Starship Asterisk*

APOD for 5/16/07 (http://antwrp.gsfc.nasa.gov/apod/ap070516.html) shows a simulated dark matter ring around Galaxy Cluster CL0024+17. The work is the product of Jee and Ford, et al, of Johns Hopkins University.

In a separate NY Times article at http://www.nytimes.com/2007/05/16/scien ... ubble.html, it was reported "Richard Massey, an astronomer at the California Institute of Technology, said the findings were facing skepticism within the astronomical community."

Why is the work by Gee and Ford considered controversial?

I'm no expert on lensing but I've heard some grumblings about this from people who are

What is wrong with gravitational lensing? I think its wonderful!

I am just a passing reader and enjoy APOD very much. The article of 2007 May 16 has a very good point of view: We see the picture and start to wonder why there are several similarities between some of the bluish galaxies.

I think the answer to the question about controversy is one of certainty-level. Starting with the distance to the nearest stars much of what we know is deductions, which will be questionable for years or rather centuries to come. Some are more certain than others, of course;-)

But gravitation itself is an area of intense study and I see this posting as an outcome of this study. Look "around" in the sky. Find more gravitational lenses, find more gravitation signs and we will know more.

So, is this kinda like a black hole? Aren't black holes and dark matter the same?

There's nothing wrong with gravitational lensing. The controversy stems from the details of how this particular piece of work was carried out

sacolton wrote:So, is this kinda like a black hole? Aren't black holes and dark matter the same?

No.

Look closely at the center of the image, at the 4 large elliptical galaxies and you will see a small blue galaxy in the middle of the pack. (It is shaped like a "B" of sorts.) This is one of the galaxies that are being lensed by the cluster. There are 4 or 5 occurances of this galaxy within the inner portion of the large dark matter ring.
It shows up really well in the center of this high resolution image of the cluster http://www.spacetelescope.org/images/la ... c0709b.jpg

cosmo_uk wrote:There's nothing wrong with gravitational lensing. The controversy stems from the details of how this particular piece of work was carried out

That doesn't surprise me. If I understand the technique, they were trying to figure out, based on the distortions, where the dark matter was located. It's like looking at a fun-house mirror and trying to figure out what shape the mirror is by what you're reflection looks like...only harder because you can't actually see the mirror. I imagine it's incredibly math intensive, and that it may even be possible for different matter distributions to result in similar lensing effects (the same issue occurs in the field of cryptography).

If the findings are upheld, they are important because they aren't consistent with most MOND theories. According to MOND, the dark matter effect should only be associated with visible matter, and this appears to be focused in some cases in regions devoid of visible matter.

The bullet cluster is another observational case I think is inconsistent with MOND.
http://antwrp.gsfc.nasa.gov/apod/ap060824.html

Thanks for the hilarious description of what constitutes the controversy:

"... looking at a fun-house mirror and trying to figure out what shape the mirror is by what you're reflection looks like ..."

I like that description.

I think the bullet cluster is a much more convincing argument for DM

My problem with discussions on dark matter is that in my heart of hearts I want to believe the math could be wrong more than I want to believe something that can't otherwise be detected has so much mass that it's bending light.

What if the generally accepted math that describes the falloff of gravity over distance is off a little bit? Perhaps the formula is so close to reality at short distances as to be verified by all known experiments, yet off when it comes to loooong distances. Perhaps whomever set down the laws of the universe couldn't make it all work out quite right without adding a fudge factor.

Or maybe there are just more black holes or cold gas out there than are being accounted for.

There are a lot of "givens" that this result is being based upon.

-Noel

NoelC, you're basically describing MOND. It could be analagous to relativity. Newton's laws described everything well at the velocities we were used to, but as we encountered higher velocities, especially being able to measure the speed of light, we ran into problems. There might be something similar with distance and gravity. This bothers cosmologists and astrophysicists, as did relativity back in the day, partially because it screws up to varying degrees a lot of existing work, partially because it's just bizarre and unexpected. However, as I mentioned before, this particular observation either contradicts MOND, or makes it twice as bizarre.

