Re: Dark matter/energy - is it real?

:: James Nicoll
:: What we'd see added to the mass of the Sun is the mass of the halo
:: streaming between the Sun and any given planet. We may be able to
:: make a stab at what this might be:

: Erik Max Francis <max@xxxxxxxxxxx>
: Except that there'd also be an almost identical amount streaming past
: the planet on the other side from the Sun. The forces would almost
: exactly balance.

Look at it this way. If the halo is centered on the galaxy, and is
roughly spherical, with density similar at points of equal distance
to the center of the galaxy, it will act as if it were a point mass at
the galactic center, that gets bigger the further you go away from that
center. This is because the gravity of any single shell-shaped region of
the halo cancels in its interior, while the gravity of any sphere-shaped
region behaves the same as a point mass at the phere's center. The halo
can be considered entirely in terms of such regions, hence the "adjustable
point mass" result. And hence, the utility for explaining why orbital
velocity doesn't drop off with distance from the galactic center.
It's really a far more elegant proposal than Shawn gives it credit for.

Locally in the solar system, it will look roughly like you're in free
fall around a 6x-as-massive galaxy. The only local effects would be
tides from this, which are very small.

: Say there's five times as much dark matter: that's 10^39 kg. Say it
: is in a torus about ten thousand light years wide, centered twenty
: five thousand light years from galactic core.

Hm. I thought it was more spherical-like, henc the "lookatitthisway" above.
But in any event

: A sphere 10 AU in radius should have about a thousand tonnes
: of dark matter in it.

also suffices to dispose of the matter on local scales.


Wayne Throop throopw@xxxxxxxxx http://sheol.org/throopw
.

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