Dark Matter Clumping: Unanswered Questions

[bigmig]

This has been discussed in other topics but not without leaving some issues unanswered. So DM has only gravity and does clump. Clearly even if it reached stellar density it would not intiate the usual nuclear 'burn' point because there are no electromagnetic/nuclear charges involved. But if it cannot be held apart by radiation pressure then why would it not carry on becoming denser - even to black hole proportions? I've read various vague notions of 'the particles pass through each other' etc but the consequences of gravitational attraction are increasing density or orbiting behavoir. This outcome has not been observed to be the case with DM so what's happening?
Thanks.

 

[BillSaltLake]

A related, but opposite, question is "why does DM clump at all"? Compare the behavior of a hydrogen atom that is in space but is attracted to a planet a few thousand miles away. The atom may be attracted and will then pick up significant speed before hitting the atmosphere or surface. Then it will stop relative to the surroundings. Thus baryonic matter will clump.
DM particles don't stop like that. If a DM particle is at rest with respect to a large nearby mass, the DM will accelerate and pass through, reaching max speed in the center and then going all the way through. It will stop at an opposite point the same distance away as origionally. The only energy loss (probably) is gravitational radiation, which in most cases is negligible (I think it would work out to only a small loss within billions of years for a planet the size of Earth). Then this "orbit" (straight line in this example) will repeat periodically. The DM particle's average distance from the planet center is only somewhat decreased by this attracton, but if the planetary mass continues to increase, the average DM particle distance will decrease further.

 

[bigmig]

I see. This would then be the sort of invisible cloud of DM in and around galaxies detected by gravitational lensing observations. The DM being in various forms of near perpetual 'orbit' like what you describe. So I guess the decay is so slight that dense clumping has not yet happened? Wonder if anyone has modeled this for various parameters of 'mass' and decay energy to see how it should have played out over time - observations at increasing distances (ages) would show potential anomolies in these 'clouds' of DM, wrt the baryonic matter behaviour that is.

 

[phyzguy]

There are a large number of people doing such simulations, and they reproduce the statistical behavior of our current universe pretty well. This is one of the (many!) reasons that most astrophysicists believe that the Lambda-CDM model is a good model and that dark matter really exists. I believe that these simulations assume that the dark matter does not decay, that it interacts with itself and with baryonic matter only through gravity, and that the dark matter particles are 'cold', meaning non-relativistic. Unfortunately, these simulations do not constrain the mass of the dark matter particles.

Here are some links to two different modeling runs:

http://www.mpa-garching.mpg.de/galform/millennium/
http://arxiv.org/abs/astro-ph/0504097
http://astro.kias.re.kr/Horizon-Run/

 

[BillSaltLake]

I know it's been modeled with DM that is without interactions (other than gravity), for example here:
http://www.mpa-garching.mpg.de/galform/millennium/0504097.pdf
This seems to jibe well with observation.

 

[bigmig]

Thanks for that. I should have made clear that I meant decay of the DM 'orbits' over time not actual DM particle decay.

 

[Chronos]

Dark matter is believed to be weakly collisional, which implies dark matter halos surrounding galaxies should have a density profile - i.e, overdense regions tend to form. One of the seminal papers on this phenomenon is by Navarro, et al -

The Structure of Cold Dark Matter Halos
http://arxiv.org/abs/astro-ph/9508025

This is the basis for the NFW profile that is still widely used in dark matter modeling. Another interesting discussion can be found here -

http://scienceblogs.com/startswithabang/2010/06/convincing_a_young_scientist_t.php

 

[bigmig]

..'related, but opposite, question is "why does DM clump at all"?' .. that's right Billsaltlake. However weak the interaction, it has reached the point of clumping, presumably from a more diffuse state. And therefore (assuming the attraction continues) on its way to some even more dense 'clumpy' state. So the observations over various distances of DM infused objects should show this clumpiness process evolving over time? Again, presumably then, a clump being the sum of the attraction forces of the individual DM particles (however weak) should display greater net attraction force - and grow/attract other clumps.
Sorry, I'm now 'rambling' too far ahead of the evidence - but interesting speculation nevertherless.

