity?

Does this mean that the DM that passed trough the larger nebula is still bound by gravitational physics and would be slowed by the intersection through the large nebula?

In other words, DM still behaves by the laws of gravity and has gravitational attraction and can also be attracted?

Finally, if the definition that DM is collisionless, that puts the kibosh on the theory that at least some of DM is made up of brown dwarfs, white dwarfs, hydrogen, and washed up medical waste.

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Guru
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#4
In reply to #3

Re:Collisionless

08/18/2006 8:31 AM

Collisionless means something that is very weakly interaction with other matter, like the neutrino - but the neutrino is too light to make up the required mass-energy (27% of the total energy, the other 73% being dark energy).

Cold dark matter (CDM) is the model where the dark matter is explained as being cold (i.e. not thermalized), non-baryonic, collisionless dust. This component makes up 22% of the energy density of the present universe.

Some 4% is CDM made up of normal matter and energy that makes up the atoms and photons that are the building blocks of planets, stars, and gas clouds in the universe, but is dark (non-radiating). This includes black holes and brown dwarfs.

So, only 1% of the total energy of the observable universe is visible (radiating) stuff. (It is believed that the BB could not have made more than 5% of the crtical energy as baryons).

And yes, all CDM interacts gravitationally, so they do get accelerated/slowed down by gravity. This is why their effects can be detected. See this Wiki Article for a reasonably detailed discusssion. (They talk about 74% dark energy, but this varies from paper to paper.)

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Guru
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#5
In reply to #4

Re:Collisionless

08/18/2006 9:08 AM

Thanks.

So, how can collisionless matter act as a gravitational lens yet not be slowed down by the intersection of the nebulas like the nebula itself?

I am assuming that the interaction of the two nebulas is a result of gravitation and not particles slamming into each other physically.

Maybe the nebula is much more gravitationally massive and the CDM less so and what we see is the difference in the effect of each. So the nebula will act as a much stronger gravitational lens, but you get a secondary lensing effect in front of the nebula that is weaker, but distinct enough to be detected.

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Guru
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#6
In reply to #5

Re:Collisionless

08/18/2006 10:04 AM

I think the ordinary matter is slowed by physical collisions with gas and dust! The CDM goes right through the lot and is also apparently much more massive than the gas and stuff - its the main gravitational lensing agent!

Gravity, however, works equally well on CDM and ordinary stuff, so the CDM is also slowed down by the gravity of the intervening cluster.

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