Questions about Dark Matter

[MeteorWayne]

That's one of the stupidest things I've ever seen written in the physics forum, and that's saying a lot!!

[Saiph]

Gravitational lensing is a very localized and specific effect. Sure, it happens everywhere, but so weakly it doesn't have any real impact on observations. For instance the sun's observable gravitational lensing effects occur from stars seen right past it's edge,a nd only during a solar eclipse.

So, except in some specific cases such massive galaxies with another right behind it, BH's right in the line of sight. there are far larger things to worry about.

Sorta like worrying about a loose stitch in your shirt, instead of the bear trying to eat you...

[Fallingstar1971]

OK so far so good.....I only have one question left

If the sum of the stars in a galaxy have more mass than the black hole, then why does the black hole occupy the center of the galaxy?

I would think that the black hole would be orbiting the stars combined mass on the outskirts of the galaxy.

Or am I thinking of it wrong. Individually, each star would have less mass than the black hole. Is this why there is a super massive black hole at the heart of most galaxies, and not on the outside? One star at a time and it seems to make sense, but when thinking of the combined mass of all the stars, it seems to get confusing

Star

[MeteorWayne]

Actually the black hole is orbiting the same thing that the stars are...the center of mass of the whole galaxy.

The reason it's at the center, and orbits so close to the center of mass of the galaxy, is that's where the greatest concentration of mass was, so that's where it formed.

[Saiph]

part of the problem with BH's is people tend to think of them as special. They aren't.

Ask you question alittle differently: If the sum of the stars in a galaxy have more mass than that single star, then why does that single star occupy the center of the galaxy?

Answer: Because it formed there.

Why is it so large? Because there is a lot of stuff there, and so it formed large, and keeps growing large, because there's a lot of stuff there.

The only difference between stars and BH's is their lifetimes. Stars, by comparison to a BH, are very short lived.


Your question can also lead into the idea of galaxy formation. Did a BH act as a nucleation site (or a seed) for the gases of a galaxy to cluster around? If so, that explains why we find a large one in the center of all galaxies. But did the BH have to be there FIRST, or did one just occur early from the first stars, and just tend to be in the center...because that's where lots of matter is?

This is a big question in modern astronomy.

[michaelmozina]

Could this be a new "force"? Does Dark matter only manifest on the grandest scales? Or is this something that we may someday be able to capture in a lab?

Star

I really liked Saiph's explanations by the way. He seemed to touch on all the key points. The only other thing I would mention is that technically DM is not a 'new force' per se but it might be a new form of matter. Assuming that a new form of matter is in fact involved in these rotation patterns it is likely that the LHC experiments could/should observe them. It's not a given that we will find new forms of matter, but we should be able to produce them in controlled experiments if they exist.