dougettinger wrote:1) Why are the spirals superficially anomalous? Because they should not exist when using normal gravity equations? or because of the unexpected velocity distribution?
The unexpected velocity profile is the anomaly. It is superficial because it can be readily explained by unseen mass. There is no problem with the underlying theories of gravity- they are used, unmodified, to explain what we see.
2) In still am not sure in which way a galaxy rotates ? In the direction of the spirals ? or opposite the direction of the spirals?
It rotates the way you would expect, with the spirals. Keep in mind that the galaxy rotates faster than the spirals, however. That is, stars in their orbits approach spirals from behind, pass through them, and continue on to the next spiral. You can talk about a galaxy rotating, and define a direction, but the galaxy itself doesn't rotate at a fixed speed. As you'd expect, it rotates faster near its center than it does at the periphery. You can imagine it sort of winding itself up- indeed, until the dynamics of spiral arms were better understood, it was an open question why the spirals didn't get wound up over time.
3) You mentioned that dark matter has an observed apparent distribution around galaxy clusters. I thought dark matter could not be observed ? only postulated ? Does this mean scientists need dark matter to explain galaxy clusters, too?
Of course dark matter is observed. Otherwise, we wouldn't be developing theories around it. How is observing in the "light" of gravity different than observing in the light of EM? Many things in the Universe are observed indirectly.
The structure of galaxy clusters is consistent with a large amount of dark matter around them. Observations of the movement of galaxies in clusters, as well as the nature of gravitational lensing caused by galaxies and galaxy clusters require dark matter (or something that behaves like dark matter) in order to be explained.
[quote="dougettinger"]1) Why are the spirals superficially anomalous? Because they should not exist when using normal gravity equations? or because of the unexpected velocity distribution?[/quote]
The unexpected velocity profile is the anomaly. It is superficial because it can be readily explained by unseen mass. There is no problem with the underlying theories of gravity- they are used, unmodified, to explain what we see.
[quote]2) In still am not sure in which way a galaxy rotates ? In the direction of the spirals ? or opposite the direction of the spirals?[/quote]
It rotates the way you would expect, with the spirals. Keep in mind that the galaxy rotates faster than the spirals, however. That is, stars in their orbits approach spirals from behind, pass through them, and continue on to the next spiral. You can talk about a galaxy rotating, and define a direction, but the galaxy itself doesn't rotate at a fixed speed. As you'd expect, it rotates faster near its center than it does at the periphery. You can imagine it sort of winding itself up- indeed, until the dynamics of spiral arms were better understood, it was an open question why the spirals didn't get wound up over time.
[quote]3) You mentioned that dark matter has an [u]observed[/u] apparent distribution around galaxy clusters. I thought dark matter could not be observed ? only postulated ? Does this mean scientists need dark matter to explain galaxy clusters, too?[/quote]
Of course dark matter is observed. Otherwise, we wouldn't be developing theories around it. How is observing in the "light" of gravity different than observing in the light of EM? Many things in the Universe are observed indirectly.
The structure of galaxy clusters is consistent with a large amount of dark matter around them. Observations of the movement of galaxies in clusters, as well as the nature of gravitational lensing caused by galaxies and galaxy clusters require dark matter (or something that behaves like dark matter) in order to be explained.