I've a problem with dark matter, and it's this: if it has mass and responds to no other force than gravity, plus there is about 20 times as much of it in the universe than ordinary matter, then it should collapse into massive clumps in the same way ordinary matter collapses into stars and other bodies.

There's a limit on star size, stars above a certain mass go supernova very quickly. However, dark matter would not be limited by this.

In fact I can't see why it doesn't collapse into black holes straight away as there is no radiation pressure to prevent it.

Also, are we in danger of passing near (or even through) a massive dark-matter object with all the disruption that would cause?

"normal matter" collapses into stars because it interacts in ways besides gravity. Things collide, and group together for non-gravity reasons. What could make a dark matter particle change it's orbit, and descend into a gravity well?

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Or another way to say this is, the ability to interact with light is not usually a way to prevent collapse (via radiation pressure), it is more often a way to promote collapse (by allowing energy to be extracted). That's very important for making stars.

This is interesting, in deed??? According to the big bang, hydrogen atoms were the first and only atoms to exist and then gravity collapsed the hydrogen to create stars and then these stars exploded to spread other elements throughout the universe which then collapsed to form new stars and on and on ...

Are you telling me that hydrogen can form "giant" molecules to aid the gravitational collapse? I think you had better revise that statement to be more clear in your intent.

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My invisible elf ??? Why he is made of dark matter and lives off of dark energy !!!
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I said nothing about giant molecules. My statement does not need modifying. I am asking you to not put words in my mouth.

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Maybe I was hasty in my reply but I think the conclusion is a valid point. Your statement seems to indicate that electromagnetic forces aid in the collapse of matter into stars. There are, afterall, only 4 forces to the physical world and since you seem to have eliminated gravity as the final cause of collapse then we are left with the next long distance force: electromagnetic.

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My invisible elf ??? Why he is made of dark matter and lives off of dark energy !!!
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I suppose that you could call the mechanism electromagnetic, but normally we talk about it on a macroscopic scale. The cloud's atoms/particles collide. This material gets hot and radiates heat away. This lost energy permits further collapse. Dark matter doesn't do this. Gravity plays a role and I never said it didn't.

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I won't get into an argument over semantics and I do not want to hijack this thread but here is how I came to my conclusion on "giant" molecules.



This statement indicates electromagnetic forces:



This statement minimizes the role of gravity:




This statement negates the role of gravity since gravity is all that dark matter can resort to:

- - - - - - - - - - - - - - - - - - - - - - - - -

Here is my own question: If two dark matter particles collide is heat generated?

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The cross-section for collision of two dark matter particles should be very small, making such collisions much less likely than for charged particles (It might be similar to the cross-section for neutrons, or much smaller, I don't know). In any case if they do collide, what happens depends on what they are. In the case of particles that are their own anti-particle, they would annihilate and give off gamma rays.

Some of the experiments that are happening looking for Dark Matter particles involve the possibility that they might rarely collide with normal matter and create a small amount of heat.

The point about dark matter is that the interactions are very rare, not that they are impossible. Neutronos are considered to be Dark Matter (hot dark matter), but we do sometimes see evidence of neutrinos.

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Let me attempt to illustrate the role of cooling in collapse of matter, and I hope similarly cool this thread. If two ordinary matter particles approach each other closely, they will lose energy only through nongravitational means - gravity being a conservative force. If the particles are charged, the electrostatic acceleration will lead to loss of some of their kinetic energy as electromagnetic radiation. If this happens within a large ensemble, the ensemble will shrink under its mutual gravity as the particles have now lost some of their internal energy within the system (that is, they have cooled). The great majority of such processes are collisional 2-body ones, therefore depending on the density of each kind of particle involved. A major complication comes from the fact that quantum mechanics gives threshold energies for important things to happen - hydrogen ionization, collisional excitation of ions of oxygen and carbon which are then de-excited by radiation... meaning that some of these mechanisms happen only at certain temperatures and certain chemical compositions. The collapse of a normal baryopnic object will accelerate - collapse increases the mean density which increases the cooling rate which allows the collapse to go faster, increasing the mean density...

