Detecting dark matter

[I really should be reading this paper on pulsar parallaxes, so let me
be brief. Again, I'll recommend highly a recent issue of Science
dealing with the dark side.]
"g" == greywolf42 writes:

g Joseph Lazio wrote in message
g ...

g 2) Even if we required gravity as the be-all and end-all of
g cosmology, we don't have any problems at all with cluster motions
g outside of the big bang. It is the fact that the observations of
g apparent motion (...) explicitly contradict the big bang that gives
g the problem.

Nope. Ignore the Big Bang. Just look at the motions of the
galaxies in the Coma cluster (...). Treat it as an
isolated system. What's the mass?

g Unknown without first assuming the big bang. If no big-bang, then
g there is no assumption that redshift is pure speed. If redshift is
g not pure speed, then there is no way that redshift implies
g distance. No distance estimate, no speed estimate. Hence no mass
g estimate available.

Ignore the systematic redshift of the Coma cluster. Take the average
redshift of all of the galaxies in the Coma cluster and subtract that
from the individual redshifts. You'll be left with a residual
redshift. For some galaxies, it's positive, some it's negative.
Equate this residual redshift with a velocity. Assume that the
cluster is a long-lived object (otherwise why are we seeing it?) and
apply the virial theorem.

As Zwicky found in the 1930s (at least some of his papers describing
this on are ADS, freely available), you'll find that this analysis
requires a lot more mass than the individual galaxies would
contribute. Now take into account the mass of the gas that's emitting
in X-rays. You still don't have enough to explain the mass derived
above.


It's also worth pointing out that we need dark matter to exist
because we've detected some. Both neutrinos and black holes are
dark matter. Neither exist in a sufficient quantity to explain
all of the dark matter required, but both exist. If we have two
examples of dark matter, it is not unreasonable to suggest that
there might be a third example of dark matter.

g Neither neutrinos nor black holes are dark matter. Dark matter is
g not merely matter that cannot be seen. Dark matter is -- by
g definition and theoretical requirement -- unable to interact with
g matter EXCEPT by gravity. [...]
No, your earlier definition was correct. Dark matter is matter
that does not interact via the EM force. Neutrinos are dark matter
because they interact only via the weak force and (presumably)
gravity. As for your assertion that black holes don't exist,
what's Sgr A*?

g Not a black hole. "I don't know" is a valid answer. My guess
g would be a magnetic pinch effect.

Why would a magnetic pinch effect affect stars? Haven't you been
trying to convince us that stellar motions aren't affected by magnetic
fields? While "I don't know" can be a valid answer, within the
context of general relativity, there is an explanation for Sgr A*.
Thus, you have to show why "I don't know" is a better explanation than
the explanation suggested by GR.


Second, the kinetic energy in the rotation is of orders of
magnitude larger than the magnetic energy density. How can the
magnetic field be comparable in influence to the rotation?

g Non sequiteur. The EM FORCE is stronger on gas molecules than the
g gravitational FORCE. Rotation is the result, not a competing
g effect.
'Fraid I still don't understand. If the total energy in the
magnetic field is orders of magnitude smaller than the kinetic
energy of rotation, how does the former produce the latter?

g Again, non sequiteur. "Energy" doesn't move matter. FORCE moves
g matter. There are two forces: EM and gravity here. Each can
g "drive" the rotation. EM force is stronger on gas molecules than
g gravitation in free space.

I'm assuming that the magnetic energy acts like a potential energy.
In your scheme, conversion of the magnetic energy to kinetic energy
would produce rotation. This is similar to how the potential energy
of a ball at the top of a hill can be converted to kinetic energy of
the ball at the bottom of the hill. Indeed, conversion of magnetic
energy into kinetic energy of *charged* particles is seen all the time
in the Earth's magnetosphere. You still haven't shown how magnetic
energy can be converted into kinetic energy of *neutral* gas.

I'm off to read some real astronomy ....

--
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