Starship Asterisk*

I'm confused about the APOD for today, 9-17. In order to be able to even artificially add a color to dark matter in a photo, don't we have to know something about what dark matter is?

I confess I am really pretty ignorant about most astronomy matters, so I apologise for that in advance. APOD's my home page because I love learning about space...I guess I just thought dark matter was theoretical?

On the nature of Dark Matter and Dark Energy, we are literally feeling our way in the dark, because we don't have any in the laboratory.

Once we get some to examine, things will be different. All the pure speculative theory will sort itself out against the available facts. Dark Matter and Dark Energy may turn out to be as mundane and easily understood as peanut butter and chocolate. They may also disappear like the Ether of pre-modern Physics in favor of a better theory that comes to "Light". Until then all we can do is measure things indirectly and infer.

Today's APOD ... http://antwrp.gsfc.nasa.gov/apod/ap080917.html ... and explanation, at least for me, leave quite a few questions to be answered. We obviously didn't record Dark Matter (because we can't) and then paint it a different color. As I understand it, we inferred the position of the Dark Matter from the disagreement between the gravitational lensing data and the X-ray data, and then we painted the calculated area. So maybe we need to know something about Dark Matter to color Dark Matter, but no - we don't need to know much about Dark Matter to color the photo.

The nature of Dark Matter is the subject of theory. Its existence is unquestionable. We know it is real because its influence disturbs our observations and theories which we are still trying to get tied up into a neat package. Incorporating Dark Matter and Dark Energy into what we thought we already knew is not a small drop in the bucket of progress in Physics - this is a Double Whopper.

craigellachie wrote:I confess I am really pretty ignorant about most astronomy matters, so I apologise for that in advance. APOD's my home page because I love learning about space...I guess I just thought dark matter was theoretical?

I was the same way until recently(about the dark matter)

The APOD picture is two superimposed pictures in false color of the energy levels of gases between stars. The placement of the Dark Matter is implied by the energy differences. The blue represents the Dark Matter and the cooler temperatures that remained bound the the clusters. Because Dark Matter doesn't interact with normal matter, it met no resistance and stayed bound to the clusters. The pink is the normal matter that collided as the two clusters passed each other creating higher temperatures/energies and stripping the normal (gassy) matter from the clusters, as the "Blue" areas drift apart the "Pink" area will remain behind because it's inertia has been robbed and converted to heat.

Sorry, its not easy to justly sum this up in one paragraph.

Dr. Skeptic wrote:Sorry, its not easy to justly sum this up in one paragraph.

Like Fermat said, "I have a neat little proof but it won't fit in the margin of this page." Or the answer might be 42. Seriously, you understate the scope of the summation task. But after I read your explanation 3 or 4 times more, I see that it does explain why the blue and the pink are where they are. I suppose it also explains why the pink is centered on a relatively vacant area - am I correct in thinking that the clusters are now past the center of the collision and that the pink is collision residue?

Correct. Well put.

Dark matter doesn't interact with normal matter except through gravity. (Just thought I'd clarify.)

The picture shows where dark matter has to be to give us the gravitational lensing effects we observe. The interesting thing is that when the galaxies collide, the dark matter just slips right through itself and through the ordinary matter, and out the other side. The only interaction seems to be through gravity. So what acts like that? Try to picture it. It's weird and wonderful.

What I'm picturing is almost like stuff that's in another dimension close to the ones we experience. It doesn't share our space, so it doesn't bump into stuff here. It just attracts it (and is attracted) gravitationally somehow. (/very unrigorous vague ideas)

It would be interesting to find out if dark matter is made up of point sources or not. How could we test that?

Tatiana wrote:It would be interesting to find out if dark matter is made up of point sources or not. How could we test that?

Remotely? Difficult until you create a new method. If gravity is all we can see of it, that's all we can measure, and a gravity detector with a resolution of 1E-19 meter at a distance of millions of light years isn't on the shelf at Radio Shack this season.

In the Laboratory? Not currently possible. However, we don't need to go to far away places to get some Dark Matter to bring to the lab. Presumably there's plenty of it closer, but we need two new methods: one to contain and transport it, and one to analyze it once we have captured it.

By inference? That's a good possibility, like the inference used to locate Dark Matter from the gravitational lens observations. But you need to match available observation methods with a new theory that will test for the answer to your question, so put your visualization circuits in overdrive.

A thought experiment! Not hard science, but modern physicists love them. Okay, I'm done thinking now, and here's what I saw:

Apart from an occasional singularity, the dimensions of space and time are smooth and continuous. Everything else comes in discrete pieces, so Dark Matter and Dark Energy come in discrete pieces (particle-like and ray-like entities) too.

Dr. Skeptic wrote:Sorry, its not easy to justly sum this up in one paragraph.

How about "something that our equations say is there, but we have no idea what it is"

craigellachie wrote:I guess I just thought dark matter was theoretical?

So is normal matter. We just have a bit more facts at hand to support it.

I hope dark matter doesn’t take the same highway as planetary nebulae… say we discover the name isn’t all that appropriate. What if it turns out to be bright when immersed in water? Or sub-atomic particles that glow under magnification?

I suggest we start calling it “what’s the matter”. That will leave the final naming convention open to a more descriptive designation.

For example: What's the matter in this image... wait... that could be confusing...



... maybe we should call it "God's matter" in line with the elusive boson.

I thought that the image showed the bigger size (Volume) of the dark matter distribution around the 2 clusters of galaxy colision. I didnt thought that the dark matter was only on each side of it but all around the centered colision. Meaning that there are dark matter all around those 2 cluster galaxies colision. Dark matter in front and behind the colision too.

So, is dark matter all around that giant colision or is it realy only on each side?

http://antwrp.gsfc.nasa.gov/apod/ap080917.html
http://antwrp.gsfc.nasa.gov/apod/ap080823.html

Reading the explanation from both of the above APODs I get the impression that the hot x-ray emitting gas of the extra-galatic normal matter is centered around the area of collision of the clusters. The extra-galatic dark matter seems to be more closely associated with the center of mass of the colliding clusters, displaced somewhat towards the center of collision.

Keep in mind the dark matter is only indirectly observed by its effects on gravitational lensing, while the x-ray emission is a direct result of the collision of normal matter.

However, Doum's observation of the dark matter being all around the collision center may be the correct one. I had thought that since dark matter does not interact with normal matter, it would pass through relatively unscathed, while the normal matter would get bound up in the collision. Instead, it may be that the dark matter has gravitationally bound the normal matter, centralizing it. On the otherhand, in the Bullet Cluster example, the normal matter seems to be re-seperating, getting pulled apart by the dark matter as the individual clusters continue on their courses.

For another view of a dense galaxy cluster and xrays, see Abell 1689:
http://www.space.com/imageoftheday/imag ... 80918.html
http://antwrp.gsfc.nasa.gov/apod/ap040627.html