
Originally Posted by
Tim Thompson
I think it has been proven, and beyond question. Spiral galaxies always have a central bulge. They never have an off-center bulge. But if the bulge moved differently from the disk, then there should be spiral or disk galaxies with bulges all over the place. So observation of bulges only in the center should be enough to disprove the hypothesis that disk & bulge move separately.
Also notice that the redshifts of elliptical & spiral galaxies in a cluster are not systematically different. So you can't single out spiral or disk galaxies. They are not systematically different, at least on the matter of redshifts.
And finally, for galaxies that are close enough, this hypothesis can be (and has been) tested by direct observation. We derive the rotation curves for galaxies by measuring the differential redshift across the disk, for edge on or nearly edge on spirals. The redshift is a simple Doppler shift, and invariably one side of the galaxy is redshifted, and the other side blue shifted, with respect to the central bulge. If the central bulge moved independently, it would not be systematically in the middle of the Doppler shift of the galaxy.