Dark Matter Distribution

Rotation speeds in galaxies are just one way the presence of DM is inferred. There are a half dozen different kinds of astrophysical observation supporting it and some of them detect NON-UNIFORM DISTRIBUTION.

Contour maps of the uneven density of DM clouds have been made using the technique of "weak gravitational lensing" which measures the optical distortion of background shapes. the amount and orientation of distortion corresponds to the density of a region of the cloud.

It is a fascinating feature that the unevenness is on a larger scale than with ordinary matter.

On the scales we are used to, DM tends to be uniformly spread out! But at much larger scales, like surrounding a galaxy or like in amongst a cluster of galaxies, or in even larger scale filament structures it shows the effects of having self-gravitated and fallen together into over-dense regions. It can clump, but only on larger scales than we are used to. why???
You might want to pursue this with further questions.

It has to do with DM's limited means of dumping excess energy. when things fall together they can't stay together unless they can somehow get rid of the excess kinetic energy they have acquired in falling! But to dump excess energy usually requires some kind of interaction. What means would DM have? Somebody here should pursue this question and think about it carefully and stubbornly. the answer has to do with the expansion of geometry itself---expanding distances. this can bleed off kinetic energy. Fascinating business. If you pursue the question, good luck to you.

 

I think you probably did not understand. I did not say that expansion leads to NON-clumping of DM.
I wanted to suggest the OPPOSITE. Expansion contributes to the ability of DM to clump.
It is probably essential, without it DM would be unable to condense into clouds. (the patches of higher density which we observe/infer in various ways)

think about it.

When ordinary matter falls together by selfgravitation it can get HOT and radiate off its heat (interacting with the electromagnetic field)

This is what allows ordinary matter to condense into lumps (stars galaxies dustclouds clusters...)

If something falls into a cloud and it cannot dump the kinetic energy it will just pass on thru and out the other side!!!

If two things collide and cannot dump the excess kinetic energy they will just bounce away with the same speed. there will be no accumulation.

Dark matter cannot interact with the EM field and therefore it cannot get hot and radiate heat or light or Xrays etc. those are ways that ordinary matter gets rid of energy when it falls together into clouds.

This DISABILITY is what prevents DM from condensing down to the scales we are used to.

What mechanism do you think bleeds off kinetic energy from DM and thus allows it to gather into regions of higher density. It is LESS able to gather and the regions are LARGER than we are used to, but it can still gather somewhat. How? Try to guess. If you can think of some mechanism, please say.

 

One way of thinking about it is that take two bricks. Throw them at each other. They clump. Take a water hose and spray it at each other. It will go "splat".

Take two flashlights or lasers and "cross the streams". Nothing happens.

If you had two clumps of dark matter and then throw them at each other, they won't go "splat". They'll pass right through each other.

The result of this is that it's easier for ordinary matter to form clumps since ordinary matter goes "splat" and dark matter doesn't. It's *possible* for dark matter to form clumps, but it's a lot harder.

One other thing. If you point a high power laser or proton beam at me, bad things will happen since my body interacts with lasers. Dark matter (and neutrinos) don't much interact with ordinary matter. In 1987, the earth was hit by a massive neutrino pulse from supernova 1987A. No one except a few physicists noticed, and they noticed because about *15* atoms changed in a ten second window. Same with dark matter. We could be swimming in the stuff and not notice although there are a lot of experiments right now trying to detect it.

 

I ain't no Einstein, buddy.

 

...and you feel the need to alert the forum to this why?

 

Just so as not to leave anyone in suspense I'll set out an idea of how DM can have its excess kinetic energy bled off, allowing it to accumulate in clouds and such.

The big factor is expansion itself--it drains momentum from stuff. Brian Powell could probably explain this better than I. I understand that Steven Weinberg discusses how this happens in his Cosmology textbook, but I haven't read his section on it. Lineweaver refers to it in a paper I just looked at.*

As far as we know DM only interacts via gravity. Imagine a cluster of DM particles falling together. Random gravitational encounters between particles swap energy back and forth---you know the "slingshot effect", a gravitational interaction where one item whips around another, slowing it down and gaining momentum. This has been responsible for cleaning a lot of stuff out of the solar system--things can pick up escape velocity by such encounters.

So what appears to have happened when DM clouds form is that some of the DM is "sacrificed" to carry away excess energy. It gets flung out of the condensing cloud and carries away the unwanted surplus energy---this allows the remaining DM to settle down and accumulate.

So then what happens to this extra energetic DM that has been flung out, as the cloud gathered? It eventually gets slowed down (relative to universe rest) by expansion itself. So then wherever it ends up when it has slowed down enough it can can find another condensing cloud, falling together under its own gravity. Maybe this time it can be one of the particles that settles down and finds a home in the cloud instead of being one of those flung out by the ensuing gravitational interactions.

The good thing about this mechanism is that it does not require ordinary matter. It allows DM to collect and form structures entirely of its own accord. There isn't very much ordinary matter, comparatively speaking, so it's important that DM can condense to some extent without help.

But once ordinary matter has formed structure there is also the possibility that DM falling into some overdense region will interact GRAVITATIONALLY with the ordinary and will give up some kinetic energy to the ordinary, which can then do all the usual stuff: undergo collisions, get hot, RADIATE it away. So that is a second mechanism---ordinary matter serving as an intermediary to dissipate energy.

Brian Powell knows more about this so he may wish to add or correct this.

*To me it's intuitive (and analogous to the redshifting of light by expansion) because separate neighborhoods where clouds are condensing are getting farther apart so a particle flung out of one and kicked over towards another locale finds it backing away from him and arrives there with diminished speed and an enhanced ability to be gathered in.

 

Hey, I'm like, sharing, dude.

 

One thing that surprises people is that much of the complex and exciting parts of cosmology don't happen at the big bang, they happen much later. From a theory point of view, the big bang is "easy." You just have a bunch of hot smooth gas. A lot of the early universe is also "easy", you have a bunch of hot smooth gas with very small fluctuations and those fluctuations can be treated as "corrections" to the basic assumption of smoothness. This sort of view gives you excellent agreement with cosmic microwave background, and one reason that you do get good agreement is that the CMB was emitted early in the universe when there wasn't enough time for anything complicated to happen.

Once you wait a while, then things get more complex because effects that don't matter in the early universe take over. Also some of the assumptions that you make to simplify the calculations start to break down.

One other active topic of research is the "back reaction problem." Put simply, most cosmology calculations assume that you have a general expansion, and that local effects don't have any reaction to the general expansion. It makes the equations simple, it's very easy to justify in the early universe, but people wonder if it stays true through out the life of the universe.