~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.