Team Maps Dark Matter in Startling Detail

Johns Hopkins University ^ | 09 December 2005 | Staff

[libsrscum]

Well, that clears that up!

(I mean that affectionately!) Thank you for the information!

[VadeRetro]

Photons are collisionless, at least with respect to each other.

[longshadow]

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.

Neutrinos have an extremely low rate of interaction with matter; see reply #20.

[longshadow]

thanks for filling in the details.

I sure hope there isn't a quiz on this next period.

;-)

[gcruse]

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

I understand.  It's the contrivance of a supernatural, also invisible, man in the sky to fill in the gaps of otherwise sensible theories that made an atheist out of me. However, there is much more evidence for dark matter than deity.

[stevio]

Not wanting to start anything but, why do you say what you do in your tag line? Do you think this is all some sort of grand accident?

[Alamo-Girl]

Thanks for the ping!

[ImaGraftedBranch]

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.

I think we tend to get in trouble when we use one hypothetical particle to explain the existence of another hypothetical particle. I agree that the Higgs should exist, but for now it is purely hypothetical.

[FreeKeys]

Well, that clears that up!

LOL & residual spasmodic chuckling. Anyway, I'm as open to the idea of other dimensions as an amateur can be...

[VadeRetro]

Not to divert this thread, I most recently answered that here.

[Physicist]

I think we tend to get in trouble when we use one hypothetical particle to explain the existence of another hypothetical particle.

I was asked for real examples of collisionless particles. Z particles are NOT hypothetical. In theory, the collisions of Z particles will be mediated by the Higgs. If there is no Higgs, then my example is even better.

[onedoug]

Thanks always. As Hamlet's Laertes, "I will the effects of this good lesson keep."

[GOPJ]

Thanks for the ping.

[Physicist]

No, no, that's a really good question.

The first too-simple answer is that they can't...and the second too-simple answer is that--to a certain extent--they do!

Suppose you had a point-like collection of many of these dark matter, and another one comes tooling along. Of course the incoming particle will be attracted to the larger mass. Regular matter would stick together and clump up, but not these. Even if the incoming particle falls dead-center, it'll pass right through and keep going.

If it has enough energy, it'll escape. But if it doesn't, it'll get to some maximum distance, fall back through, reach the same maximum distance in the opposite direction, and fall back through again, repeating forever, the perfect harmonic oscillator.

But that's a bit contrived. In the real world, what you'd have isn't a single, large clump with some test particles falling directly at it, but a diffuse gas of particles falling every which-way. If there happend to be a local area of higher density (and for this purpose a clump of regular matter will do), the nearby dark matter particles will bend their trajectories towards it, and eventually, the combined effect of all the purely elastic collisions will be to condense the dark matter. On a large scale, it actually will form clumps, and you can see that that has happened in the pictures from this group.

But in order to condense, a gas must somehow lose its gravitational potential energy. If all collisions are elastic, all they do is to redistribute the energy, and not shed it...so how can this be? Well, don't forget that the dark matter particles also undergo these extremely weak, elastic collisions with regular matter as well. If the dark matter is hotter than the regular matter, then on average, the dark matter will be losing kinetic energy to the matter, which will in turn radiate some of it away as photons. In this way, the dark matter slowly cools and condenses.

One more thing. If a collection of dark matter does, by itself, form a black hole, as could certainly happen, it will be indistinguishable from one formed by regular matter, except that it won't have a magnetic field. But the main features--the Schwartzschild radius, the event horizon, the lensing--will be the same.

[unlearner]

Apparently I overlooked your comments. I just noticed I had not viewed them. Sorry, didn't intend to ignore you.

I am not unwilling to embrace the existence of dark matter, just feel it is a contrivance. It seems an ad hoc theory modification.

I think it is at least as reasonable to reopen the files on the constancy of the speed of light. (We know experimentally that it is a constant. The question is if it is a changing constant.) Is there a mechanism that determines the velocity light travels? That is where we should look for answers to the mystery of an accelerating universe, in my opinion.

"there is much more evidence for dark matter than deity."

Not really. I've met the "Deity". I've never come across any dark matter. Maybe it gravitates to liberal social functions (where I am persona non grata).

[Paul Ross]

Bump!