Verifying

On 11/11/2017 9:50 PM, jacobnavia wrote:
Le 10/11/2017 à 08:10, Jos Bergervoet a écrit :

We know what the rotation curves are, so the acceleration of
stars on average is known, and it is known that this does not
fit with the gravity of known matter in the galaxies. So you
do not need the observations as you describe here to get this
information. What we do *not* know is:
1) Is there more matter than the known matter, so stronger
gravity and therefore restoring agreement with the movement?

That would be a nice solution. And if space between the stars wasn't
empty but filled with some kind of very thin gas?

Yes, but then this gas would have to be undiscovered until now
for some reason. Which means it has to consist of things we can't
easily see, e.g. particles like axions or sterile neutrinos.
It can also be a collection of massive, as of yet undiscovered,
black holes. Actually they are now starting to be discovered by
LIGO. But you'd still need quite a lot of them to have enough
extra mass..

2) Is there another force that adds to the effect of gravity
so together they give agreement with the motion?
The first possibility leads to the hunt for dark matter, the
second to the search for a "fifth force"

Dark matter was supposed to be in some kind of "halo" outside the
galaxy.

No, dark matter is supposed to form a cloud with the galaxy
immersed in it. Probably densest in the galaxy itself but
also extending to some region (halo) outside the galaxy.

The stars then, are pulled by the outside.

No they are not! Whatever the distribution in the radial
direction, gravity *always pulls inwards*, except if the
distribution is completely confined to a shell, in which
case in the empty inner region it gives gravity zero, but
is *still* not pulling outwards! (Do your 1/r^2 exercises
again, for a distributed mass with spherically symmetric
distribution, only a function of r.)

A fifth force would
have a vector centered in the center of the milky way.

Yes, if it is to explain the rotation curves. But a cloud
of dark matter would also pull inwards.

Is it possible then, to figure out this from the orbit of a known star,
say, the sun?

No, a fifth force or a cloud of unseen mass will both give
more inward attraction (to the galactic center) for this
star, you can't see the difference.

The observations with telescopes as discussed above will not
help with these questions at all, they will just reproduce the
already observed disagreement between motion and the gravity
of known matter. Which then leaves us again with the same two
questions.

A fifth force would perturb the orbit of the sun in a different way than
matter in a halo.

No, as proposed solutions for the motion of the stars,
both these things of course are supposed to influence the
orbits of the stars in the same way (giving them more
inward acceleration).

Besides, even if it is weak it has been there since
eons. After all this time (age of the sun around 5GY) some perturbation
should be observable.

Of course. The whole structure of the universe (zooming
in from the largest levels) would have to agree with any
new force, or new kind of matter we introduce! It looks
like this: http://www.atlasoftheuniverse.com/universe.html
Shouldn't an analysis of the orbit of the sun give an answer to that?

[[Mod. note --
Interstellar space is indeed filled with a "very thin gas":
https://en.wikipedia.org/wiki/Interstellar_medium
https://en.wikipedia.org/wiki/Intracluster_medium
This is already included in counts of "known matter".
-- jt]]

And it should not only be in agreement with the orbit of
the sun, but with all galaxies, clusters, filaments, voids,
and anything else we observe in the universe.

--
Jos