Interaction between dark matter and normal matter

[phinds]

Sometime during the last couple of weeks, during which I've been doing a lot of poking around on this forum and various other sources on the internet, I ran across something to which, unfortunately, I have lost the reference. It may even have been off of a link somewhere in this (cosmology) forum.

What it said, and I did not expect this, and don't understand it, is that there have been measurements of the state of things around a pair of galaxies that collided sometime in the past and the results were that while the normal matter of each galaxy was, on average, slowed by the merging of the two galaxies (which I would expect) and the result was a single misshapen galaxy but that the globes of dark matter from each moved out past the merged galaxies thus resulting in 3 blobs, the merged galaxies and 2 blobs of dark matter.

Since

• dark matter seems to interact gravitationally in the same way that normal matter does
• even a galaxy has very low density, so the slowing of the galaxies during the collision is NOT due to stars smashing together

why does the dark matter not stick around in exactly the same way? The implication seemed to be, and this may be where my lack of knowledge keeps me from seeing this, is that the NON-gravitational interaction of the galaxies did not affect the dark matter so, by inertia, it kept going.

This does not make sense to me. It is possible that I somewhat misremember the EXTENT of the lack of interaction of the two types of matter, but what I am SURE I do not misremember is that there were, exactly as I decribed above, 3 blobs (this was shown graphically ... as I recall, the whole thing was a cartoon type illustration, all of which made sense to me except for this one part.)

Thanks for any help you can give me to understand this phenomenon. If the whole thing seems preposterous, I'll dig around more and see if I can find the reference to get more information.

 

[phyzguy]

You may be talking about the collision of the Bullet cluster - two galaxy clusters that are in the process of colliding. As you said, the dark matter appears to interact only gravitationally, so the dark matter clouds just "fall through" each other and are not slowed by any type of friction. What you may be missing is that in a large galaxy or galaxy cluster, most of the ordinary matter is not on the form of stars, it is in the form of hot, ionized gas( plasma). In a large cluster, the stars represent only about 1% of the total matter. The collisions in this hot gas cause friction, which heats and slows the two gas clouds, so they merge much more easily that the dark matter clouds, leading to essentially the "three blobs" you are talking about. We know this is the case, because we can see these hot gas clouds due to the X-rays they emit. The Chandra space telescope has many X-ray images of colliding galaxies and galaxy clusters.

 

[phinds]

What you may be missing is that in a large galaxy or galaxy cluster, most of the ordinary matter is not on the form of stars, it is in the form of hot, ionized gas( plasma). In a large cluster, the stars represent only about 1% of the total matter.

AHA ... THAT'S the piece of information I was missing. I'm just getting into this stuff, so my knowledge is moderate in a few places, totally missing in others.

Thanks for setting me straight on that.

 

[narrator]

Which prompts other questions.

Galaxies must have "collided" many times in the past. If this separation effect is part of the process, there would be galaxies around in which less than expected dark matter effects are evident. And possibly a much smaller number of other galaxies which have picked up these hitchhikers, accumulating more DM than is usual. Could these separated "blobs" end up circling their galaxies in a similar way that planets find themselves doing around stars? Or is there a process which reunites them with their parent galaxies (some sort of gravitational equilibrium)? If the hitchhiker idea is plausible, then many galaxies would show deviation either side of the expected effects of DM, and there would be areas of "empty" space with a high density of DM (creating a whole bunch of effects).

 

[phinds]

Which prompts other questions.

Galaxies must have "collided" many times in the past. If this separation effect is part of the process, there would be galaxies around in which less than expected dark matter effects are evident. And possibly a much smaller number of other galaxies which have picked up these hitchhikers, accumulating more DM than is usual. Could these separated "blobs" end up circling their galaxies in a similar way that planets find themselves doing around stars? Or is there a process which reunites them with their parent galaxies (some sort of gravitational equilibrium)? If the hitchhiker idea is plausible, then many galaxies would show deviation either side of the expected effects of DM, and there would be areas of "empty" space with a high density of DM (creating a whole bunch of effects).

