Verifying

[jacobnavia]

If there is anew kind of attractive force, unknown to us, that pulls
matter together, we should be able to see some effects of this feeble
force in our environment.

Now, consider the earth moon system. We have a mirror there, that can
reflect any laser beam to the earth.

The orbit of the moon should be affected if the geometry works. When the
moon is moving away from the center of the galaxy it should feel a
braking effect. Wehn it is approaching, an accelerating effect.

Over time these effects could accumulate. Contrary to the noise, the
acceleration should always have a vector pointing to the Sagitarius
constellation.

We have extremely precise clocks, and we have even detected
gravitational waves. I do not think that detecting this force is very
difficult, if it can be done at all, of course.

And if it is not detected, it is surely an argument againt that supposition.

There many reasons that the experiment could fail. Braking effects of
the tides, and many others, influences of other planets, etc. Taking all
that into account is a considerable work.

How big is that force?

Those stars must be pulled somehow, by this attractive force, so that
their rotation doesn't go down as distance increases. This force holds
the galaxy together.

But as distance increases, the force decreases. Nuclear force is very
strong but doesn't reach very far. Gravity is much weaker but reaches
farther, much farther.

But gravity decreases as the square of distance, and doesn't reach as
far as this new force, even feebler than gravity.

But it is feasible for astronomers to catch it now, they have all the
required technology.

Nobody is looking however.

Looking for perturbations in the orbits of the planets that point to the
center of the galaxy.

[Jos Bergervoet]

Post by jacobnavia
If there is anew kind of attractive force, unknown to us, that pulls
matter together, we should be able to see some effects of this feeble
force in our environment.

Only if it is strong enough at the (relatively) short distances
of "our environment".

Post by jacobnavia
Now, consider the earth moon system. We have a mirror there, that can
reflect any laser beam to the earth.

We also have satellites and spacecraft at larger distances.

...

Post by jacobnavia
We have extremely precise clocks, and we have even detected
gravitational waves. I do not think that detecting this force is very
difficult,

So you have information as to whether it is strong enough at the
relatively short distances of our environment, as mentioned above?

...

Post by jacobnavia
But gravity decreases as the square of distance, and doesn't reach as
far as this new force, even feebler than gravity.

OK, you propose a force decaying more slowly than 1/r^2

Post by jacobnavia
But it is feasible for astronomers to catch it now, they have all the
required technology.
Nobody is looking however.
Looking for perturbations in the orbits of the planets that point to the
center of the galaxy.

You are mistaken, people are constantly looking. Remember all
the activity related to the Pioneer anomaly, or more recently
(and based on the orbits of the planets) the search for planet
nine.

But all this analysis of forces (at the relatively short distances
of our environment) just hasn't (yet) provided any evidence for
the long-range force that you propose! You should be more patient.
(At least for planet nine now the evidence is mounting..)

[Richard D. Saam]

Post by Jos Bergervoet
You are mistaken, people are constantly looking. Remember all
the activity related to the Pioneer anomaly

In reference to the Pioneer Anomaly
the most definite conclusion is based on the JPL report:
http://arxiv.org/abs/1204.2507

[[Mod. note -- Published as Phys Rev Lett 108, 241101.

The same authors' (Rutyshev et al) previous paper,
arXiv:1107.2886 = Phys Rev Lett 107, 081103,
is also essential reading.

The thermal modelling is discussed in more detail in
Rievers et al, New J Physcs 11, 113032
http://dx.doi.org/10.1088/1367-2630/11/11/113032
and the same authors' (Rievers et al) later paper
arXiv:1104.3985 = Annelen der Physik 523, 439
http://dx.doi.org/10.1002/andp.201100081
-- jt]]

It does not reflect the thermal and Doppler solutions over time.
With time, the constant Doppler solution
dominates the decaying thermal solution
as the Pioneers approach interstellar space.
The Doppler solution is real
versus
the 1000s of calculated decaying internal finite element
diminishing contributions.

