Team Maps Dark Matter in Startling Detail

Johns Hopkins University ^ | 09 December 2005 | Staff

[PatrickHenry]

The universe is always newsworthy.

[PatrickHenry]

SciencePing

An elite subset of the Evolution list. See the list's explanation at my freeper homepage. Then FReepmail to be added or dropped.

[A.Hun]

Have they decided on the fate of Hubble yet?

[Doctor Stochastic]

...Advances in computer technology now allow us to simulate the entire universe...

In a coarsely gridded sort of way.

[PatrickHenry]

Don't be such a spoilsport.

[adorno]

Invisible, yet undoubtedly there — scientists can measure its effects — its exact characteristics remain elusive.

Unlike normal matter particles, physicists believe, they do not collide and scatter like billiard balls but rather simply pass through each other.

Collisonless? Invisible? Apparently do not interact with other matter, including dark matter? Yet, dark matter shapes most of the universe?

I'll bet that changes in these theories will be forthcoming in the near or slightly more distant future. Tough I probably won't be around to read about it.

[RightWhale]

Griffin wants to send up the Shuttle for Hubble repairs, but the Shuttle schedule is rapidly evaporating. It may become impossible.

[PatrickHenry]

Collisonless? Invisible? Apparently do not interact with other matter, including dark matter? Yet, dark matter shapes most of the universe?

I post; you decide.

[VadeRetro]

The universe is always newsworthy.

Another story about the same dumb ol' universe. Doesn't anyone ever broaden out a little?

[PatrickHenry]

The universe -- love it or leave it!

[VadeRetro]

Doesn't anyone ever just once want to get out and take a look around outside?

[adorno]

I post; you decide.

Hey, I wasn't knocking you or the post.

I was just skeptical about some of the theories in the article. Nothing more.

[longshadow]

Apparently do not interact with other matter, including dark matter? Yet, dark matter shapes most of the universe?

Of course they "interact with other matter" -- that's the point of the article: the gravitational interaction of dark matter with other matter (including dark matter) in the universe. The simulation of that predicted gravitational interaction produces a distribution of matter that matches what we see.

[adorno]

Of course they "interact with other matter" -- that's the point of the article: the gravitational interaction of dark matter with other matter (including dark matter) in the universe. The simulation of that predicted gravitational interaction produces a distribution of matter that matches what we see.

Ok, so dark matter has gravitational properties. Yet, whatever the dark matter particles look like, they are invisible and collisionless. It is the collisionless part that would seem to indicate that dark matter is mass-less. If dark matter is mass-less and collisionless, how can it also contain the property of gravity?

[longshadow]

It is the collisionless part that would seem to indicate that dark matter is mass-less.

I'm not aware of any theoretical requirement for dark matter to be massless in order to be "collisionless," but since I am neither a particle physicist nor play one on TV, I'm pinging someone who is, in case he can shed more light on the subject.

[LLoyd George]

"it is very challenging to verify the simulation results observationally"

Really? (Not surprisingly!)

[unlearner]

"Dark matter [is] invisible, yet undoubtedly there"

You can count me as a doubter. Dark matter is a contrivance to fill the gaps in otherwise sensible theories.

I find it much easier to believe the fine structure constant and light velocity have changed over the history of the universe.

To me this is a more rational explanation of measurements that seem to indicate an accelerating universe and gravity with no apparent matter.

[VadeRetro]

If dark matter is mass-less and collisionless, how can it also contain the property of gravity?

While we're waiting for the pros to turn up, another amateur opinion.

Dark matter has to have some kind of effective mass or it wouldn't be invoked to explain what it explains. It just doesn't have a gas pressure. Gas pressure (the effect of particles colliding) works against clumping. If particles that had gas pressure were all there is, space would look a certain way.

Having some of the mass in a space unable to bump into other particles, even its own kind of particles, helps stuff (including the stuff that does have collisions) clump up gravitationally better. Space looks a little different.

Which is why dark matter was hypothesized in the first place. The gravity wells of galaxies and eventually other spaces were deeper than we could explain without something invisible and very different from normal matter made of protons and neutrons.

Meanwhile, most of the mass-energy of the universe is something called "dark energy." That's a different can of worms yet.

[libsrscum]

Anyone know if the property of "collisionless" has been postulated or observed for anything other than dark matter? I guess this is the first I've heard of the term.

[Physicist]

Non-interacting particles aren't required to be massless. They are required to be chargeless. A gas of (massive) Z bosons will be largely non-interacting, because they don't exchange photons, gluons, Z's, or W's, but they will exchange Higgs bosons. A gas of (neutral, massive) Higgs bosons will likely also interact with difficulty, because they only will exchange Z's. (The strength of both interactions will depend on the Z-Higgs coupling, which IIRC is a free parameter of the Standard Model).

Unfortunately, Z's and Higgs bosons are extremely unstable, so collecting a gas of same is a tall order.

Neutrinos are stable and are practically non-interacting, but they are also practically massless.

In practice, all the other particles we have measured carry some other kind of charge, be it electromagnetic, weak or strong, but nothing in principle prevents there from being another type (or even class) of particles that don't interact except gravitationally. The universe could be brimming with them, but there's no way to detect them in the laboratory, just because of the fact that they carry no Standard Model charges. The only way to detect them would be through gravitational means.

[Geek alert: If there are extra dimensions, the possibility exists that a high-energy electron-positron collider could produce massive Kaluza-Klein "tower" graviton states. These in turn would couple to the gravitation-only dark matter states. You wouldn't see those dark-matter-producing collisions directly, as the final-state dark matter particles would escape, undetected as usual, but you could see a faint impression of them through Bremsstrahlung radiation from the initial-state electron and positron. This would manifest itself as events with a single, hard gamma ray, the distribution of which would be peaked in the forward direction (along the beam axis).]