Purely gravitational collapse is extremely slow even on cosmological timescales, proceeding by statistical changes in the population of various orbits ("dynamical friction"). Lacking (as far as we can tell) any nongravitational interactions to cool the motions of whatever it's made up of, dark matter cannot collapse in the way ordinary matter does.

Those were two good replies that make sense.

After antoniseb's first reply thoughts inside my head were colliding quite exponentially which raised concern within my consciousness about my mental state and its continued viability.

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Thanks everyone, so the idea is, the dark matter cannot loose kinetic energy in the same way as ordinary matter. Initially the DM particles are moving too fast for them to have any real probability of staying together long enough to initiate growth of clumps.

What we seem to be saying however is DA cannot change its kinetic energy. When subatomic or atomic particles collide, that's not really the word. They interact via the electromagnetic force (or perhaps also the strong or weak nuclear forces). If DM particles are immune from these forces, can they really be said to "collide"?

And since they are immune from electromagnetic radiation, can the concept of temperature apply to dark matter?

So what initiates the large-scale structure in the DM computer models? Shouldn't it stay evenly spread out ?

Temperature for Dark Matter refers to how fast the particles are moving. We say dark matter is hot if it is not gravitationally bound, and moves close to the speed of light. It is "cold if it is slow enough to be gravitationally bound, such as orbiting in a galaxy or a cluster of galaxies.

Some theories of dark matter allow it to rarely and infrequently interact with itself, or with normal matter, so there might be a minor concentration of Dark Matter in the center of the Sun, or in neutron stars. This is conjecture till we actually start observing Dark Matter particles. I mention this so that you will know that there are a lot of aspects that aren't nailed down yet, and neither you nor I can say something categorical with it being certain to be right.

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Dark matter particles do interact gravitationally (well, we figure they must since they interact with ordinary matter that way). Two-body gravitational interactions don't change the center-of-mass energy, but in multibody systems they can have net effects in redistributing the energy. One useful example is of a single particle picking up kinetic energy from an encounter with a bound pair (at the expense of the pair's binding energy). In a large system, such collisions will, over time, gradually shrink the inner regions as an envelope of only slightly-bound particles carries the former energy of core particles, the basic reason being that there is slightly more phase space available for scattering into higher-energy orbits than lower-energy ones. All this was (thoughtfully!) worked out decades ago in the context of stellar dynamics, since stars can be considered a collisionless system except in a few very confined contexts.

I'd have thought the gravitational interractions of sub-atomic particles would be very weak, so weak the age of the universe would be insufficient for significant change. Note the dynamics were worked out for stellar sized objects, for which the dominant force IS gravity.

However, the observations suggest galaxies formed early in the history of the universe.

I guess it's all been worked out though.

What are the densest and largest DM concentrations likely? What would life be like either within or near to one of these?

Yes, I think the gravitational interactions must be just each particle with the average field of all the other particles, so like stellar dynamics without the two- and three-body effects. But stars rarely literally collide, whereas particles (which are much more densely packed) do, even DM particles. So the collisional effects that ngc3314 is referring to are not in themselves gravity, and the stellar dynamics example is merely intended to provide an analogous way to think about it. The system is collisionless in the sense that orbits are completed without a collision, but it may still be relaxed in the sense that eventually collisions do occur, and there has been plenty of time to reach a fixed temperature. I think galactic DM models typically assume a uniform temperature, though I've never really understood just why you can do that. If you do that, you get a density that falls like 1/r^2, which is what you need to get a fairly constant orbital speed.

Obviously I need to read up on it a bit more! I do have this feeling though that they're making things up to make the theory fit, and perhaps have not thought through all the repercussions fully.

I don't think it's entirely unlikely enough ordinary matter will be found to make DM unnecessary. For example, in the last few months alone we have had the diameters of galaxies doubled because of the discovery of their faint outer regions, and also two new satellite galaxies of the milky way.

OK there's still a long way to go, but if things keep being discovered at that rate it might not be long before DM is redundant.

And it is conceivable that you are right, but I predict that if you do that research you mention, the more you know, the more you will come to accept dark matter.