This JPL analysis failure unnecessarily negates a major contribution
to science.
I am not talking about new physics but a confirmation of space time.
The JPL report has a geocentric/heliocentric view
and dismisses the universal view.

The JPL analysis is based on incomplete analysis of the data
assuming the decay for Pioneer acceleration aP

daP/dt = -k*aP model one

This fits the assumption that
the thousands of analyzed aP thermal components
are tied to the RTG half life with aP decay approaching zero with time.

A better fit to trajectory data is the 'aPinfinity' effect
as the Pioneers approached interstellar space:

daP/dt = -k*(aP - aPinfinity) model two

Initially, as the Pioneers pass Jupiter,
the thermal emission overwhelms
the anomalous acceleration (aPinfinity)
making it statistically insignificalt in this initial trajectory phase
but diminishes with time(model one)
with aP decay approaching aPinfinity
with time (model two)
as the Pioneers probe interstellar, intergalactic space
on leaving the solar system
with diminished internal thermal and external solar wind
considerations previously considered.

Statistically significant aPinfinity values are:
Pioneer 10 aPinfinity 7.0x10^-10 m/sec^2
Pioneer 11 aPinfinity 8.2x10^-10 m/sec^2

These values may represent a constant for interstellar medium
within some standard deviation
or actually represent different values based on
differing Pioneer 10 and 11 directional probes of interstellar medium.
The interstellar medium may not be uniform.

Logically, in as much as aPinfinity
is a measure of space-Pioneer momentum transfer (spacetime viscosity)
then all transiting object motion would be affected.
This has implication for galactic and planetary system formation
as reflected in accelerations associated
with the Tully Fischer relationship.

Apparently there is still some unpublished Pioneer data
to further test this hypothesis.
Considering this data's importance to the scientific community,
it should be published.

RDS

[Richard D. Saam]

Post by Richard D. Saam

Post by Jos Bergervoet
You are mistaken, people are constantly looking. Remember all
the activity related to the Pioneer anomaly

In reference to the Pioneer Anomaly
http://arxiv.org/abs/1204.2507
[[Mod. note -- Published as Phys Rev Lett 108, 241101.
The same authors' (Rutyshev et al) previous paper,
arXiv:1107.2886 = Phys Rev Lett 107, 081103,
is also essential reading.

This reference is key;
Note Figure 1
The top panel represents modeling
daP/dt = -k*aP model one
and the bottom panel
daP/dt = -k*(aP - aPinfinity) model two
It is clearly evident that model two predominates.

Post by Richard D. Saam
The thermal modelling is discussed in more detail in
Rievers et al, New J Physcs 11, 113032
http://dx.doi.org/10.1088/1367-2630/11/11/113032
and the same authors' (Rievers et al) later paper
arXiv:1104.3985 = Annelen der Physik 523, 439
http://dx.doi.org/10.1002/andp.201100081

Presented data was below 45 AU where thermal effects predominate.

Post by Richard D. Saam
-- jt]]
It does not reflect the thermal and Doppler solutions over time.
With time, the constant Doppler solution
dominates the decaying thermal solution
as the Pioneers approach interstellar space.
The Doppler solution is real
versus
the 1000s of calculated decaying internal finite element
diminishing contributions.
This JPL analysis failure unnecessarily negates a major contribution
to science.
I am not talking about new physics but a confirmation of space time.
The JPL report has a geocentric/heliocentric view
and dismisses the universal view.
The JPL analysis is based on incomplete analysis of the data
assuming the decay for Pioneer acceleration aP
daP/dt = -k*aP model one
This fits the assumption that
the thousands of analyzed aP thermal components
are tied to the RTG half life with aP decay approaching zero with time.
A better fit to trajectory data is the 'aPinfinity' effect
daP/dt = -k*(aP - aPinfinity) model two
Initially, as the Pioneers pass Jupiter,
the thermal emission overwhelms
the anomalous acceleration (aPinfinity)
making it statistically insignificalt in this initial trajectory phase
but diminishes with time(model one)
with aP decay approaching aPinfinity
with time (model two)
as the Pioneers probe interstellar, intergalactic space
on leaving the solar system
with diminished internal thermal and external solar wind
considerations previously considered.
Pioneer 10 aPinfinity 7.0x10^-10 m/sec^2
Pioneer 11 aPinfinity 8.2x10^-10 m/sec^2
These values may represent a constant for interstellar medium
within some standard deviation
or actually represent different values based on
differing Pioneer 10 and 11 directional probes of interstellar medium.
The interstellar medium may not be uniform.
Logically, in as much as aPinfinity
is a measure of space-Pioneer momentum transfer (spacetime viscosity)
then all transiting object motion would be affected.
This has implication for galactic and planetary system formation
as reflected in accelerations associated
with the Tully Fischer relationship.
Apparently there is still some unpublished Pioneer data
to further test this hypothesis.
Considering this data's importance to the scientific community,
it should be published.
RDS