 

[phyzguy]

..'related, but opposite, question is "why does DM clump at all"?' .. that's right Billsaltlake. However weak the interaction, it has reached the point of clumping, presumably from a more diffuse state. And therefore (assuming the attraction continues) on its way to some even more dense 'clumpy' state. So the observations over various distances of DM infused objects should show this clumpiness process evolving over time? Again, presumably then, a clump being the sum of the attraction forces of the individual DM particles (however weak) should display greater net attraction force - and grow/attract other clumps.
Sorry, I'm now 'rambling' too far ahead of the evidence - but interesting speculation nevertherless.

I don't understand what you are saying here. DM definitely interacts through gravity, that's how we know it exists. Gravity, being an attractive-only force, will cause regions of higher density to grow denser and regions of lower density to grom less dense. The only question is whether there are other, dissipative interactions between the DM particles, and my understanding is that today's observations are consistent with the only interaction being gravity.

 

[bigmig]

As I said I'm rambling ahead of the evidence with a few unfinished thought trains and need to go back to some of the research cited above to get a better grasp of the issues.

 

[bigmig]

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

Thanks 'Billsaltlake' and 'phyzguy' for the reference below, it takes care of most of my question (for now!)
'http://www.mpa-garching.mpg.de/galform/millennium/0504097.pdf'

 

[Wez]

Just a thought, but we conceive what we see in our universe is sitting on a fabric of space time, but a fabric has two sides, so what sits on the other side of the fabric ?, I would suggest an oppersite, but that in turn must have a fabric, but not ours so it too sits on a fabric of it's own and so on, at diminishing "influential " returns so is it possible dark matter is a single or multiple reflective universe(s) , what I'm suggesting is there are more than one universe that is some how linked, the mass we desribe as dark matter is a matress of multible time space fabrics that were born at the same time or perhaps and more likely at a different time but some how there is a "physics" why they still influence each other hence the " dark matter" or for that dark energy is perceived to exist.

 

[Slinkey]

Maybe I'm missing something here (which is quite possible I might add) but we have inferred the existence of dark matter due to the velocities of stars at the edges of galaxies being too fast. We see this in contrast to the velocities we see in the solar system, for example, where the velocity of planets fall in line with Keppler's Law and the farther a planet is from a star the slower it traverses its orbit around the star and hence the planets have different orbital velocities.

But why would this contrast actually exist? I don't understand how there can be so much dark matter around a galaxy such that the stars do not orbit the galactic centre in line with Keppler, yet there is not enough dark matter to influence the planets around stars so that Keppler's law applies. If dark matter is affecting the velocites of the stars themselves around the galactic centre then why does this effect not transmit to planets around stars as well?

 

[phyzguy]

Maybe I'm missing something here (which is quite possible I might add) but we have inferred the existence of dark matter due to the velocities of stars at the edges of galaxies being too fast. We see this in contrast to the velocities we see in the solar system, for example, where the velocity of planets fall in line with Keppler's Law and the farther a planet is from a star the slower it traverses its orbit around the star and hence the planets have different orbital velocities.

But why would this contrast actually exist? I don't understand how there can be so much dark matter around a galaxy such that the stars do not orbit the galactic centre in line with Keppler, yet there is not enough dark matter to influence the planets around stars so that Keppler's law applies. If dark matter is affecting the velocites of the stars themselves around the galactic centre then why does this effect not transmit to planets around stars as well?

This is a good question. It has to do with the low density of dark matter and the huge difference in volumes between the solar system and the galaxy. The volume of the solar system inside Pluto's orbit is ~10^39 m^3, while the volume of the galaxy inside a radius of 10 kpc is ~10^62 m^3. Assume for the sake of argument that the density of dark matter in the galaxy is ~.01 Msun/pc^3. It isn't a constant density, but this will illustrate the point. Then the total mass of dark matter inside Pluto's orbit is ~10^-13 Msun, which is completely negligible. On the other hand, the total mass of dark matter inside a radius of 10 kpc is >10^10 Msun, which is comparable to the mass of stars. The dark matter density falls of slowly as you go to larger and larger radii, so it becomes more and more dominant the further out you go.