There have been a great many searches done, to find baryonic components of DM; some examples:

-> one of the first uses of the HST was to look for (red) dwarfs in the (nearby, Milky Way) halo (IIRC, Bahcall was the PI) - it's a very nice piece of work (so typical of John), you might like to read up on it. It didn't rule out such faint stars, but it did put an upper limit on how much of the MW's DM halo could be in the form of such stars (very little, IIRC).

-> MACHO, OGLE, and other transient lens searches - these are sensitive enough to detect 'compact' masses (or any kind!) down to ~Jupiters, out as far as the LMC (or was it the SMC?). Again, they didn't rule out some clumps (Jupiters to dark stars) as a component of the MW's DM halo, but they did put limits on it (again very little can be in this form).

-> ISO, XMM-Newton, Chandra, COMPTON, INTEGRAL, Spitzer, ... have all looked for photonic signatures of baryonic matter, in galaxies, halos, galaxy groups, clusters, ... in the form of gas (hot, cold, and anything in between), dust, grit (stuff that's bigger than dust), ... Again, some baryonic matter was found, but nowhere near enough to account for more than a tiny fraction of the observed DM.

-> A regular BAUT contributor also used an ingenious method to look for DM, in galaxies, in the form of dust (by observing galaxies 'back-lit' by more distant galaxies - though AFAIK the primary aim wasn't to search for 'DM as dust'); again, very little of the DM can be in such a form.

So what (baryonic matter) can be there, that hasn't been detected yet? If you can come up with some form ...

(Note that some kinds of populations of black holes is one possibility ... but they're not exactly 'baryonic', are they!)

Does Dark Matter generally occur around galaxies, or is it kinda evenly dispursed between empty space and galaxies?

As far as we can see (to date), most DM is in rich clusters, where it is concentrated more or less towards the centre (there are some quite decent contraints on the radial profile, from a variety of sources).

For galaxies, it seems there is also a fairly well established radial profile (in general, integrated over many galaxies; an individual galaxy's profile may be significantly different), though I personally am less knowledgable about this.

We are just beginning to be able to determine, observationally, the extent to which the distribution of DM in galaxy halos deviates from being spherical.

I can't answer the question about the distribution of DM in poor clusters, or in non-cluster galaxy groups.

Nor do I know if it's been determined what the density of DM is, outside clusters and groups (e.g. in voids, or along walls).

BLACK MATTER? This to me is a very interesting subject! I have not yet heard anything that has satisfied my wondering mind! But...I do have an opinion. I have submitted my opinion a few times at various places and or Blogs, and nobody has replied yet!? And Probably won't.

Most of my satisfing conclusions have come from simple LOGIC! We all have accepted the exsistence of infinity. At least I have. It would be impossible for it NOT to exist! There is no perimeter to the infinite Universe, that too, would be impossible! If there were, then infinity would not be in any equation!

It is my opinion, that the Black Matter is nothing more than.....
"The Color Of Infinity" Why? Because if traveling at the speed of light from any given solid matter, (such as Earth) in any selected direction, FOR EVER, avoiding any and all solid matter on this journey...you would never escape from the color of Black, as you view around 360 degrees! So BLACK MATTER (that which we see at night between the stars and planets) is not an object, or a thing, it is the absence of solid matter and light! Light can only come from some form of solid matter or gases that harbors in or around solid matter!

Furthermore, the concept of "The Universe is expanding" is absurd!! Expanding, clearly suggests the BLACK MATTER is getting bigger? If this would be true, then it means, infinity is not yet finished being as big as it can get! Come on...Let's get real! Expanding only means that all solid matter is doing nothing more than moving further away from each other!

What do ya think about that, HUH??

"Expanding" is just a word. You're right, the upshot is, all the distant galaxy clusters are getting farther away from each other, and the words you use to describe that are quite a matter of taste. The most common description is that space is expanding.

Your comment says "NOTHING" to me! ALL words are just a word! We use words to communicate!? Words are Definitive, Discription, Explaining, etc.