[jacobnavia]

Post by Jos Bergervoet
So you have information as to whether it is strong enough at the
relatively short distances of our environment, as mentioned above?

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. Then, we subtract
gravity effects and we obtain the force that is necessary to accelerate
that star to its observed speed at 60000 light years from the center.

Supposing a roughly linear decay, here at only 30000 light years from
the center we should have half of that.

Looks simple but can't be that simple. I am surely missing something,
but what?

[[Mod. note -- There are a couple of things:
1. A solar-type star at a distance of 60,000 light-years has an
apparent magnitude of around 21, which is faint enough that
getting a good spectrum will take a lot of big-telescope time.
2. Once you get that spectrum you get the star's radial velocity
(its velocity along our line-of-sight to the star).
But if you want all 3 components of its vector velocity you
also need to measure its velocity perpendicular to our line-of-sight.
That means doing high-precision astrometry to measure how its
position gradually drifts relative to other more distant objects,
with corrections for the Earth's motion around the center of our
own galaxy. (This will be on the order of micro-arcseconds/year.)
This star is too faint for Gaia to give good data (Gaia's error
bars are up to 200 microarcseconds/year at magnitude 20), in
fact I can't think of any current telescope/detector that could
do relative astrometry at that accuracy level on an object that
faint in a reasonable amount of telescope time. :(
3. Suppose we somehow managed to measure all 3 components of the
star's vector *velocity*. That doesn't tell us anything about the
star's gravitational *acceleration* (we don't know that it's moving
in a circular orbit about the center of our galaxy!), which is the
quantity which is actually influenced by dark matter/modified gravity.
-- jt]]

[jacobnavia]

Post by jacobnavia

Post by Jos Bergervoet
So you have information as to whether it is strong enough at the
relatively short distances of our environment, as mentioned above?

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. Then, we subtract
gravity effects and we obtain the force that is necessary to accelerate
that star to its observed speed at 60000 light years from the center.
Supposing a roughly linear decay, here at only 30000 light years from
the center we should have half of that.
Looks simple but can't be that simple. I am surely missing something,
but what?
1. A solar-type star at a distance of 60,000 light-years has an
apparent magnitude of around 21, which is faint enough that
getting a good spectrum will take a lot of big-telescope time.

We have big scopes now. Or just choose a nearer one. The farther you go,
the more the discrepancyy between gravity and its observed speed should
be, as we read from the speed charts of stars around the center.

The difference is bigger when you get away from the galaxy, at the
outskirts.

Post by jacobnavia
2. Once you get that spectrum you get the star's radial velocity
(its velocity along our line-of-sight to the star).
But if you want all 3 components of its vector velocity you
also need to measure its velocity perpendicular to our line-of-sight.

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.

The center, we, and that star are in the same plane. Are those
corrections really necessary?

