~Maestra Myers

The spreading infrared luminosity from the central cloud of hot gas in between the two clusters of Abell 520 could actually be causing the gravitational light bending. In the 1919 solar eclipse study, it could have been the hot spreading luminosity from sun that causing the observed light bending an not the yet-to-be-specified property of the mass of the sun that is causing the warping of the nearby space that the background starlight would appear as bent.

I have been trying for years to get my experiments replicated so someone besides me would start to believe that spreading infrared radiation is gravitationally attractive. And now maybe Mahdavi et al have found evidence that collaborate my experiments ,which in a simple, inexpensive way demonstrate that spreading heat is attractive.

I think I can answer some, but not all, of your questions.

1. A cluster of galaxies is a group of galaxies that are gravitationally bound to each other.

2. Abell 520 apparently formed from the collision of two or more clusters of galaxies. If the the various elements of the conglomerate remain gravitationally bound together, they will have formed a larger cluster of galaxies (Abell 520).

3. Each galaxy, or particle of dark matter or gas within a cluster before the collision would have its own motion relative to the other parts of the cluster added to the motion of the whole cluster relative to the rest of the universe. After the collision, there would also be a tendency to orbit the center of mass of Abell 520. This tendency to orbit the center of mass of Abell 520 would apply to all three elements, the galaxies, the dark matter, and the gas. But each object’s motion before the collision would also affect that object’s trajectory after the collision.

4. I can’t tell the Milky Way stars from the galaxies either, the astronomers may have to use the spectral shifts to determine which is which.

5. The conundrum is why the dark matter and the galaxies aren’t in the same place as each other the way they are in the Bullet Cluster.

I’m not on completely firm ground here, but I think that the distribution of mass in the galaxy is inferred from the studies of the gravitational lensing. Wherever we infer more mass than is accounted for by the galaxies and the gas, the artist with the blue paintbrush has painted in some dark matter. The lensing also allows us to infer the location of the center of mass of the whole system, so I guess this is what they mean by the “center.”

The most clumped thing is the red, hot gas. The blue dark matter spans more space and more protrusions. Wouldn’t that make the dark matter orbiting the hot gas? At least by the reasoning used in this thread.

Also, I am having trouble discerning what are stars vs galaxies, and which are galaxies that are clustered vs. ones that may be at different distances. Frankly, I don’t know what makes this a “galaxy cluster”.

I do agree with MichaelS. Friction sucks energy out of orbits, and thus causes them to get smaller. I don’t see how the lack of friction would be aiding dark matter to clump more than regular matter. It would be the other way around.

Maybe I’m mis-understanding what you meant, but friction forces are what usually drop things into increasingly-lower orbits. For instance, the ISS will eventually hit the Earth because of air resistance; if you put dark matter into that orbit it would sit there for a very long time (assuming dark matter really does interact only through gravity). I know in this case, it will take longer for the friction to slow anything down since the orbiting stuff is slowing itself down, but I think the same principal applies.

XX said: “The problem with ‘dark matter’ is that, since it is undetectable, except through gravity… it is therefore unseeable and unprovable. This strikes me as very similar to the concept of ‘God’.”

Your exception negates that entire “problem”. Can you see air? Of course not, but you still know it exists because you see and feel its effects. Likewise, we can’t see dark matter, but it appears that we can see its effects, so we think it exists. Maybe it’ll turn out to be something different than what we think right now, but it’s not some hypothetical, whimsical “dark matter will allow your non-existent soul to enter an imaginary playground if you believe in it when you die” type of thing.

That could be right, Kyle – I don’t know any more details than were given in the picture and the accompanying remarks. What keeps that explanation from being “simple”, from many peoples’ perspective, is that it seems to imply a mechanism by which the dark matter and the galaxies, the two of which were presumably together (in basically the same place) in each cluster before the collision, could get separated from each other after the collision. The only known force that would act on either the galaxies or the dark matter is gravity.

Some people are suggesting some sort of gravitational slingshot mechanism. We use something like this to direct some of our spacecraft. But these people can’t get the details (abode of the devil)to work out.

Others are suggesting that maybe the dark matter interacts with itself in some way other than gravitationally. That would lead us into new physics.

There has been some speculation about this on bautforum.com, some in the Universe Today Stories forum and some in the Astronomy forum.

1. The dark matter in the center of the cluster is opaque and is blocking the light from the central galaxies.

2. The galaxies have congregated in orbit around a central mass of dark matter.

Doh!!!

Gary 7

GOD DID IT!

Mighty: You’re in good company in your skeptical viewpoint. Richard Feynman was famous for his attitude of always questioning perceived authority. Even old, tried, true and trusted experiments weren’t beyond his pervue. That attitude led him to a Nobel prize,,,
,,,but dark matter fits our observations quite well. I much prefer it to tossing out theories that have been shown to work so reliably. We stand upon the shoulders of giants and hope,,,they don’t have a bad itch,,,

Gary 7

I’m selling some. It comes in a Klein Bottle.