Dark matter became a lot more palpatable as I learned a little more about the various particles out there besides the electrons, photons, and quarks we're familiar with, and extreme case effects among those particles like superfluidity and superconductivity...those two both relating to electromagnetic effects.

Cold gas and black holes seem unlikely. We're talking over 5 times as much mass as what is visible and 50 times what is contained in actual stars (~90% of the visible mass is gas and dust). Even cold gas counts among the visible, because just by being warmer than absolute 0 it has to radiate weakly. I believe one of the main uses of observatories like the gigantic Parkes Radio Telescope is mapping out this cold gas and dust based on its low frequency emissions, for example.

Black holes likewise become visible when they interact with other matter, for example, emitting x-rays from an accretion disc.

These black holes and cold gas would have to be distributed in similar haloes that dark matter is posited to form around galaxies to provide the observed influence.


It's all so complicated...and we haven't even touched on dark energy!

BMAONE23 wrote:Look closely at the center of the image, at the 4 large elliptical galaxies and you will see a small blue galaxy in the middle of the pack. (It is shaped like a "B" of sorts.) This is one of the galaxies that are being lensed by the cluster. There are 4 or 5 occurances of this galaxy within the inner portion of the large dark matter ring.
It shows up really well in the center of this high resolution image of the cluster http://www.spacetelescope.org/images/la ... c0709b.jpg

Actually I see a whole bunch of bluish colored galaxys smeared in the arc shape characteristic of gravitational lensing. Is it possible that this amount of 'lensing' may be accounted for by the gravitational field of the central cluster of galaxys? And is the central cluster closer to us so as to have such an effect?

Actually I see a whole bunch of bluish colored galaxys smeared in the arc shape characteristic of gravitational lensing. Is it possible that this amount of 'lensing' may be accounted for by the gravitational field of the central cluster of galaxys?

It is not possible for only the luminous matter to account for the amount of lensing seen, there must be some unseen mass, either that or the stellar populations of the galaxies must be extremely strange (you have to assume a reasonable mass to light ratio to work out how much mass is implied by the light you see). Some of the unseen mass is in the form of the hot x-ray emitting gas between the galaxies, the rest, presumably is dark matter.

AZJames wrote:

BMAONE23 wrote:Look closely at the center of the image, at the 4 large elliptical galaxies and you will see a small blue galaxy in the middle of the pack. (It is shaped like a "B" of sorts.) This is one of the galaxies that are being lensed by the cluster. There are 4 or 5 occurances of this galaxy within the inner portion of the large dark matter ring.
It shows up really well in the center of this high resolution image of the cluster http://www.spacetelescope.org/images/la ... c0709b.jpg

Actually I see a whole bunch of bluish colored galaxys smeared in the arc shape characteristic of gravitational lensing. Is it possible that this amount of 'lensing' may be accounted for by the gravitational field of the central cluster of galaxys? And is the central cluster closer to us so as to have such an effect?

How come the other galaxies don't seem to be repeated? The bluish galaxies have enough differences as to cause suspicion of lensing. Maybe we want to find that missing matter too much.
Orin

The 'bluish galaxies' happen to be the only ones properly aligned with the gravitational lens and ourselves for us to be able to see them being lensed.

Qev wrote:The 'bluish galaxies' happen to be the only ones properly aligned with the gravitational lens and ourselves for us to be able to see them being lensed.

I still think the bluish galaxies have enough differences to to be suspect of being lensed. How can we be certain of lensing here?
Orin

orin stepanek wrote:

Qev wrote:The 'bluish galaxies' happen to be the only ones properly aligned with the gravitational lens and ourselves for us to be able to see them being lensed.

I still think the bluish galaxies have enough differences to to be suspect of being lensed. How can we be certain of lensing here?
Orin

There are quite a few, quite good, tests that you can use to see if it's lensing.

For example, there's consistency: in cases where there are multiple images of the one background galaxy, you can use a 'ray tracing' approach to reconstruct an unlensed view of that galaxy; if it's not lensing, you can't get a consistent reconstruction.