Post by jacobnavia
That means doing high-precision astrometry to measure how its
position gradually drifts relative to other more distant objects,
with corrections for the Earth's motion around the center of our
own galaxy. (This will be on the order of micro-arcseconds/year.)
This star is too faint for Gaia to give good data (Gaia's error
bars are up to 200 microarcseconds/year at magnitude 20), in
fact I can't think of any current telescope/detector that could
do relative astrometry at that accuracy level on an object that
faint in a reasonable amount of telescope time. :(

We can use stars nearer of course. For instance at mag 20 if that is the
limit of Gaia. That would be a good start.

Post by jacobnavia
3. Suppose we somehow managed to measure all 3 components of the
star's vector *velocity*. That doesn't tell us anything about the
star's gravitational *acceleration* (we don't know that it's moving
in a circular orbit about the center of our galaxy!), which is the
quantity which is actually influenced by dark matter/modified gravity.
-- jt]]

"We don't know that is moving in a circular orbit"... wow, I always
thought that they are doing so, and that the "arms" we see are density
waves in the disc of stars circling the center.

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. 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?

If we approximate the orbit by some kind of circle, we approximate the
milky way to a center of gravity around the black hole in the central
buldge, we can calculate the gravity exerced by the (I suppose known)
mass of the galaxy and the speed that a star should have isn't it?

Very roughly. A more sophisticated thing would take into account the
form of the buldge, the mass of the plane inside the orbit, etc.

Dark matter scenarios suppose some form of invisible matter outside and
propose a vector that is in the opposite direction pulling the stars.
The force the stars feel would come from outside the galaxy in some kind
of halo.

Maybe we could look in the other direction. And if the force came from
the galaxy itself?

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.

Is there any delta?
Has anyone done this calculations?

[Jos Bergervoet]

Post by jacobnavia

Post by jacobnavia

Post by Jos Bergervoet
So you have information as to whether it is strong enough at the
relatively short distances of our environment, as mentioned above?

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. Then, we subtract
gravity effects and we obtain the force that is necessary to accelerate
that star to its observed speed at 60000 light years from the center.
Supposing a roughly linear decay, here at only 30000 light years from
the center we should have half of that.
Looks simple but can't be that simple. I am surely missing something,
but what?
1. A solar-type star at a distance of 60,000 light-years has an
apparent magnitude of around 21, which is faint enough that
getting a good spectrum will take a lot of big-telescope time.

We have big scopes now. Or just choose a nearer one. The farther you go,
the more the discrepancyy between gravity and its observed speed should
be, as we read from the speed charts of stars around the center.

But all this is known already (from the smaller scopes of
some years ago).

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?
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".

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.

[Steve Willner]

Post by jacobnavia
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.

Post by jacobnavia
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.

Post by jacobnavia
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.]

Post by jacobnavia
"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.

Post by jacobnavia
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".

Post by jacobnavia
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.

Post by jacobnavia
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."

Post by jacobnavia
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.

Post by jacobnavia
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/10.1088/0004-637X/783/2/130/meta#apj490685s4
but the authors don't derive masses from the rotation curve.

[Daniel S. Riley]

Post by jacobnavia
Now, consider the earth moon system. We have a mirror there, that can
reflect any laser beam to the earth.

Five, actually, regularly pinged looking for deviations:

"Tests of Gravity Using Lunar Laser Ranging" (2010)
https://link.springer.com/article/10.12942/lrr-2010-7

"Lunar laser ranging: the millimeter challenge" (2013)
https://arxiv.org/abs/1309.6294

see also

https://en.wikipedia.org/wiki/Apache_Point_Observatory_Lunar_Laser-ranging_Operation
https://tmurphy.physics.ucsd.edu/apollo/apollo.html

The latest observations are limited by the physical characteristics
of the reflectors.

Post by jacobnavia
Nobody is looking however.

??

