Ideas about dark matter and structure

[yuiop]

kev, your conclusion is not logical. please read some of the preceding posts in this thread.

Could you be more specific? You suggest that there are methods by which dark matter can dispose of kinetic energy, but can you show a method that is purely a gravitational interaction?

If dark matter particles ineract with each other non gravitationally then do you agree that the dark matter in the original clusters of bullet cluster collision would have dispersed kinetic energy during the collision and formed a clump at collision point like the hot gas? The colliding hot gas of the colliding clusters interacted and dispersed kinetic energy to form one central clump, while the colliding dark matter haloes did not disperse kinetic energy and "passed straight though" each other. Do you agree that the bullet cluster clearly shows that hot gas has a greater tendency to clump than dark matter? In that collision the hot gas clumped while the dark matter did not. It could not be clearer.

Do you agree that for dark matter to gravitationally collapse it has to interact with itself or normal matter in order to disperse kinetic energy?

Do you agree that my demonstration of an object falling through a hole in planet will continue to oscilate forever without friction dispersing kinetic energy as electromagnetic energy?

Do you agree that dark matter is generally thought to be non interacting or weakly interacting other than gravitationally?



Some additional observations can be made from the comparison of the bullet cluster and the train wreck cluster. They appear to give a conflicting image of the nature of dark matter. In the bullet cluster massive galaxies and stars appear to be relatively unaffected by the collision of two clusters and pass straight through the collision area, as does the dark matter, leaving a region of hot gas and relatively little dark matter where the collision took place. This shows the none interacting nature of dark matter.

In the train wreck cluster, the dark matter seems to have remained with the hot gas at the centre of the collision while the massive gaxies have passed through without any dark matter attached to them. This is the opposite of what appears to have happened in the bullet cluster and it is a big puzzle. A closer look at the train wreck image seems to show that some of the dark matter has been dispersed at right angles to the trajectory of the colliding clusters. If dark matter is generated by black holes as I speculated, then one explanation is that black holes in the original clusters that collided to make the train wreck were thrown out sideways by a slingshot effect due to a very close interaction of the black holes during the collision. it is possible that the original clusters had more than one black hole each, in complex orbits around each other that were inherently unstable and easily sent off at tangents during the collision. The dark matter that seems to have been ejected sideways in the train wreck cluster is possibly generated by the slung shot orbiting black holes spiraling outwards at an unusual angle from the collison. The proof would come from locating the black holes in the bullet and train wreck clusters. My bet is that the dark matter stays with the trajectories of the black holes after the collision and that slung shot black holes will be found way off the collision axis of the train wreck clsuter. It should be noted that at this point, no one has offered a viable explanation of why the bullet cluster and train wreck cluster differ so much in their distribution of dark matter after the collsions.

 

[marcus]

Could you be more specific? You suggest that there are methods by which dark matter can dispose of kinetic energy, but can you show a method that is purely a gravitational interaction?
..

Already did. Please read earlier posts.

Let's review the basic idea in the initial post news item, so we retain focus. It is too interesting to fritter away in accidental misunderstanding.

Dark matter presumably starts out more or less uniformly distributed, and self-gravitating clouds of it clump---there are various obvious mechanisms by which a condensing cloud can eject excess kinetic energy and angular momentum---mechanisms involving only gravitational interaction.

One point is that clouds of dark matter may be able to coalesce without the help of ordinary matter.

Another point of the research news item is that for some unexplained reason there may be a minimal size of around 10 million solar.

Here's an excerpt from the article cited in the initial post:
==quote==
...Since dwarf galaxies are mostly dark matter – the ratio of dark matter to normal matter is as large as 10,000 to one – the minimum-mass discovery reveals a fundamental property of dark matter.

... “This helps us better understand the particle that makes up dark matter, and it teaches us something about how galaxies form in the universe.”...

The scientists say clumps of dark matter may exist that contain no stars...
==endquote==

when one first imagines ways in which coalescing dark matter could eject excess kinetic, one very likely thinks of mechanisms involving ordinary matter----by gravitational interaction the dark transfers kinetic to the ordinary, and the ordinary radiates it away by various means (this was discussed earlier in thread).
But now it looks like we have to consider ways that the dark matter could eject excess kinetic energy by itself----without the help of ordinary matter!

There may be processes we don't know about, but one obvious mechanism is that by gravitational interaction some of the particles acquire above average kinetic, and fly away. That leaves the remaining cloud with below average.