Nereid wrote:

orin stepanek wrote:

Qev wrote:The 'bluish galaxies' happen to be the only ones properly aligned with the gravitational lens and ourselves for us to be able to see them being lensed.

I still think the bluish galaxies have enough differences to to be suspect of being lensed. How can we be certain of lensing here?
Orin

There are quite a few, quite good, tests that you can use to see if it's lensing.

For example, there's consistency: in cases where there are multiple images of the one background galaxy, you can use a 'ray tracing' approach to reconstruct an unlensed view of that galaxy; if it's not lensing, you can't get a consistent reconstruction.

http://www.mpa-garching.mpg.de/~hilbert ... P_2006.pdf

That's just it. I see a lot of the blue blobs. Maybe that's what you mean by consistency; but they just don't look alike to me. If this is lensing than there must be a lot of distortions. I guess I was looking for more perfect duplications. There must be another factor causing the distortions!?!
Orin

This tutorial might help ...

Nereid wrote:This tutorial might help ...

OK; Thanks! I see that distortions are the norm according to that info. Live and learn!!!
Orin

I don't know anything. That said, as a kid, I "knew" that for a galaxy to rotate around a central point, (like our solar system does around the massive sun) something must be massive enough to be "the galaxy's sun" How else? Now we have evidence of supermassive black holes @ the core, right?
I know, get to the point, here is my idea. Flamers, on your marks!
What if-- there was no dark matter?
I know that the "?measured?" matter doesnt produce for the proven theories about gravity.
So, we came up with DARK MATTER to explain what we dont know about gravity.
If Dark Matter has the same pull, then does it make sense to see dark matter grav lensing? How can it bend light (or block)if we cant see it?
Our planet is made up of a lot of parts, little pieces of matter all together. Collectively, they keep us glued to terra firma.
What if--Gravity didn't ADD UP? 3+3=6
Maybe it adds together. 3+3+Stronger by proximity. Or 3+3*prox? Or scarier, 3*3*prox?
Maybe gravity multiplies with proximity to matter, and gravity waves send a "bow shock" like our sun's magnetosphere pushing out? That might be able to do it.
Matter, Dark matter. Matter, Anti-matter. It's not like it's rocket science or anything. Figure it out ffs.
just a theory. I paint cars.

8) As in most 'articles' about dark matter, 'dark matter' can be replaced by 'plasma ******' and that makes much better sense. In this case: If 2 groups of galaxies crash, the reflections would probably be in a growing sphere. But the computer modeling shows a ring. Really wild that the ring could be seen only where we are. Much more likely would be a number of plasma sheaths radiating around - similar to the cells on the sun's surface. In this case we could be looking down in ONE of the rather large plasma sheaths. Even those 'spokes' in the image make more sense in plasma than in 'dark matter.' The gravitational lense effects is visible, but if the images are gentling, it's because of plasma, not dark matter. The astronomers could take better time looking for the beams going up and down from the center of our galaxy.

lankytom wrote:8) As in most 'articles' about dark matter, 'dark matter' can be replaced by 'plasma ******' and that makes much better sense. In this case: If 2 groups of galaxies crash, the reflections would probably be in a growing sphere. But the computer modeling shows a ring. Really wild that the ring could be seen only where we are. Much more likely would be a number of plasma sheaths radiating around - similar to the cells on the sun's surface. In this case we could be looking down in ONE of the rather large plasma sheaths. Even those 'spokes' in the image make more sense in plasma than in 'dark matter.' The gravitational lense effects is visible, but if the images are gentling, it's because of plasma, not dark matter. The astronomers could take better time looking for the beams going up and down from the center of our galaxy.

This certainly seems intuitively plausible, doesn't it?

However, AFAIK, none of the 'plasma' approaches work ... in fact, they fail rather spectacularly.

Why?

One reason is that the estimated mass of the DM is a) mostly between the galaxies in the cluster, and b) invisible - if it were plasma, it'd be lit up ~10+ times more brightly than we actually see it to be.

And those are just the direct observational problems with any kind of 'plasma' explanation ...