Doesn’t the dark matter theory also predict that there should exist galaxies devoid of dark matter? I mean, if u look at the famous picture of the bullet cluster, u see the heated gas separated from dark matter. Won’t that gas eventually cool down and form its own stars and galaxies, without dark matter? What kind of galaxies will be the result of that? Are people looking for such galaxies?

When I went to Carlsbad Caverns when I was 12, at the end of the tour I felt kind of let down. As amazing as it was, it was no longer legendary. It was now finite. It just made me want to discover more, stranger caverns.

I suppose that if I’m honest with myself, my gut reaction to even a good, sound theory is “Hmm. Is that all? I wonder what ELSE might work?” Although I am not a scientist, I understand that that’s how many scientists think.

But the main problem with neutrinos is that they have very low rest mass, so act like “hot” dark matter, and don’t clump at all. We need “cold” dark matter, that does clump gravitationally, but doesn’t interact and doesn’t make light, yet rules the dynamics of the universe (with dark energy, but that’s another thread). Is this so far-fetched of an idea? Not if you look at the way we probe our universe: we only use light! So is it any surprise that if our universe is ruled by something that doesn’t make light, we wouldn’t know about it until we started looking closely at its gravitational influence?

As for the idea that it is bad science to try to postulate new types of matter to help allow our present theories to be correct, I have two words for you: positrons and neutrinos. Both of those particles were postulated to make an elegant theory correct, before they were ever detected. That doesn’t mean every new particle that would be nice to exist really does (say, the Higgs), but it doesn’t make it bad science to look for it, even to expect it to exist, in the absence of any preferred model. That’s precisely where we find ourselves today, and we are indeed looking for the dark matter in laboratories, just as we did with positrons and neutrinos.

As for Abell 520, I have no idea what the dynamical explanation there is. But I also had no idea what the explanation was for the weird dynamics seen in recent supernova explosions– yet no one said “the theory of gas dynamics must be wrong, we need to look for new forces”. We figured that the dynamical puzzle would be solved, and that’s where my money is here. I could be wrong, of course! That’s the fun of science, and horse racing. But I do know the best theory of the alternatives when I see it.

To Gary 7: I am aware of the non-falsifiable situation and that’s essentially what I was pointing out. But I was just adding my two cents, admittedly more from gut reaction rather than knowing any of the math, that as I hear string theory explained it seems to acquire new assumptions in order to make the previous assumptions work. But the math is beautiful, I’m told.

Good analogy to the neutrino, but still, could such particles collect in such concentrations so as to create a sizable gravitational field and yet still not interact physically? In other words, how can it have mass yet not interact? I am really in the dark about that, I’d like to know if there is a plausible hypothesis.

As to “not privy to…”, sure, I admit I didn’t follow a career in science because of the math. So even though I have these questions (as the one I just stated above) I have to take it on faith that scientists know what they’re talking about.

Brian: Perhaps MOND explains one set of observations, and something else–Dark Matter, for instance–explains others?

Thomas Siefert: Sure, I read the whole thing, which prompted my comments, based on the fact that Phil’s posting was about problems with the theory. Note that I didn’t say outright that I didn’t believe in Dark Matter, as others before me did. I was just examining my reactions to what I read. Although it’s just possible I was making loaded statements in order to provoke a reaction.

I take no pride in being a holdout, but I always have that little voice in the back of my head that reminds me that scientists are prone to much of the same limitations as us regular folk…such as following the flock, linear thinking, and having biases. Also, that there could be much we don’t yet know. Ever read 19th century explanations for volcanism? I love hearing alternate theories, even if they are somewhat suspect, because it gives rise to the occasion to re-examine the prevailing view, and to find reasons to defend or question it.

Dark matter theory does seem to be somewhat robust these days, I’m just not one to jump on the bandwagon right away…I’m probably just hoping that if I wait long enough, something stranger will come along.

Birds from dinosaurs, anyone?

Well, people look for explanations to things. A number of years ago, we saw that stars in galaxies and galaxies in galaxy clusters weren’t following the paths we expected them to on the basis of Newton’s Laws, Einstein’s Theory of Relativity, and the distribution of luminous mass that we could see. They moved as if some extra, unseen, matter was pulling on them. You can Google up more info for yourself.

An alternative theory, called Modified Newtonian Dynamics (MOND), proposed that the observed motions of the stars in the galaxies could be accounted for if the old f=ma that we all learned in high school was modified. MOND worked pretty well at expaining the motions of stars in galaxies, but had trouble explaining some other things. MOND was never able to explain gravitational lensing, for instance, or the observations of the Bullet Cluster, or, more recently, a galaxy cluster called CL 0024+17. Again, I suggest Google. More and more precise experimental measurements of f=ma are beginning to impact the MOND discussion also.

In 2004, someone proposed a theory called Tensor-Vector-Scalar gravity (TeVeS), which has had some success matching observations. So if you don’t like dark matter, you can root for TeVeS or come up with your own alternative theory – but again, in the long run, the theory that best matches the observations will win out.