(same mechanism is presumed to have operated in the early solar system---some junk escaped as it settled down, carrying away excess energy---think of the slingshot effect)

The puzzle that seems so interesting IMO is this minimum of 10 million solar. It might be false. But suppose the research is born out and there really is this minimum. What does this say about dark matter?

 

[yuiop]

...

Do you agree that the bullet cluster clearly shows that hot gas has a greater tendency to clump than dark matter? In that collision the hot gas clumped <EDIT>slowed down</EDIT>, while the dark matter did not. It could not be clearer...

Let's focus on the above statement. Do you agree that if the above statement is true and hot gas has a greater tendency to clump than dark matter (as the bullet cluster seems to show) then that fact is hard to reconcile with the idea that dark matter is the cause of hot gas clumping in the early universe?


P.S.

Do you agree that the difference between the http://cfa-www.harvard.edu/hea/cos/cg.html" collisions presents a major problem for our current understanding of dark matter?

 

[marcus]

kev's personal ideas about dark matter and structure

kev has some personal ideas about dark matter which don't fit into a thread I started about minimal galaxy mass----a recent research paper on observed dwarf clouds of dark matter (around 10 million solar mass).

I can't respond to kev's personal ideas in that thread without derailing it, because they are too unrelated to the topic. So I [EDIT: support the action of moving his posts to start this new] thread for kev, so he can discuss his ideas.

I will fetch some quotes from the other thread, where they are off topic. One idea is that dark matter is actually produced by supermassive black holes. Ordinary matter collects and undergoes collapse, eventually forming black holes, which somehow produce dark matter. And that is where dark matter comes from---and why it is found in haloes around galaxies (which, after all, contain supermassive black holes.)

Another idea is that the primary puzzle we should focus on is the dissimilarity between two collisions between clusters of galaxies----the bullet and the trainwreck. Unfortunately these are processes involving tens of trillions of solar masses, not tens of millions.
Collisions of clusters of normal-size galaxies (trillion-solar scale) are a whole other topic from the existence of these curious dwarf clouds (million-solar scale). So kev should have another thread to discuss his ideas about bullet & trainwreck cluster collisions.

I will go fetch some quotes from the other thread, to start this one.

 

[cristo]

I've moved posts to this thread, thus they are out of order.

 

[marcus]

In this post in the other thread, kev asked that we focus on an earlier statement of his. It isn't something I'd want to focus on in the other thread, but people may wish to discuss it here. I might comment too!

... Do you agree that the bullet cluster clearly shows that hot gas has a greater tendency to clump than dark matter? In that collision the hot gas clumped while the dark matter did not. It could not be clearer.

Do you agree that for dark matter to gravitationally collapse it has to interact with itself or normal matter in order to disperse kinetic energy?

...

Let's focus on the above statement. Do you agree that if the above statement is true and hot gas has a greater tendency to clump than dark matter (as the bullet cluster seems to show) then that fact is hard to reconcile with the idea that dark matter is the cause of hot gas clumping in the early universe?P.S.

Do you agree that the difference between the http://cfa-www.harvard.edu/hea/cos/cg.html" collisions presents a major problem for our current understanding of dark matter?

Part of what you say is obvious and part is apparently based on misunderstanding. The example of bullet cluster collision is well-known. Ordinary matter has electromagnetic interaction, can turn collision energy into heat, and can radiate it off. Two clouds of ordinary matter can collide inelastically and fuse into a single cloud, radiating off the excess energy. Ordinary matter has an easier time clumping.

But dark matter can nevertheless clump. Computer simulations show it coalescing into clouds of about 1 milion solar. And then these mini-clouds coalesce to form larger clouds. Gravitational interaction allows excess energy to be ejected. Also space expansion cools, albeit slowly as Oldman points out.
hard to reconcile with the idea that dark matter is the cause of hot gas clumping in the early universe?
Well it is certainly possible to reconcile. Maybe you just need to try harder
I wouldn't restrict it to hot gas. Any kind of ordinary matter. The general proposition is that dark matter can collect into clumps and can help ordinary matter to do so. Evidently people run computer simulations where this turns out to happen spontaneously---primarily thru gravitational interaction on the dark matter side, I would suppose.

Bullet cluster, as I say, is well-known and doesn't preclude this (though I grant that you may think it does.)

 

[marcus]

Here is another of kev's ideas, brought from the other thread so it can be discussed, if people want, without interfering with the topic of dwarf dark matter clouds, in the other thread (and the thread's general bias towards conventional science.)

