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  #11  
Old November 12th 17, 07:52 AM posted to sci.astro.research
Jos Bergervoet
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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

  #12  
Old November 13th 17, 01:51 AM posted to sci.astro.research
root[_2_]
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jacobnavia wrote:
[[Mod. note -- 19 excessively-quoted lines snipped here. -- jt]]
Dark matter was supposed to be in some kind of "halo" outside the
galaxy. The stars then, are pulled by the outside. A fifth force would
have a vector centered in the center of the milky way.


There would be no gravitational effect on bodies within such
a halo shell. That was a first year physics problem.

[[Mod. note -- 24 excessively-quoted lines snipped here. -- jt]]
  #13  
Old November 15th 17, 10:55 PM posted to sci.astro.research
Steve Willner
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In article ,
jacobnavia writes:
Suppose some star S at 60 thousand light years from the center of the
galaxy. A normal star whose mass can be accurately determined.

Its speed can be measured, and its mass is known.


You don't need the mass. At these scales, stars are effectively
massless "test particles," and only accelerations are relevant.

Then, we subtract gravity effects


The "gravity effects" depend on the mass distribution, which is
unknown. (This is what jt was getting at.) In other words, if you
knew the accelerations and assumed a gravity law, you could derive
the mass distribution of the Milky Way. Or if you knew the mass
distribution from other data, you could derive the gravity law. But
you can't derive both. What you could do is look for the _simplest_
set of assumptions that would explain all the accelerations. That
would be a huge advance, of course.

As jt also mentioned, the observables (in principle!) are the three
spatial coordinates and the three components of velocity. With GAIA
and ground-based radial velocity surveys, those will soon be measured
for millions of stars! What you want, though, are the
_accelerations_, which can't be directly measured. However, the
statistical distribution of velocities should at least put
constraints on _either_ the mass distribution _or_ the gravity law,
and the task again will be to look for the simplest assumptions that
explain the data.

In article ,
jacobnavia writes:
Mmm the galaxy has a plane of rotation. Th center of the galaxy, that
star and we are rotating around the same central object, the galaxy, in
a plane.


Any three points define a plane, but neither the Sun nor an arbitrary
star is in general moving in that plane. To put it another way, the
Milky Way disk has a finite height, and the halo is roughly
spherical, and neither stellar component has motions perpendicular to
the vector towards the Galactic center.

[Moderator's note: A pedantic note to avoid posts pointing this out:
any three points define a plane if they are not colinear. -P.H.]

"We don't know that is moving in a circular orbit"... wow, I always
thought that they are doing so,


I'm not sure why you thought that. The orbits are roughly
elliptical, generally with modest eccentricities, but the ellipses
are not closed as for solar system planetary orbits. That's because
the mass distribution of the Galaxy is not spherically
symmetric.

and that the "arms" we see are density
waves in the disc of stars circling the center.


Indeed so. These density waves perturb the stellar orbits. The bar
is also an important perturber.

You might look up "Local Standard of Rest" and "Solar Motion".

The stars must be doing "some" kind of circle around the center since
the form of the galaxy (a rotating plane of stars ) indicates so.


The flatness of the disk indicates that the orbital inclinations are
small (not zero, though) but says nothing about eccentricities. In
fact, you could even have large inclinations if the eccentricities
were also large, but that's not what's observed.

I even thought that the sun was rotating about 1 rotation per 250
million years, so it is around 20 galactic years old. I thought
that the orbit was a circle. Is that not correct?


The time scale is about right (I get 220 Myr), but "circle" is an
approximation even rougher than "ellipse."

Dark matter scenarios suppose some form of invisible matter outside


Not only "outside." The morphology of the putative dark matter halo
is unknown, but simulations say it ought to be concentrated toward
the Milky Way center, roughly but not exactly spherically symmetric,
and more extended than the stellar distribution.

A second star we could use of course, is the nearest one, the sun. The
sun's orbit could tell us about the force effects here. Here we have
more data and less problems than with the star's far away. We know very
precisely the distance to the center, and the mass between us and the
center, so the effects of gravity could be calculated much more easily.


The mass is derived from the solar motion; we don't know it
independently. Unless I'm missing something. One standard reference
on the subject is at
http://iopscience.iop.org/article/10...ta#apj490685s4
but the authors don't derive masses from the rotation curve.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA

  #14  
Old November 18th 17, 09:45 AM posted to sci.astro.research
jacobnavia
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Posts: 105
Default Verifying

I would like to thank all people (and the moderator) that took the time
to answer my (rather silly sometimes) questions.

jacob
 




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