[Earthscienceguy]

MOND theory, really.   

My one self has a real problem with being squeezed down to nothing but information on the surface of a black hole.

And my other self does not like being shadow of my other self. 

I understand that MOND can describe the angular momentum of galaxies.  But describing the universe as bits of random information one the surface of a black hole does not sound any better than imaginative matter that we cannot see.

[Janus]

Given that we only have about 100 years' observations of individual stars in distant galaxies, why do we think that they are in stable orbits and not just rushing towards the galactic centre from deep space? How good is the historic velocity data? Given that 100 years is a fleabite in galactic history, how accurate is the acceleration measurement?

While we don't have observations for any single galaxy that has lasted more than 100 yrs, we do have observations of different galaxies at differing distances from the Earth, and thus viewed from different points of the universe's history spread over billions of years.    If galaxies were not relatively stable in terms of star orbits, we should see galaxies in various stages of collapsing in on themselves or flying apart.   Closer galaxies should appear, on average to be in the later stages of this than further galaxies.  Even a difference of 1 billion ly should produce a noticeable difference in the appearance of galaxies.  As it is,  It doesn't.   That means that over a billion yrs or so, galaxies have not evolved much. 

As for the stars rushing in from deep space.  When we measure the orbital velocities of those outer stars we are looking at galaxies that are rotating more or less edge-on* to us.  We then compare the Doppler shift between the stars on one side Which  would be moving away and the stars on the other side, which are moving towards us.  The average between these readings should work out to be the recessional velocity of the galaxy as a whole, while the difference between this average and the velocities we record gives us the orbital velocities at along a line perpendicular to the radial line to the center.
we can verify the galaxy as a whole velocity by looking at the center point of the galaxy where the majority of the orbital star motion is at a right angle to our sight-line and doesn't significantly contribute to the Doppler shift readings.
Now if stars wee rushing in or out from the center of the galaxy, then, when looking straight in at the center, we would see a wide variation in Doppler shifts around the average, which we don't measure. 

* if we were looking "straight down" at a galaxy, all the orbital velocities would be at a right angle to our line of sight, and we would not be able to get direct Doppler shift measurements of them.  Exactly edge on would have us looking through the disk at the stars we are trying to measure the Doppler shifts for, meaning we would get all those other star Doppler shifts mixed in.
A slight angle is best, where we can look directly at the part of the disk we are measuring, but it still has a significant velocity component along the line of sight.  We can then use a little trig to work out the  actual velocity perpendicular to the radial line. ( for example if the galaxy is tipped towards us at 10 degrees, and we measure an Doppler shift equating to 200 km/sec, we can calculate the actual orbital velocity as being 203 km/sec at  a 45 degree tilt, that same 203 km/sec orbital velocity would give us a Doppler shift reading of 143.5 km/sec.)

[PmbPhy (OP)]

Quote from: alancalverd on Yesterday at 22:29:51How good is the historic velocity data? Given that 100 years is a fleabite in galactic history, how accurate is the acceleration measurement?

Its the doppler that's measured and from that the speed.

Yes, but the hypothesis of dark matter derives from acceleration, not speed. So you need two credible Doppler measurements some time apart.

The magnitude of the speed remains constant since the acceleration is centripetal acceleration and as such acceleration is through changes in direction. As such a single Doppler measurement will give you acceleration. Recall that a = v²/r

[alancalverd]

Only if r is constant, i.e. you know that the orbit is stable. So the question is to what degree of accuracy can we say that  dr/dt = 0 for any of these objects? Given that r ≈ 10⁵ light years or so, I wouldn't have thought that even 100 years of good observation of the doppler shift of one star would convince anyone that it was in a stable elliptical orbit at r.

But astronomers are known to be clever.

[PmbPhy (OP)]

Only if r is constant, i.e. you know that the orbit is stable.

Not true. The formula is an expellant approximation if the orbit is not perfectly circular. The direction of motion of the stars can be determined by the doppler.

Determining the rotation curve is not a simple matter. Note that the rotation curve is flat too and that says a lot. Its not determined from a single star but many stars. You do know what a galaxy looks like, don't you? What does is appearance tell you about the motion of stars? What do you think Doppler analysis tells us?

[PmbPhy (OP)]

Only if r is constant, i.e. you know that the orbit is stable.

Not true. The formula is an expellant approximation if the orbit is not perfectly circular. The direction of motion of the stars can be determined by the doppler.

Determining the rotation curve is not a simple matter. Note that the rotation curve is flat too and that says a lot. Its not determined from a single star but many stars. You do know what a galaxy looks like, don't you? What does is appearance tell you about the motion of stars? What do you think Doppler analysis tells us? That curve is for all galaxies for the most part.

[alancalverd]

Bedtime! Back to basic newtonian physics tomorrow!

But what concerns me intellectually is the invocation of another aether: stuff with no mechanical or electrical properties, but mass. Or at least some gravitational property that is shared by objects with mass, in exact proportion to their inertial mass, which this stuff apparently does not have!

[PmbPhy (OP)]

Bedtime! Back to basic newtonian physics tomorrow!

But what concerns me intellectually is the invocation of another aether: stuff with no mechanical or electrical properties, but mass.

You mean like black holes, brown dwarfs, etc? They do have mechanical properties.

[Janus]

Bedtime! Back to basic newtonian physics tomorrow!

But what concerns me intellectually is the invocation of another aether: stuff with no mechanical or electrical properties, but mass. Or at least some gravitational property that is shared by objects with mass, in exact proportion to their inertial mass, which this stuff apparently does not have!

What makes you believe that dark matter wouldn't have an equal gravitational and inertial mass?

[yor_on]

Janus

" While we don't have observations for any single galaxy that has lasted more than 100 yrs, we do have observations of different galaxies at differing distances from the Earth, and thus viewed from different points of the universe's history spread over billions of years.    If galaxies were not relatively stable in terms of star orbits, we should see galaxies in various stages of collapsing in on themselves or flying apart.   Closer galaxies should appear, on average to be in the later stages of this than further galaxies.  Even a difference of 1 billion ly should produce a noticeable difference in the appearance of galaxies.  As it is,  It doesn't.   That means that over a billion yrs or so, galaxies have not evolved much.  "

Doesn't that depend on what 'origin' one presume for this universe? If I assume it to 'originate everywhere', then the universe should be isotropic and homogeneous, with all galaxies being more or less equivalent.
=

ah, ok, you were thinking of it in terms of the time passed since some origin, and now it makes sense to me :)
Need some free time here ::))

[Janus]

Doesn't that depend on what 'origin' one presume for this universe? If I assume it to 'originate everywhere', then the universe should be isotropic and homogeneous, with all galaxies being more or less equivalent.

"At this moment", yes.  However we aren't seeing the galaxies as they are at this moment, but how they were when the light we are seeing left them.  Thus, if our galaxy and Andromeda  were the same age, we would be now seeing Andromeda when it was 2.5 million years younger than our galaxy.  The light from a galaxy that we measure as being 1 billion ly away, would be seen as it was when it was 1 billion years younger than it is "now"  As we look further and further way, we are looking at younger and younger versions of the universe.