Interesting questions ... I just assumed that the picture being discussed was just after the collision (large amount of time by our standards but modest on a cosmological scale) and that gravitational equilibrium WOULD be restored eventually, with the 3 blobs converging. This may have been an unwarrented assumption on my part but the apparent lack of anyone (that I am aware of) having detected the kind of effects you talk about would seem to suggest that my assumption was probably correct. I too look forward to anyone with more knowledge jumping in with comments.

 

[phyzguy]

Interesting questions ... I just assumed that the picture being discussed was just after the collision (large amount of time by our standards but modest on a cosmological scale) and that gravitational equilibrium WOULD be restored eventually, with the 3 blobs converging. This may have been an unwarrented assumption on my part but the apparent lack of anyone (that I am aware of) having detected the kind of effects you talk about would seem to suggest that my assumption was probably correct. I too look forward to anyone with more knowledge jumping in with comments.

Not unwarranted assumptions at all - this is exactly what happens. The separation of the dark matter and ordinary matter is only transitory. Gravity quickly pulls them back together and re-establishes equilibrium.

 

[twofish-quant]

Galaxies must have "collided" many times in the past. If this separation effect is part of the process, there would be galaxies around in which less than expected dark matter effects are evident. And possibly a much smaller number of other galaxies which have picked up these hitchhikers, accumulating more DM than is usual. Could these separated "blobs" end up circling their galaxies in a similar way that planets find themselves doing around stars? Or is there a process which reunites them with their parent galaxies (some sort of gravitational equilibrium)? If the hitchhiker idea is plausible, then many galaxies would show deviation either side of the expected effects of DM, and there would be areas of "empty" space with a high density of DM (creating a whole bunch of effects).

Yes. The main effect that allows you to see dark matter is gravitational lensing. If you have a blob of dark matter, it will bend the light of quasars going through it, which gives you a map

http://www.space.com/3319-astronomers-create-3d-map-dark-matter.html

One thing that we have been able to establish is that the "blobs" of dark matter are galaxy sized or larger. There was an idea about a decade ago that the blobs of galaxies would be planet or star sized, but that idea is dead, since you should see a distant star go "blip" if you have a star sized chunk of dark matter move in front, and we haven't seen that, although a lot of the effort in thinking about how planet sized blobs of dark matter would affect light from distant stars turns out to be useful in detecting Earth like planets.

There are also a ton of experiments trying to see dark matter interacting with normal matter on earth...

http://en.wikipedia.org/wiki/Weakly_interacting_massive_particles

 

[Tanelorn]

"The main effect that allows you to see dark matter is gravitational lensing"

When thinking about the observation of DM in the colliding clusters, has some kind of "memory effect or artifact" of the curvature of space time, due to the effect of the movement of ordinary matter in the clusters passing through each other been ruled as a candidate for DM? Perhaps, for some reason, it takes time for the curvature of space to correct itself? One possibility might be newly created ionised hydrogen atoms left behind after collision?

 

[twofish-quant]

When thinking about the observation of DM in the colliding clusters, has some kind of "memory effect or artifact" of the curvature of space time, due to the effect of the movement of ordinary matter in the clusters passing through each other been ruled as a candidate for DM?

One problem is the "duck factor." If you have a memory or artifact effect that looks like dark matter and acts like dark matter (i.e. it looks like a duck and acts like a duck), then pretty quickly it becomes a duck.

One possibility might be newly created ionised hydrogen atoms left behind after collision?

There are reasons why we think that dark matter isn't baryonic.

Also right now with dark matter, we are more interested in figuring out what it isn't than what it is. Saying think of what it could be isn't that useful since it could be a lot of things. What are the interesting arguments are those that say that it *can't* be something.

 

[Chronos]

WMAP has ruled out baryonic matter as a significant contributor to the 'dark' matter component of the universe.

