On 11/9/2017 10:00 AM, jacobnavia wrote:
Le 08/11/2017 à 20:33, jacobnavia a écrit :
Le 05/11/2017 Ã 09:05, Jos Bergervoet wrote:
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.

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.

--
Jos