 

[phinds]

Just a thought, but we conceive what we see in our universe is sitting on a fabric of space time, but a fabric has two sides, so what sits on the other side of the fabric ?, I would suggest an oppersite, but that in turn must have a fabric, but not ours so it too sits on a fabric of it's own and so on, at diminishing "influential " returns so is it possible dark matter is a single or multiple reflective universe(s) , what I'm suggesting is there are more than one universe that is some how linked, the mass we desribe as dark matter is a matress of multible time space fabrics that were born at the same time or perhaps and more likely at a different time but some how there is a "physics" why they still influence each other hence the " dark matter" or for that dark energy is perceived to exist.

wez, welcome to the forum. You might want to actually READ the rules you agreed to when you signed on. I think most people don't at first (I know I didn't) but there is a definite rule against overly speculative personal opinions and that's what you have in the paragraph above. The moderators have a low tolerance for this sort of thing.

 

[marcus]

Bigmig, Phyz, Bill, Phinds...

I don't think anyone has mentioned that expansion of distance by itself can drain momentum (and KE) from particles. We already know it drains energy from light---the CMB has been redshifted by factor of 1100 so a CMB photon has lost 99.9% of its energy.

So a dark matter particle can lose kinetic energy gradually over a long period of time just because of the geometry expanding. It slows down relative to universe rest.

The significance for clumping I guess you could say is this. Clouds, when they condense, need a way to dump energy. Ordinary matter has collisions and gets warm and radiates heat. But DM cannot.

DM can interact gravitationally with itself by "gravity bootstrap" encounters that give some DM particles excess velocity so they escape, while other particles lose velocity and are then more securely bound.

The DM particles that carry away the excess energy then may travel for a long time until they are slowed down by expansion. then they may participate in formation of another cloud.

The point is that condensing DM has a way to dump excess energy analogous to ordinary matter radiating heat. It can do it entirely by gravitational interaction---with a portion of the DM carrying away excess energy.

So you don't HAVE to assume some other mechanism---although I don't doubt that some of the simulations do assume some other kind(s) of interaction besides gravitational.

 

[bigmig]

Marcus I was intrigued by the SA article dealing with models of space-time, and ending with speculation on a possible fractal/size dependent dimensionality to space-time you referenced in your reply (hopefully still above). To save me wading through lots of stuff have you any references for more recent development of these ides?

Thanks.

 

[phinds]

The DM particles that carry away the excess energy then may travel for a long time until they are slowed down by expansion

Marcus, I'd appreciate some small expansion (no pun intended) on that statement regarding the bolded part. I don't get how the movement of a DM particle through the expansion slows it down. Or is is just that is that it is traveling away from its point of origin less rapidly than other further away objects which means it slows down in a relative sense? Seems to me only gravity would actually slow it down in any absolute sense, relative to its point of origin.

 

[BillSaltLake]

On average, the momentum of particles (both with and w/o mass) decreases over time, and is ~proportional to the inverse of the expansion parameter. This means that particles with mass will slow down relative to the CMB frame.

 

[phinds]

On average, the momentum of particles (both with and w/o mass) decreases over time, and is ~proportional to the inverse of the expansion parameter. This means that particles with mass will slow down relative to the CMB frame.

OK, I can accept that what you say is true (thanks for the response, by the way) but I still don't get the ramifications. Let me rephrase the question.

A DM particle is in a galaxy and is expelled outward at a given speed relative to the edge of the galaxy. Are you saying that DUE TO EXPANSION its speed relative to that edge decreases? Are you even SAYING that its speed relative to that edge decreases? I can't get how to relate that to "slowing down relative to the CMB"

 

[BillSaltLake]

At the present value of H₀~ 1/t₀, if a DM particle is traveling at speed v relative to the CMB, then (exclusive of the gravitational attraction to nearby galaxies, or assuming the DM is far from any galaxy) after 1.4 Gyr, the speed will be ~0.9v. That is, 10% loss in speed at a 10% later time.

 

[phinds]

At the present value of H₀~ 1/t₀, if a DM particle is traveling at speed v relative to the CMB, then (exclusive of the gravitational attraction to nearby galaxies, or assuming the DM is far from any galaxy) after 1.4 Gyr, the speed will be ~0.9v. That is, 10% loss in speed at a 10% later time.

Again, you have given an answer relative to the CMB, which does nothing (as far as I can tell) to answer my question about the speed relative to the edge of the galaxy from which it was ejected? I thank you for taking the time to answer me, but I think either you are completely missing the point of my question or I am completely missing the point of your answer.