 

[twofish-quant]

WMAP has ruled out baryonic matter as a significant contributor to the 'dark' matter component of the universe.

Yup. For a non-technical reason why. Take two bricks and toss them together. They tend to create lumps. Also if you hit a brick you'll find that they conduct sound waves. Now if you toss a bunch of neutrinos or photons together, they don't form lumps, and if you shout into a flashlight beam, you'll find that light doesn't conduct sound all that well.

By looking at the galaxy distribution and the distribution of radiation from CMB, you figure out that 1) it's not very lumpy and 2) dark matter acts as if it doesn't conduct sound very well. Therefore it's not "ordinary matter."

 

[Tanelorn]

Twofish said: "One problem is the "duck factor." If you have a memory or artifact effect that looks like dark matter and acts like dark matter (i.e. it looks like a duck and acts like a duck), then pretty quickly it becomes a duck."

I agree, I was just trying to find an explanation that didnt require creating something new. In a homogenous universe I don't understand why we can't find even one particle of DM near our location.

Chronos, I will have to revisit the reasons why WMAP rules out baryonic matter. Hey I only get one hour a week on this and my memory isn't what it used to be! :)
I am very glad that you all take the time to discuss these questions on this site.

 

[phinds]

I don't understand why we can't find even one particle of DM near our location.
.

If you could figure out some way to "find" one, you would be rock star among physicists. Since it doesn't interact in any way with normal matter and it doesn't emit or reflect ratiation and no one know what it is, how would you suggest it be found?

 

[narrator]

If you could figure out some way to "find" one, you would be rock star among physicists. Since it doesn't interact in any way with normal matter and it doesn't emit or reflect ratiation and no one know what it is, how would you suggest it be found?

It must react with normal matter in some way, for it to be affected in the above scenario where galaxies collide.

 

[phinds]

It must react with normal matter in some way, for it to be affected in the above scenario where galaxies collide.

Yes, it reacts gravitationally with normal matter in exactly the same way that other normal matter does, and I stated that in the OP but neglected to repeat it in the response which you are commenting on. Unfortunately it seems that in the absence of any OTHER kind of interaction, no one has figured out a way to "see" it. Of course if we ever figure out what it IS, we'll have a better ability to focus the search, but I think even now the search is hot and heavy for the most likely candidates.

 

[twofish-quant]

If you could figure out some way to "find" one, you would be rock star among physicists. Since it doesn't interact in any way with normal matter and it doesn't emit or reflect ratiation and no one know what it is, how would you suggest it be found?

You figure out what something is, by figuring out what it isn't.

What you do is to make some guesses as to what dark matter could be, look for those guesses. If you find it, then you win. If you don't find it, then you also win since you've established that dark matter *isn't* what you were searching for.

Here is a recent survey

http://lss.fnal.gov/archive/2009/conf/fermilab-conf-09-486-a.pdf

And here are some of the experiments under way...

http://xenon.physics.rice.edu/
http://cdms.berkeley.edu/
http://www.antares.physik.uni-erlangen.de/Members/motz/public-material/indirect-dark-matter-search-with-antares-1/

 

[twofish-quant]

Unfortunately it seems that in the absence of any OTHER kind of interaction, no one has figured out a way to "see" it.

No. You can see dark matter by gravitational lensing.

http://www.astro.gla.ac.uk/users/martin/outreach/lensing.html

Of course if we ever figure out what it IS, we'll have a better ability to focus the search, but I think even now the search is hot and heavy for the most likely candidates.

It's the other way around. We look for lots of things. If we can't find that, then we know what it isn't. Once you've cross off all of the things that it can be, then you are left with what it can be.

It's like a game of twenty questions.

 

[phinds]

No. You can see dark matter by gravitational lensing.

Yes, I understand that that's one of the main ways that its existence was confirmed experimentally, but that does nothing to tell us what a single dark matter particle is, or even whether or not it is a single particle. That is, it doesn't tell us what it IS, it just tells us one of the things that it DOES.