Here is an additional thought. If dark matter particles can only interact gravitationally with normal particles and with other dark matter particles then dark matter can not gravitationally collapse. The reasoning goes like this. Imagine an object that has been dropped down a hole drilled through a planet. It falls straight through until it pops out the other side and then turns around and falls back to its original starting point and then continues to oscillate indefinitely like this, in a manner that can be described as simple harmonic motion. I am of course assuming there is no atmosphere to create friction and damp the oscillation eventually bringing the object to rest at the centre of the planet. That is sort of the point of this post. Without some non-gravitational interaction causing friction there is no damping and no gravitational collapse, particles just pass straight through and continue to oscillate forever.

[EDIT] This I think is basically what oldman was referring to in relation to the virial theorem and the requirement for kinetic energy to be dispersed in order for collapse to take place. [/EDIT]

So none-interacting dark matter evenly distributed in the early universe shows no tendency to clump like normal matter. Weakly interacting dark matter might have some tendency to clump but not to the extent of normal matter. This is partly supported by the observation that the dark matter of galaxies tends to extend way beyond the radius of normal luminous matter in a galaxy.

It also raises the question as to why dark matter seems to be centred on galaxies and does not remain evenly distributed throughout the universe. Since science does not seem to have a definite answer as to what exactly dark matter is composed of, I hope I am allowed to make this speculation. It has been observed that the amount of dark matter in galaxies is related to the size of the supper massive black holes at their centres, so is it possible that dark matter is generated by those black holes? One idea is inspired by discussions of Hawking radiation. Virtual particles that pass through a event horizon take on "real" qualities and real particles can become virtual. What if black holes somehow radiate virtual particles?

The half life of virtual particles is determined by the Heisenberg uncertainty principle and this is typically extremely brief period. What if these ejected virtual particles are gravitationally accelerated to extreme velocities by the black holes so that there half life is greatly extended by time dilation and with their extreme speeds they can survive to great distances before expiring? This would create a sphere around the black hole (and galaxy) that has a definite radius that can be calculated from the expected energies of the virtual particles and the size of the black hole, the radius being determined by the expected half life of the relativistic virtual particles. This idea has an analogue in the virtual particles that are thought to be responsible for the strong force in the nucleus of atoms and this idea nicely explains the very limited radius of the strong force. The volume and surface area of the black hole presumably determines the amount of virtual particles that are generated by the black hole as random production of virtual particles is related to the volume of vacuum under consideration. So in this idea, rather than clumping of dark matter seeding the clumping of normal matter, normal matter clumps according to small random fluctuations in the mass distribution of the early universe (possibly assisted by primordial black holes created during the big bang that would survive much longer due to the great amount of back ground radiation preventing their immediate evaporation). Once normal matter clumps and initial black holes form, the generation of dark matter by those black holes accelerates the clumping and structure forming process, possibly exponentially. Just food for thought. I hope it is of interest and inspirational to someone ;)

Whole thing based on a misconception. the example of a particle falling back and forth thru the Earth has no bearing, since it does not reflect the complicated gravitational interactions going on in an N-body system. grav'l interactions can give some particles greater than average energy and they can fly out---removing energy from the cloud.

so your premise---that dark matter cannot condense into clouds----is unfounded. the rest of the speculation is a way of explaining the observed existence of clouds of dark matter (on the false assumption that they cannot form by themselves)

 

[marcus]

I've moved posts to this thread, thus they are out of order.

Thanks! The order doesn't matter. Main thing is people can now discuss kev's ideas if they want, without being off topic in other thread.

 

[oldman]

... grav'l interactions can give some particles greater than average energy and they can fly out---removing energy from the cloud.

so your premise---that dark matter cannot condense into clouds----is unfounded.

Marcus: Is this really the consensus view on how collapsing dark matter is virialised by gravitational interactions alone? Sounds to me rather like the virialisation of globular clusters on a vaster scale, and perhaps a bit of a stretch.

If there are other web-accessible references about how dark-matter-collapse is enabled I'd appreciate being steered to them.

Kev: Your object oscillating in a hole through a planet is a good example of a general need to remove kinetic energy to enable systems to condense , see my post #4 of Marcus' thread More clues about structure formation ... . The virial theorem just particularises this general phenomenon, as indeed you guessed in your edit.

 

[oldman]

This thread has the potential to become quite interesting.

Going back over posts I was struck by something Marcus said:

ordinary matter has an easier time clumping

. Later, he dismissed the premise

---that dark matter cannot condense into clouds----(as) unfounded

. Maybe so, but there is always a matter of degree with such concusions.

Dark matter is thought to be very different from ordinary matter. I think it would be surprising if the differences didn't have consequences as far as structure formation goes. What about a compromise?