 

[BillSaltLake]

Suppose that the DM particle and galaxy are a bound system, such as an elliptical orbit (which includes the straight-line case). My understanding of this is that the orbit will remain the same over time to first order, other than energy loss from gravitons. The expansion makes the orbit slightly larger than without expansion, and if H is changing, this will make the tiny diameter adjustment change with time. However to first order in time, the DM orbital speed remains the same.
If however the DM particle energy is sufficient that it can escape the galaxy, then the excess momentum can be drained away so it may be captured by another galaxy, although I haven't defined what the value of the "excess momentum" is. Any free-moving massive particle will slow down.

 

[phinds]

Suppose that the DM particle and galaxy are a bound system, such as an elliptical orbit (which includes the straight-line case). My understanding of this is that the orbit will remain the same over time to first order, other than energy loss from gravitons. The expansion makes the orbit slightly larger than without expansion, and if H is changing, this will make the tiny diameter adjustment change with time. However to first order in time, the DM orbital speed remains the same.
If however the DM particle energy is sufficient that it can escape the galaxy, then the excess momentum can be drained away so it may be captured by another galaxy, although I haven't defined what the value of the "excess momentum" is. Any free-moving massive particle will slow down.

Once again, I don't see how any of what you said has any bearing on my question, which is why would the expansion of the universe make the DM particle slow down relative to the galaxy from which it was ejected.

Marcus, since this questions was address to one of your remarks, perhaps you could jump in here and help me out.

 

[George Jones]

So a dark matter particle can lose kinetic energy gradually over a long period of time just because of the geometry expanding. It slows down relative to universe rest. ...

The DM particles that carry away the excess energy then may travel for a long time until they are slowed down by expansion. then they may participate in formation of another cloud.

A DM particle is in a galaxy and is expelled outward at a given speed relative to the edge of the galaxy. Are you saying that DUE TO EXPANSION its speed relative to that edge decreases? Are you even SAYING that its speed relative to that edge decreases? I can't get how to relate that to "slowing down relative to the CMB"

Think on a larger scale, i.e., something like this.

Suppose galaxy A and galaxy B both move with the Hubble flow, i.e., they both zero peculiar velocity. Dark matter is ejected from A and travels to B During this travel, the universe expands. When it gets to B, the speed of the dark matter with respect to B is less than the speed of dark matter with respect to A when it left A.

Now, the above scenario is artificial because we to consider a time while structure is forming, but the idea is the same. Dark matter is ejected with a certain speed relative to the Hubble flow. As it travels, its speed relative to the Hubble flow coincident with it drops. Eventually, it moves slowly enough with respect to the Hubble flow that it can again participate in structure formation.

 

[marcus]

Once again, I don't see how any of what you said has any bearing on my question, which is why would the expansion of the universe make the DM particle slow down relative to the galaxy from which it was ejected.

Marcus, since this questions was address to one of your remarks, perhaps you could jump in here and help me out.

Hi George, Phinds, Bill.
somehow I missed this. But George has already answered with a complete picture and I think it confirms what Bill said earlier about expansion draining momentum from particles during a long trek.
So I will just give my excuses for missing I got preoccupied in another subforum, and also had to help my wife fix supper.

George's explanation is clear concise straightforward. I wish there were a FAQ about this very thing: dark matter dynamics and structure formation. It's beautiful that DM can play such an important role at organizing the ordinary matter for us. I don't recall that there is an FAQ on it, maybe there is though.

For some computer sims of DM helping to form structure, google "Smoot TED" and you get a video of a 20 minute slide talk by Nobelist George Smoot explaining early U structure to a general audience.

 

[phinds]

So if I understand it right, it in fact does NOT slow down relative to its point of origin, much less do so due to the expansion. It ARRIVES at B going slower than the speed it was traveling at when it left A because the expansion "pulls it backward" so to speak. But relative to A it is traveling much faster than when it left A --- that is, it is moving away from A much faster than it was when it left A, because from A's point of view, the expansion is pulling it AWAY from A. This is what I thought in the first place, and why I was questioning the statement that it slowed down due to expansion.

As seems to often be the case, it's a matter of the frame of reference. I've been asking from A's point of view about a statement made from B's point of view.

PLEASE tell me I've got this right.

Thanks