[thedoc (OP)]

Mike Garrard asked the Naked Scientists:
   
Please can you explain the current theories on why we don't detect dark matter stars?

Why doesn't the dark matter halo of one galaxy start pulling the visible matter out of another colliding galaxy?

Why can't we detect any local  solar system effects?
 
When dark matter is mentioned, the proof always appears to be the rotational pattern of galaxies.  Surely there is more?
 
Thanks
 
Mike Garrard

What do you think?

[yor_on]

Nice question.

Don't think anyone seen it yet.
Glast hasn't at least, as far as I know?

First read this. http://www.pbs.org/wgbh/nova/physics/blog/tag/glast/ numbering different approaches.

Then have a look here http://www.nasa.gov/mission_pages/GLAST/news/dark-matter-insights.html

And then for the technical aspects of what Glast can do http://www.particle.kth.se/~tomiy/material/LicentiateThesis_TYlinen.pdf

[graham.d]

Mike Garrard asked the Naked Scientists:
   
Please can you explain the current theories on why we don't detect dark matter stars?

Why doesn't the dark matter halo of one galaxy start pulling the visible matter out of another colliding galaxy?

Why can't we detect any local  solar system effects?
 
When dark matter is mentioned, the proof always appears to be the rotational pattern of galaxies.  Surely there is more?
 
Thanks
 
Mike Garrard

What do you think?

1. Dark matter does not seem to interact with other matter (at least not very much), or with itself, except gravitationally. Star formation relies on aggregation of material, which, in turn, relies on other forces, electromagnetic, strong forces and weak forces to bind the aggregate together and absorb and radiate the incident kinetic energy. Without this, material will remain in complex eliptic orbits roughly spherically dispersed around the centre of mass.
2. There would be a general gravitational attraction on another galaxy from the composite normal + dark matter components in a galaxy but remember that galaxies are mostly empty space. Galaxies would mostly pass through each other. The inverse square law means that the strong effects that would cause large disturbances only occur when a star in one galaxy passes close to the star in another. Because dark matter seems thinly dispersed, the way it acts is not so dramatic.
3. Local effects of dark matter are hard to detect because the density is very, very low.
4. The effects on galaxy rotation is very strong evidence indeed. The Virial Theorem relates the kinetic and potential energies of bodies in a closed system. Both these can be fairly well determined by relating the doppler effect from radiation from stars in relation to their distance to the galaxy centre and the brightness of the star. The type of star is also relevent. It turns out that, if all the matter in a galaxy were made of visible material, either the virial theorem is wrong, which is very unlikely, or there is other matter we can't see. More careful modelling of this gets a good fit with measurements if the distribution of the dark material is evenly distributed in a roughly spherical shape centred on the galactic centre and extending quite away beyond the visible edge of the galaxy.

This is my rough understanding of it anyway. Hope it helps.

[yor_on]

Quite concise Graham

Did you see this?

"The team examined two years of LAT-detected gamma rays with energies in the range from 200 million to 100 billion electron volts (GeV) from 10 of the roughly two dozen dwarf galaxies known to orbit the Milky Way. Instead of analyzing the results for each galaxy separately, the scientists developed a statistical technique -- they call it a "joint likelihood analysis" -- that evaluates all of the galaxies at once without merging the data together. No gamma-ray signal consistent with the annihilations expected from four different types of commonly considered WIMP particles was found.

For the first time, the results show that WIMP candidates within a specific range of masses and interaction rates cannot be dark matter. A paper detailing these results appeared in the Dec. 9, 2011, issue of Physical Review Letters. "The fact that we look at 10 dwarf galaxies jointly not only increases the statistics, but it also makes the analysis much less sensitive to fluctuations in the gamma-ray background and to uncertainties in the way the dark matter may be distributed around the dwarfs," said Maja Llena Garde, a graduate student at Stockholm University in Sweden and a co-author of the study.

For any given properties of a dark matter particle, the distribution of the particles has a significant impact on the expected gamma-ray signal, a wrinkle that often is handled inadequately, if at all, in previous studies."

If there is 'dark matter' it seems to act very inconspicuous with ordinary matter?

[Soul Surfer]

In order to condense into a small volume gravitating matter has to loose energy this is because as things collapse they move faster and faster and unless they hit something or radiate they just whip by and shoot out the other side.  Ordinary matter can do this by radiating electromagnetic energy and particles, that is getting very hot, and can therefore collapse to form stars.  For particles that interact only by gravity to lose significant amounts of energy by gravitational wave radiation they need to get extremely close (down around the planck limit).  This is an extremely improbable interaction and so almost never happens so dark matter cannot condense into star sized objects only quite cold galaxy sized objects.

