A dark matter disk in our Galaxy

September 16, 2008

(PhysOrg.com) -- An international team of scientists predict that our Galaxy, the Milky Way, contains a disk of ‘dark matter’. In a paper published in Monthly Notices of the Royal Astronomical Society, astronomers Dr Justin Read, Professor George Lake and Oscar Agertz of the University of Zurich, and Dr Victor Debattista of the University of Central Lancashire use the results of a supercomputer simulation to deduce the presence of this disk. They explain how it could allow physicists to directly detect and identify the nature of dark matter for the first time.

Unlike the familiar ‘normal’ matter that makes up stars, gas and dust, ‘dark’ matter is invisible but its presence can be inferred through its gravitational influence on its surroundings. Physicists believe that it makes up 22% of the mass of the Universe (compared with the 4% of normal matter and 74% comprising the mysterious ‘dark energy’). But, despite its pervasive influence, no-one is sure what dark matter consists of.

Prior to this work, it was thought that dark matter forms in roughly spherical lumps called ‘halos’, one of which envelopes the Milky Way. But this ‘standard’ theory is based on supercomputer simulations that model the gravitational influence of the dark matter alone. The new work includes the gravitational influence of the stars and gas that also make up our Galaxy.

Stars and gas are thought to have settled into disks very early on in the life of the Universe and this affected how smaller dark matter halos formed. The team’s results suggest that most lumps of dark matter in our locality merged to form a halo around the Milky Way. But the largest lumps were preferentially dragged towards the galactic disk and were then torn apart, creating a disk of dark matter within the Galaxy.

“The dark disk only has about half of the density of the dark matter halo, which is why no one has spotted it before,” said lead author Justin Read. “However, despite its low density, if the disk exists it has dramatic implications for the detection of dark matter here on Earth.”

The Earth and Sun move at some 220 kilometres per second along a nearly circular orbit about the centre of our Galaxy. Since the dark matter halo does not rotate, from an Earth-based perspective it feels as if we have a ‘wind’ of dark matter flowing towards us at great speed. By contrast, the ‘wind’ from the dark disk is much slower than from the halo because the disk co-rotates with the Earth.

“It's like sitting in your car on the highway moving at a hundred kilometres an hour”, said team member Dr Victor Debattista. “It feels like all of the other cars are stationary because they are moving at the same speed.”

This abundance of low-speed dark matter particles could be a real boon for researchers because they are more likely to excite a response in dark matter detectors than fast-moving particles. “Current detectors cannot distinguish these slow moving particles from other background ‘noise’,” said Prof. Laura Baudis, a collaborator at the University of Zurich and one of the lead investigators for the XENON direct detection experiment, which is located at the Gran Sasso Underground Laboratory in Italy. “But the XENON100 detector that we are turning on right now is much more sensitive. For many popular dark matter particle candidates, it will be able to see something if it’s there.”

This new research raises the exciting prospect that the dark disk – and dark matter – could be directly detected in the very near future.

Citation: Monthly Notices of the Royal Astronomical Society paper

Provided by Royal Astronomical Society

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3 / 5 (2) Sep 16, 2008
This is counter intuitive. Am I correct in thinking that they suggest that the dark matter is an envelope (or bubble) surrounding the milky way disk. Why would it not be pulled into the much denser disk of normal matter? What opposing forces hold it out there. It is symmetric about the plane of the disk, so its net gravitational effect on the disk is cancelled out. Wouldn't the halo of dark matter be accompanied by a halo of visible matter?
Sep 16, 2008
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not rated yet Sep 16, 2008
How would this disc of dark matter affect the supposed density of the interstellar medium in our general galactic vicinity? Say within 25-50 light years?
1 / 5 (1) Sep 16, 2008
To assume we know "anything" is a reach. To assume we know "everything" is rediculous. At our learning rate in another 1,000 years we should know "something". Another decade would then be required to begin "understanding"!
4 / 5 (1) Sep 16, 2008
This is counter intuitive. Am I correct in thinking that they suggest that the dark matter is an envelope (or bubble) surrounding the milky way disk. Why would it not be pulled into the much denser disk of normal matter? What opposing forces hold it out there. It is symmetric about the plane of the disk, so its net gravitational effect on the disk is cancelled out. Wouldn't the halo of dark matter be accompanied by a halo of visible matter?

I'm not a million percent sure, but It my understanding that other normal matter forms rotating planar disk like orbit because its most stable. Collision between all the matter cause its even out into ecliptic plane. Having stuff orbiting from millions of different angles causes crashes and collisions until only the most stable position remains. These collisions in normal matter are mediated through Electomagnetism, Nuclear Forces, Gravity etc

Dark Matter supposedly only interacts strongly through the gravitational force, so its slips and slides through normal matter without colliding and being forced into an orbit along the ecliptic. I don't believe its supposed to even collide with itself other than the interaction through gravity....

Anyway, thats my crappy understanding of it.
not rated yet Sep 19, 2008
So,...(I do mention dark matter in this ramble so, patience) lemme go ahead and ask this, the risk of ridicule that asking it poses aside; if we, and our universe, were a simulation on some advanced being's technology platform, IF,....we were, what would it look like to us? I mean; imagine for a second, sentient and conscious beings in a simulation on a screen or in a virtual/generated world. To them, would it look as though time began in an instant out of nowhere (uh, Big Bang?),...and would the matrix or scaffold on which/in which these simulated being existed be invisible to them (uh, Dark matter?), ...and wouldn't the force that moved elements around in this simulated universe be necessarily hard for the beings in it to comprehend or detect (uh, Dark Energy),...items/matter being "deleted" or leaving the sim go down a,...say it with me now; "Black Hole"....perhaps. ..and, in fact, when these beings did the math that they could about their "known' universe, it did not ad up. It should not be speeding up, there should be more matter,...there should be an explanation for the odd quantum observations but there aren't. oh,...and, gravity? If we were looking at a simulated universe, gravity would be pretty frickin' easy to explain, wouldn't it?

The possibly now contemplated by theories such as string and brane that there are other dimensions,...uh,...like maybe, REAL reality versus ours? Remember Michio Kaku's story of the Carp in the pond when he was a boy... to them, the carp, below the waters surface IS the universe, and above it an impossible to understand dimension (at least impossible for the Carp physicists to understand)

There is no real reason that the theory of everything Has To Be really difficult for humans to grasp or discover. There is nothing that obligates the universe to be difficult for us to comprehend,....unless.....we were a simulation. Then it would be very important for it to be difficult, wouldn't it?

Now, before everyone accuses me of being a nut who has played to much Spore or SIMMS,...I have never played either. Just wondering about this. Why does all of this difficult to understand stuff seem so easy to understand, if we cogitate it in the context of a simulated universe.

Like a local pastor says here; not a sermon, just a thought (or two)

not rated yet Sep 23, 2008
I don't buy into the simulation argument. The simple reason is that in designing a simulation you want to do impossible things because its fun and generally they are hackable.
What really blows my mind is how ordered the Universe is. How it adheres to laws of mathematics and generally everything makes perfect predictable sense. (Ok - lets set aside Schrodingers Cat for a minute). The other thing is that computer simulations for us cannot produce truly random events. Whereas our universe is full of perfectly random events. Of course there might be a purely mathematical way of synthesizing random numbers, but we don't have it yet. All our random number generators rely on physical observations or physical randomness.

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