-dan

[[Mod. note -- One particularly fascinating part of the lunar-laser-ranging
story is the unexpectation degredation of the retroreflectors, apparently
due to thermal gradients caused by lunar dust coating the reflector surfaces.
I think this was first published in https://arxiv.org/abs/1003.0713
and there's a nice summary and update in section 4.2 of the 1309.6294 paper.
-- jt]]

[Richard D. Saam]

Post by jacobnavia
If there is anew kind of attractive force, unknown to us, that pulls
matter together, we should be able to see some effects of this feeble
force in our environment.
Now, consider the earth moon system. We have a mirror there, that can
reflect any laser beam to the earth.
The orbit of the moon should be affected if the geometry works. When the
moon is moving away from the center of the galaxy it should feel a
braking effect. Wehn it is approaching, an accelerating effect.
Over time these effects could accumulate. Contrary to the noise, the
acceleration should always have a vector pointing to the Sagitarius
constellation.
We have extremely precise clocks, and we have even detected
gravitational waves. I do not think that detecting this force is very
difficult, if it can be done at all, of course.
And if it is not detected, it is surely an argument againt that supposition.
There many reasons that the experiment could fail. Braking effects of
the tides, and many others, influences of other planets, etc. Taking all
that into account is a considerable work.
How big is that force?
Those stars must be pulled somehow, by this attractive force, so that
their rotation doesn't go down as distance increases. This force holds
the galaxy together.
But as distance increases, the force decreases. Nuclear force is very
strong but doesn't reach very far. Gravity is much weaker but reaches
farther, much farther.
But gravity decreases as the square of distance, and doesn't reach as
far as this new force, even feebler than gravity.
But it is feasible for astronomers to catch it now, they have all the
required technology.
Nobody is looking however.
Looking for perturbations in the orbits of the planets that point to the
center of the galaxy.

Mond derived a relationship

M = v^4/(a0*G)

to explain the Tully Fischer relationship
Baryonic mass M ~ (galactic rotation v km/sec)^4
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics
with the physically dissatisfying modified gravity explanation for an
anomalous acceleration 'a0'.

But assume the observed velocity dispersion
represents a tangential velocity(v) around galactic center
that is related to a radial velocity(vr = constant*v)
that is a measure of mass inflow to the galactic center
whose galactic time(t) is measured by a/v.

This is dimensionally expressed as:

v^2 = G*M/R = G*M/(vr*t) = G*M/(vr*vr/a)
= G*M/(constant*v*constant*v/a)

then

M = constant^2*v^4/(a*G)

with the same dimensional form as MOND
but with a more physically satisfying explanation

The above formula dimensionally matched to the experimental data
baryonic mass M ~ (galactic rotation v km/sec)^4
for the baryonic Tully-Fisher relation
http://arxiv.org/abs/1512.04543 figure 2
with constant ~pi
indicates reasonable acceleration(a) values
on the order of 10^-9 cm/sec^2
and galactic times on the order of a billion years.

So 'a' can be interpreted as a deceleration
within galactic time-frame (t)
due to a drag like force (m*a)
on baryonic objects of mass m (dark matter?)
m*a ~ vacuum mass density * object crossection
optically unseen due to their
small size and diffuse distribution.

The implication is that these objects are much smaller than planets
but make up most of the galactic mass(M).

[jacobnavia]

Post by Jos Bergervoet
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
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?

Post by Jos Bergervoet
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. The stars then, are pulled by the outside. A fifth force would
have a vector centered in the center of the milky way.

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

Post by Jos Bergervoet
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. 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.

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]]

[Jos Bergervoet]

Post by jacobnavia

Post by Jos Bergervoet
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
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..

Post by jacobnavia

Post by Jos Bergervoet
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.

Post by jacobnavia
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.)

Post by jacobnavia
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.

Post by jacobnavia
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.

Post by jacobnavia

Post by Jos Bergervoet
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).

Post by jacobnavia
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

Post by jacobnavia
Shouldn't an analysis of the orbit of the sun give an answer to that?
[[Mod. note --
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.

[root]

jacobnavia <***@jacob.remcomp.fr> wrote:
[[Mod. note -- 19 excessively-quoted lines snipped here. -- jt]]

Post by jacobnavia
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]]

[jacobnavia]

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

jacob