[Mike G]

Thanks.  Unfortunately those links don't actually answer the core question.  The only repeatable evidence seems to be gravitational effects eg: orbital speeds, gravitational lensing.  Gravity pulled visible mass particles into star, solar system and galaxy sized clumps.  Why not dark matter?  What about the dark matter on the axis of rotation and above the galaxy, why is that not falling in?


@ Graham:
1. Can you support that electromagnetic, strong forces and weak forces are required to bind aggregate together?  Gravity would seem sufficient up to the point where the particles are orbiting up to the speed of light.  If we ran a model of particles where the only force was gravity, would you not expect them to clump?
2. You start with the assumption that's it's thinly dispersed then use that to explain lack of interaction for galaxies.  I'm asking why it's thinly dispersed.  Running some numbers, for lack of interaction it has to be pretty even in interstellar space.  What is your theory as to why a grativationally active particle would remain evenly distributed?  In the link, the bullet cluster is used as evidence: yet the gas clouds lag the galaxy clusters *because* they are thinly and evenly dispersed: they interacted whilst the galaxies missed each other.
3. See #2.  My question is why is the density low.
4. I'm not disputing the existance of dark matter.  I'm asking why it doesn't clump.



@ soul searcher:
You seem to agree with Graham's point #1.  I accept this argument against dark matter 'stars'.  To me there is a large gap between an argument that says you can't get the density of stars, and one that says it must be so thin as to be undetectable.  Why no solar system sized clumps? 

[Soul Surfer]

As I said its roughly galaxy sized clumps that's why galaxies are the size they are

[Mike G]

SS, that's a circular argument, galaxy sized clumps because that's the size of galaxies.  If neutral dark matter particles can coalesce should they get within the Planck distance, then that is an argument for dark matter black holes.  The parameters then become: how much dark matter can be gravitationally captured, and how much time.

Here's a thought: if the theory that colliding WIMPs change into gamma rays then maybe that thins them down to this 'undetectable at the small scale' density that is required for our solar system to work.

[graham.d]

@ Graham:
1. Can you support that electromagnetic, strong forces and weak forces are required to bind aggregate together?  Gravity would seem sufficient up to the point where the particles are orbiting up to the speed of light.  If we ran a model of particles where the only force was gravity, would you not expect them to clump?
2. You start with the assumption that's it's thinly dispersed then use that to explain lack of interaction for galaxies.  I'm asking why it's thinly dispersed.  Running some numbers, for lack of interaction it has to be pretty even in interstellar space.  What is your theory as to why a grativationally active particle would remain evenly distributed?  In the link, the bullet cluster is used as evidence: yet the gas clouds lag the galaxy clusters *because* they are thinly and evenly dispersed: they interacted whilst the galaxies missed each other.
3. See #2.  My question is why is the density low.
4. I'm not disputing the existance of dark matter.  I'm asking why it doesn't clump.

I would start by saying this is not my theory but my take on the current ideas on the subject. Nobody really understands the properties of dark matter so I think the idea is to find models that fit the observations of galaxies and galaxy clusters. These approximately spherical models of a uniform distribution of dark matter (at a low density) seem to work quite well at fitting with the Virial theorem. There may be other models that work too but the spherical model has an elegent simplicity.

Gravity is insufficient, on its own, to cause clumping; or, maybe, even to give the models proposed. Without other forces the dark matter will simply pass through other matter or other "particles" of dark matter without any interaction. They would all be in an endless dance about some centre of gravity (as the model suggests). The only potential loss of energy would be through gravitational waves, but maybe others could comment on this. It maybe that there is some interaction with other matter (or itself) but the models would suppose this to be very small.

You may be right that dark matter particles have enough interaction to lose sufficient energy to eventuall coalesce, but this may take many more billions of years and the universe is not old enough to have allowed this to happen yet.

[Mike G]

Graham, thanks.  This has helped me grasp the theories.  For this to apply even direct collisions of dark matter must be elastic with no eneregy loss or completely lacking in interaction except gravity, passing through each other as you say.  Accepting that point, a two particle system would not coalesce.  However as gravity is inverse square attraction, intuitively I would think a density of particles would still congregate about their common centre of mass as they all followed increasingly tighter local orbits.  The squared function would cause them to bind gravitationally to the particle nearest to which they pass.  End result is clumping.  What happens with the model that prevents this?

[Soul Surfer]

Nothing it depends on the initial "temperature" of the dark matter particles it is generally considered to be quite cold and the temperature of the environment for our universe around the 2-3K CMB temperature away from major sources of energy.

