Dr Van Flandern,
Dark matter is proposed as the solution to the "glactic rotation curve problem" (see drawings). Our solar system's orbital velocity is about 60 km/sec faster than expected (220 km/sec vs 160 km/sec) if the matter we can see is all there is and assuming 1/r^2 gravity.
According to the Dark Matter model, there is a halo of dark matter around a galaxy that is responsible for the difference between the expected and observed rotation curves for galaxies.
To me the word halo conjures up an image of a spherical shell, or perhaps a torus, of stuff (non baryonic dark matter is what they mostly call it) surrounding a galaxy. But we know from Physics 101 that the gravitational force field inside of a shell (or inside of a torus near its central plane) is zero.
The proposed rotation curve for this halo of DM (fig5) looks to me like the curve for a solid sphere.
So, what do they really mean when they say there is a "halo" of dark matter around a galaxy?
LB
Code:
orbital speed
^
|x
| x
| x
| x
| x
| x
+---------------------------------> dist from center
fig1 Expected Rotation Curve for a solar system
fig2 Observed Rotation Curve for a solar system
orbital speed
^
| x
| x x
| x x
| x x
| x x
|x x
+---------------------------------> dist from center
fig3 Expected Rotation Curve for a galaxy
orbital speed
^
| x
| x x
| x x x x
| x x
| x
|x
+---------------------------------> dist from center
fig4 Observed Rotation Curve for a galaxy
orbital speed
^
|
| x
| x x
| x x
| x
| x
| x
| x
| x
| x
| x
| x
| x
| x
|x
+---------------------------------> dist from center
fig5 Proposed Rotation Curve for a galaxy's DM halo
This curve, combined with the curve in fig3, produces the curve in fig4.
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