I am safe here, because I took liberties to Declare my lack of a good education and I am in no way, concerned about trying to impress anybody with Superior Grammar! I also take no offense to anything in your reply, I am pleased that you even did so!

But I would like for you to, Define "SPACE" for me. Because I am anxious to reply to your definition! In other words, when you use the word "Space" are you refering to the entire Universe or our Solar System, or maybe our Galaxy?

All I can say my friend is, Infinity DOSE EXIST!, and therefore Space it's self, cannot
expand! One other thing of interest is that, "There is absolutely NO CENTER to Space!

If you think there is a center to Space, tell me what you base it on? Because in my mind, anything that has a center, would have an outer edge of some kind, but Infinity would cancel that idea out immediately! Right?

Hi searlesgold, I don't think that Ken G was trying to impress anyone with superior grammar. He was making a very good statement about the universe expanding (or at least appearing to expand). I took it as a very friendly note. Ken G is one of our most consistantly friendly contributors.

In any case, the statement above that I quoted from your post is a classic example of something new members sometimes do. You assert that some statement that can't be proven is true, and conclude from that something that most scientists believe cannot be true. There are a lot of people here that will listen to your ideas, and quite a few that can help you in your quest for knowledge.

I think you may want to either substantiate your claim above, or back down, and tell people this is a gut-feeling, not a proven fact.

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I can certainly comment on the "center of space" idea. I agree, there is no evidence that space has any center. Yet, there is value in picturing space as expanding. What makes you think there's a contradiction there? Where in the concept of expansion is there a need for a concept of center? Generalize your thinking, expansion does not require a center, only that ther is more space between distant galaxies. (As for defining space, that would be a long exercise, I'll let the science dictionaries speak to that). Rest assured that space is a useful concept, connected to reality via a well defined measurement process-- that of measuring distances between events. It's how we keep everything from happening in the same place, in a nutshell.

Huh!
1) Infinity + one = infinity. There, "infinity" just expanded.
2) There are different "sizes" of infinity. The "infinity" of real numbers (Aleph one) is larger than the "infinity" of integers (Aleph null).
3) Just because infinity exists does not mean that space is infinite.

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T. Anderson

Let it be known that, First and Foremost, I have the highest respect and politeness required in Debates (not arguments) for all and anyone who joins in on this wonderful forum and submits an idea or offers any facts that we can enjoy!! I am here to learn and express my opinions, I am allowed to do?
I ALSO ENJOY.

Now then....your display of: Infinity+One = Infinity Expanded, is nothing more than a play on words! Infinity as in distance....Miles or Light Years has no connection to your equation! Expansion as in Balloon, that is being filled with air, there will have to be room for an expansion to prevail. BUT,....as the balloon expands, it clearly will occupy more space! With regards to this example and with the SPACE of (of, not in) the UNIVERSE, why would space need more space, if Space is Infinite!?

If you have read any of my previous Posts, remember I have only a High School Education. But this dose not disqualify me from having an opinion! However, it has been very regretful at times, to say the least! I also expressed that my opinions come from mostly "LOGIC" and a lot of reading on this subject, that has over powered me starting from a very early age.

If anyone prevents me from Posting on this forum......(which I hope never happens) I will simply find somewhere else to talk to people who have the same interest as I. It might be a good idea to get use to me and my LOGIC!
I will never be intiminated by Super Intelects when referring to nothing more than THEORIES!

Thank You!

Well, all I can say here is, if you read my Post entirely, I would like to think that you didn't over look my addmission of LOGIC! That along with addmitting,
"I only have a High School Education", and I highly regret not going to College!

I read about Facts and I wonder about Theories! If I understand correctly,
Theories are not facts, just concepts of what might be.? With this in mind, I
have said nothing that calls for me to "Step Down" from anything!

What can Logic be but anything from the gut!

In science, "here's my theory" does not mean "here's my guess".
Quite simply, logic is a set of rules for solving problems. Nothing more, nothing less. What you are calling "logic" (e.g. "from the gut") sounds more like "common sense". There is nothing that compels the universe to conform to your (or anybody elses) common sense. Science is, however, compelled to conform to the universe (whether it makes sense or not).

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T. Anderson