All condensations of matter have a gravitational escape velocity and as long as the particles are below this velocity condensation will continue.

As the particles condense the release of gravitational potential energy causes the particles to gain kinetic energy, that is move faster and become hotter.  At some point unless there is a way of getting rid of this energy the condensation will self limit by the fact that as many of the particles "evaporate" as join the condensation.  The best theoretical estimates suggest these condensations are around the size of galaxies with escape velocities in the range around hundreds of kilometres per second.

It is only the thermal electromagnetic radiation from condensing stars into the cold of space that allows stars to form in the first place.  When the universe was very young it was too hot for stars to form.

[Mike G]

Thanks, good information.  I'm still struggling though.  "Evaporate" into what?  Clearly not photons or something we can detect.  I thought the low temperature was a function of expanding space time: is the concept that the 2-3k is still sufficient for dark matter to be above escape velocities?  Is the sphere around a galaxy a dynamic oscillation of multiple particles just barely gravitationally bound, and if so, why does the visible matter end up as a disk inside a larger gravitational sphere?  Please could you give me the outline rules used in the models, for example (guessing)
* 100x mass of a proton hence bulk effects
* very low density, 10^8 less than visible matter hence no local effects
* only gravitational interaction, even with direct collisions
* all particles always moving faster than escape velocities

[Soul Surfer]

"Evaporate" the particles just escape unchanged into less densely occupied space outside the growing condensation.

I am not sure that I fully understand your request for "rules" or your suggested parameters there may be some detail that I am missing.  This is the best I can do

I made no assumptions about the masses of the particles involved although lower mass particles will be moving faster at a given temperature.

I made no assumptions about particle density.

 "local effects"   local mass concentrations will create changes in the mean density of dark matter particles.  Experiments are currently being conducted to try to detect this effect as a tiny error in the inverse square law.  however all DM particles will just move through stars etc on hyperbolic orbits

The escape velocities of smaller gravitating bodies are irrelevant because the particles cannot lose energy by collisions.  Gravitational interactions with fundamental particles could occur and trap them but they are extremely improbable because of the tiny cross sections involved.  much smaller than neutrinos and they are hard enough to detect.

[Mike G]

Latest NS podcast has Dominic quoting a paper on DM: within earth radius it implies equivalent mass of 1.5 mile diameter asteroid, pretty small.  However DM does appear to be in the plane of the galaxy.

"particles cannot lose energy by collisions": this must be a requirement to feed models that match (lack of) observation?
"tiny cross sections involved": do you mean relative to the average density, or is the theory of lack of interaction based upon the size of the particle?

[imatfaal]

Latest NS podcast has Dominic quoting a paper on DM: within earth radius it implies equivalent mass of 1.5 mile diameter asteroid, pretty small.  However DM does appear to be in the plane of the galaxy.

  I think DM physics does need to answer a lot of questions - I am not sure there is a nice compelling answer to the patchiness of DM.  We also need to start detecting it directly.  I am not a MOND-ite or denier :-) but there is still a lot of unanswered problems.

"particles cannot lose energy by collisions": this must be a requirement to feed models that match (lack of) observation?

No - I don't think so.  At a small enough level - and importantly without electromagnetic interaction - we can assume that collisions are completely elastic; ie particles will go in with an a joint amount of KE and come of the collision with same joint amount of KE.  For a cloud of particles to clump there needs to be a method of dispersing the inherent KE - particles which interact via electromagnetic force can do this, but dark matter cannot disperse its KE and thus it keeps on moving

"tiny cross sections involved": do you mean relative to the average density, or is the theory of lack of interaction based upon the size of the particle?

  I think you can read cross-section as meaning likelihood - the origins of the term are to do with area of target over total area but in nuclear/particle physics other non-classical things must be taken into account

[evan_au]

The dominant theory of dark matter particles is that they interact only by gravity.

This suggests that the only way to confine them within a small volume is if they run directly into the event horizon of a black hole, which will capture them.

[Mike G]

I'm not talking about a small volume.  Gravity manages to pull dust clouds together into stars.  Would I be right in saying that until these particles start colliding, they won't be radiating and losing the energy required to aggregate into greater density?  I'd think that would be about the size of a solar system due to the lack of visible matter in interstellar space.  The jist of the responses and the paper quoted (actually 12th Aug NS) imply it's at least interstellar sized, which then means these clumps are merged.  We have an idea of the actual density, very low.  I think that sets some boundaries on the speed*mass of the particles in order to balance the gravitational attraction at the known (?) density.  So DM is already clumped to maximum density.