It's the other way around. We look for lots of things. If we can't find that, then we know what it isn't. Once you've cross off all of the things that it can be, then you are left with what it can be.

I do understand that that is how the search is progressing, but that just keeps telling us what it isn't. How can we be sure we have ruled all but one thing by confirming that it isn't anything else. Could it not be something OTHER that the one remaining thing? Something that we've not otherwise ever experienced?

 

[narrator]

Yes, it reacts gravitationally with normal matter in exactly the same way that other normal matter does, and I stated that in the OP but neglected to repeat it in the response which you are commenting on. Unfortunately it seems that in the absence of any OTHER kind of interaction, no one has figured out a way to "see" it. Of course if we ever figure out what it IS, we'll have a better ability to focus the search, but I think even now the search is hot and heavy for the most likely candidates.

Two things here.

It's probably been asked already, so please excuse my lack of finding it. DM has been used to explain things which are light-years from us, but has there been any thing more local to demonstrate its existence? And by local, I mean anywhere from our solar-system down to atomic level.

The second thing, and again this has probably been asked. What has been done to detect its existence within this more local area? I imagine that if DM is affected by gravity, there could be a way to indirectly detect it experimentally, here on Earth.

 

[twofish-quant]

Yes, I understand that that's one of the main ways that its existence was confirmed experimentally, but that does nothing to tell us what a single dark matter particle is, or even whether or not it is a single particle. That is, it doesn't tell us what it IS, it just tells us one of the things that it DOES.

But it does tell us that it is dark matter. There are a number of proposals out there that say that dark matter doesn't exist and that what we are really seeing is modified gravity. The fact that we see gravitational lensing suggests that it really is matter.

How can we be sure we have ruled all but one thing by confirming that it isn't anything else.

We can't. :-) :-) :-)

But what happens is that you come up with a list of things that it can't be, and then you have creative folk try to figure out what it can be. After a while, you can say, it's either a duck or something that acts a lot like a duck.

Could it not be something OTHER that the one remaining thing? Something that we've not otherwise ever experienced?

Right now, we *know* that it is not a known particle. You can list all of the known particles in a list, and after you put in the properties of dark matter, and we've already crossed all of them off. So we *know* dark matter is something we haven't directly observed before.

So what you do is to list all of the properties of hypothetical particles, and then try to figure out what it can't be. We know that whatever it is, it can't react with ordinary matter at greater than X rate, because otherwise we've seen it. We know it can be a standard matter/anti-matter mix because we would have seen something.

Maybe in twenty years, we will have established that it can't be a particle at all, which would be cool, but we aren't at that point yet. It could also be that everything that we now have on the list gets crossed off, in which case we just have to think harder as to what we may have missed. Or not...

 

[Tanelorn]

twofish, If not a particle at all, then perhaps DM could be a memory affect of the curvature of space time, or curvature of the aether of space itself?

 

[Chronos]

Dark matter is like neutrinos - we were fairly certain neutrinos existed, but, it took decades to actually detect them. Dark matter is a similar case only DM is to neutrinos like neutrinos are to baryonic matter - difficult to detect. We are confident DM exists as it is the only reasonable explanation for large scale structure formation in the early universe - not to mention the bullet cluster.

 

[DavidMcC]

Dark matter is like neutrinos - we were fairly certain neutrinos existed, but, it took decades to actually detect them. Dark matter is a similar case only DM is to neutrinos like neutrinos are to baryonic matter - difficult to detect. We are confident DM exists as it is the only reasonable explanation for large scale structure formation in the early universe - not to mention the bullet cluster.

True, but the actual particles might be from some big bangs that we never saw with light!

 

[Chronos]

All the more reason to ascertain the nature of DM - which probably is not a single, unique particle, IMO.

 

[DavidMcC]

All the more reason to ascertain the nature of DM - which probably is not a single, unique particle, IMO.

I agree. It may even be that not ALL "dark matter" is as dark as the kind I'm thinking of.