Re: Dark matter hides, physicists seek (Forwarded)

"NV" == Nicolaas Vroom <nicolaas.vroom@xxxxxxxxxx> writes:

NV> "Nicolaas Vroom" <nicolaas.vroom@xxxxxxxxxx> schreef in bericht
NV> news:Thuvh.316448$HS2.6325285@xxxxxxxxxxxxxxxxxxxxxxxx

NV> In the book Galactic Dynamics by James Binney and Scott Tremaine
NV> at page 23 we can read: "It is a remarkable fact that the circular
NV> speed curves still remain flat even at radii well beyond the outer
NV> edge of the visible galaxy, thus implying the presence of
NV> invisible or dark mass in the outer parts of the galaxy (...)"
NV> What this quote implies that dark matter is the sole and only
NV> explanation beyond a certain distance.

Strictly, there are at least two possibilities: Either there is unseen
matter or the gravitational force changes on these large scales. Most
astronomers and physicists prefer the former, because it is easier to
explain all of the data with this explanation.

NV> This may be true in principle, the problem is that stil more
NV> visible matter is discovered at larger distances, making this
NV> assumption a less strict requirement. At solar scale the same can
NV> be said about the kuiper belt objects which make our solar system
NV> larger and larger with at galaxy type scales invisible objects.

Correct, but if you look at the mass of the KBOs, it's tiny compared
to, say, Jupiter, to say nothing of the Sun. Similarly, in the
outskirts of galaxies, one would have to look at the ratio of dark to
light matter. I don't remember the numbers off the top of my head,
but I'm certain that the ratio is substantial. In other words, a
substantial amount of unseen matter, relative to the matter that's
detected, is required. It's not clear how to make the unseen matter
out of anything with which we are familiar. If it were stars, we'd
see it. If it were hydrogen gas, we'd see it.


[...]
NV> However this also raises a new issue: what is the average height
NV> as a function of distance.

Here you have to be a bit more specific. Which disk? The Milky Way
has both a "thin" disk of gas and young stars and a "thick" disk of
old stars. Interior to the solar circle, the thin disk has a width of
about 200 pc while the thick disk has a width of about 2000 pc.
Outside of the solar circle, the number of young stars decreases
dramatically, while the hydrogen gas starts to increase in thickness
substantially.

NV> In Astronomy of November 1992 at page 45 in the article "The grand
NV> Illusion. What we see in the night sky is a widly misleading
NV> representation of the universe" by Ken Croswell He makes the point
NV> that "often" the brightest objects have the smallest mass and the
NV> dimmest objects have the heighest mass, which implies how
NV> difficult it is to estimate mass as a function of luminosity.

You must be misreading this or Croswell is writing about something
slightly different than to what you're trying to apply it. Take the
case of stars. The most massive stars are the brightest. A star 10
times the mass of the Sun can have a luminosity 1000 times that of the
Sun; conversely, a star 10 times less massive than the Sun can have a
luminosity 100 times less than that of the Sun.


NV> For me it is difficult to understand why the scientific community
NV> assumes dark matter in a sperical distribution as a tool to
NV> explain galaxy rotation curves and not visible/baryonic matter in
NV> the disc. In my opinion the first step should be to explain a
NV> galaxy rotation curve by only assuming baryonic matter in the
NV> bulge and the disc

You might try re-reading Binney & Tremaine. First, there's just no
way to stuff enough visible matter into a typical spiral disk in order
to explain a typical rotation curve. Second, there's an issue with
the stability of spiral disks if they are not surrounded by a large
halo of mass, which is again not seen.

NV> The second step should be define the stars that make up this mass
NV> based on a distribution as explained at page 48 of the above
NV> mentioned document. Step three should be to calculate the
NV> luminosity of this galaxy assuming that many stars are hided by
NV> other stars in the foreground. The fourth step should be to
NV> compare this luminosity which what is observed.

Alternately, if you think that hundreds (thousands?) of people who
have done this kind of work already were wrong, why don't you do it to
prove them wrong?

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