Suppose dark matter just clumps differently from baryonic matter (being unable to generate escaping radiation) rather than not at all. Nature may well then have devised an alternative end to the story of gravitational collapse. Without access to an easy method of getting rid of kinetic energy (star formation) collapse is likely to be more rapid, and to resemble to a greater degree the free-fall of Kev's object in a hole through a planet, albeit with many-body complications.

When one remembers that there are huge amounts of dark matter sloshing around the universe (five times the amount of ordinary matter), and that observations show that dark matter is indeed part of vast structures formed by gravitational collapse, namely galaxies ---it seems to me that Nature's story of collapse could well incorporate the formation of central galactic black holes out of fast-collapsing dark matter.

Or am I pushing this idea too far?

 

[George Jones]

First, a quote from Weinberg's new cosmology book, and then a couple of interesting leads.

Of course, eventually the perturbations in the matter density became strong enough for the linear approximation to break down, as shown vividly be the existence of stars and galaxies and galaxy clusters. It is believed that these structures were formed in a two-step process.¹ First, in regions where the density was a little larger than average, the cold dark matter and the baryonic matter together expanded more slowly than the universe as a whole, eventually reaching a minimum density and then recontracting. This scenario is discussed in section 8.2. If an overdense region was sufficiently large then as shown in 8.3 its baryonic matter collapsed along with its cold dark matter. Then in a second stage, after this collapse, the baryonic matter lost its energy through radiative cooling, and it condensed protogalaxies consisting of clouds of gas that eventually form stars. The cold dark matter particles could not lose their energy through radiative cooling, so they remained in large more-or-less spherical halos around these galaxies.

¹S. D. M. White and M. J. Rees, Mon. Mot. Roy. Astron. Soc. 183 341 (1978)

An interesting dark matter conference

http://www-conf.slac.stanford.edu/ssi/2007/lateReg/program.htm

was held a little over a year ago. Many of the videos are interesting, and the video of the second lecture (Aug. 3) by Risa Wechsler might be particularly relevant.

This CosmoCoffee discussion

http://cosmocoffee.info/viewtopic.php?t=367

might have some interesting references.

 

[marcus]

This thread has the potential to become quite interesting.

Going back over posts I was struck by something Marcus said:

ordinary matter has an easier time clumping

. Later, he dismissed the premise

---that dark matter cannot condense into clouds----(as) unfounded

Hi oldman,
as I think you indicated, ordinary matter does seem to have a much easier time clumping! It has great mechanisms for radiating away energy. With dark matter, condensing into clouds would seem to be more difficult but it evidently does happen.

As that passage of Weinberg points out, the condensation of dark matter is less complete.

I think there is no observational evidence to support the idea that dark matter cannot gather into clouds by itself----indeed that paper in the other thread, about dwarf galaxies, says that in some cases the visible matter was only 1/1000 or 1/10,000 of the total mass. One could suppose that, in a galaxy with so little visible matter, the dark matter must have managed most of the condensation "on its own hook"

However I would expect that wherever visible matter is present in significant amounts, gravitational interaction with visible will help the dark matter shed energy.

It should be interesting to check out the link(s) that George provided.

My present view is that not enough is known about dark matter, so that all this is pretty hypothetical. There is a fair amount of evidence that dark matter can gather into clouds (even with very little visible matter present) and therefore it must have means of getting rid of kinetic energy. We can only guess as to which means are the important ones. Gravitational interaction with out-flinging (you mentioned the analogy with globular cluster formation) is surely one possibility but we can't say for sure it is the main mechanism.

 

[oldman]

...a quote from Weinberg's new cosmology book, and then a couple of interesting leads.

Thanks for these useful references, especially the Weinberg extract. I'm still saving up for his book. It looks as if it is well worth its price.

My present view is that not enough is known about dark matter, so that all this is pretty hypothetical.

Yes, I agree with you. I am sure that there is lots of computer modelling of the collapse of various mixtures of dark and baryonic matter churning away, invoking appropriate virialisation mechanisms. Some serious computer time and complicated software must be involved.

In the meantime we can ponder the raw observed facts, which are that there do exist clouds of dark matter, which must surely be the result of gravitational collapse.

But I hope that workers in this field keep in the back of their minds that an extreme 'end game', as it were --- namely free-fall with the eventual formation of an event horizon --- is always available to Nature as a last resort, should virialisation prove to be too slow. And central black holes do nowadays seem to be accepted as a sort of 'observed' reality... Black holes made from Dark matter have a certain sort of symmetric appeal to me!