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Questions on the enigmatic rotational curve of spiral galaxies



 
 
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  #1  
Old February 22nd 09, 12:11 PM posted to sci.astro,sci.physics,sci.physics.relativity
Robert Karl Stonjek
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Posts: 196
Default Questions on the enigmatic rotational curve of spiral galaxies

I just want to add a thought experiment to help us visualise what is being
suggested/asked.

Consider a number of stars arrayed in a line across space, say a few light
years in length.

Assuming the initial condition is one of a stationary motion of each star
relative to the other, what would we expect to occur next?

I assume that the mutual attraction of the stars will cause the row of stars
to shorten until they end up clumped together.

So to make my 'barred' galaxy stable I rotate it fast enough so that the
stars on the two ends don't proceed toward or away from the rotational
centre.

Will this work? I assume it will not work close to the rotational centre
but will work further out.

There is a greater gravitational pull on objects closer to the two ends
because there is more mass between those objects and the centre, but the
rotational speed is greater as well - nicely balanced

Now we add more mass to the centre. Objects toward the ends of the arms are
going to be drawn inwardly unless the arm describes an arc. Now the pull
directly from the central mass can be added to the less effective pull of
the curved arm and the galaxy is again stable.

Why do I think this has been missed?
Models consist of known and unknown quantities, such as the numbers of and
masses of the stars that can be observed. But unobserved is the central
mass which is estimated. The central mass, I assume, is estimated at far
higher than it actually is. I don't think modellers have considered
relatively tiny central masses, as in my barred galaxy model above. Thus a
higher central mass then requires a greater mass beyond the galaxy for it to
rotate as observed.

I do not have the skills to test these ideas but I assume that either:
It has already been considered and shown to be flawed/viable or
members of this list can estimate the viability of the idea using a
simplified model (as I have suggested above).

Robert


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  #2  
Old February 22nd 09, 12:28 PM posted to sci.astro,sci.physics,sci.physics.relativity
Ian Parker
external usenet poster
 
Posts: 2,554
Default Questions on the enigmatic rotational curve of spiral galaxies

On 22 Feb, 12:11, "Robert Karl Stonjek"
wrote:
I just want to add a thought experiment to help us visualise what is being
suggested/asked.

Consider a number of stars arrayed in a line across space, say a few light
years in length.

Assuming the initial condition is one of a stationary motion of each star
relative to the other, what would we expect to occur next?

I assume that the mutual attraction of the stars will cause the row of stars
to shorten until they end up clumped together.

So to make my 'barred' galaxy stable I rotate it fast enough so that the
stars on the two ends don't proceed toward or away from the rotational
centre.

Will this work? *I assume it will not work close to the rotational centre
but will work further out.

There is a greater gravitational pull on objects closer to the two ends
because there is more mass between those objects and the centre, but the
rotational speed is greater as well - nicely balanced

Now we add more mass to the centre. *Objects toward the ends of the arms are
going to be drawn inwardly unless the arm describes an arc. *Now the pull
directly from the central mass can be added to the less effective pull of
the curved arm and the galaxy is again stable.

Why do I think this has been missed?
Models consist of known and unknown quantities, such as the numbers of and
masses of the stars that can be observed. *But unobserved is the central
mass which is estimated. *The central mass, I assume, is estimated at far
higher than it actually is. *I don't think modellers have considered
relatively tiny central masses, as in my barred galaxy model above. *Thus a
higher central mass then requires a greater mass beyond the galaxy for it to
rotate as observed.

I do not have the skills to test these ideas but I assume that either:
It has already been considered and shown to be flawed/viable or
members of this list can estimate the viability of the idea using a
simplified model (as I have suggested above).

http://www.google.co.uk/search?hl=en...tation&me ta=

All galaxies seem to contain a supermassive black hole. This seems to
be required from the point of view of stability. A number of
simulations have been performed.

http://www.mpa-garching.mpg.de/galform/

This is a website for one of the foremost groups.


- Ian Parker
  #3  
Old February 22nd 09, 01:40 PM posted to sci.astro,sci.physics,sci.physics.relativity
Robert Karl Stonjek
external usenet poster
 
Posts: 196
Default Questions on the enigmatic rotational curve of spiral galaxies


"Ian Parker" wrote in message
...
On 22 Feb, 12:11, "Robert Karl Stonjek"
wrote:
Snip

I do not have the skills to test these ideas but I assume that either:
It has already been considered and shown to be flawed/viable or
members of this list can estimate the viability of the idea using a
simplified model (as I have suggested above).

http://www.google.co.uk/search?hl=en...tation&me ta=

All galaxies seem to contain a supermassive black hole. This seems to
be required from the point of view of stability. A number of
simulations have been performed.

http://www.mpa-garching.mpg.de/galform/

This is a website for one of the foremost groups.


- Ian Parker

Thanks, Ian,
The simulations are a little to big but the paper search is more
interesting. For instance in The method of Galactic Rotation
http://adsabs.harvard.edu/abs/1996A&AS..118...59J
the authors assume an instant propagation of gravity and the gravitational
pull of the galactic arm itself appears to be left out altogether.

My quick skim of several results leads me to conclude that the Kepler model
is the first approximation and this fits well with the inner region of a
galaxy eg 1pc for the Milky Way (the supermassive black hole accounts for
the motion of the inner objects). But the bar model may be a better
approximation for the outer region ie starting with evenly spaced massive
objects across, say 20kpc (comparable to the Milky Way) and then consider
the initial conditions of a static and rotating bar and see what happens.

The simplified model is the bread and butter of physics modelling, but the
sphere or disc may not be the best simplified model in this case. The bar
may be more instructive for the outer region.

Here is my objection clearly outlined in the abstract of another paper:
"Galaxies as rotating systems involve a balance of gravitational attraction
and centripetal force. When the central mass is dominant the dynamics,
referred to as Keplerian, are that the orbital velocities are proportional
to the inverse square root of the path radius. A plot of velocity vs. path
radius is a Rotation Curve. For galaxies viewed as the disk edge we see one
end moving toward us relative to the center and the other moving away. The
rotational velocities are measured from the variation in redshift along the
galactic diameter. Rotation curves so obtained are not the expected
Keplerian inverse square root; rather, [beyond the galactic core] they are
flat. In a solid sphere all parts move at rotational velocities directly
proportional to radius. Since the observed flat curves lie between the
Keplerian inverse square root of radius and the solid's direct proportion to
radius, a mass discrepancy is inferred -- that unseen matter is dispersed
throughout the galaxy, a halo of "dark matter" that causes the curve form
exhibited -- thus the "dark matter" hypothesis. The mass discrepancy only
appears where the acceleration is less than 10-8 cm/sec2. Modeling gives an
alternative hypothesis: Modification of Newtonian Dynamics or MOND, that
gravity and or inertia are modified when g or a is less than 10-8 cm/sec2.
No justification has been developed except correlation with the mass
discrepancies. An alternative explanation is presented -- the general
exponential decay of the overall universe, which has been analyzed and
developed in several papers. The universal decay accounts for the mass
discrepancies because the effect of the decay is to make the rotation curves
appear to deviate from the form expected in a Keplerian galactic disk
although the actual rotational behavior does not."
http://www.citeulike.org/group/48/article/70704

Referring back to my illustration at the root of this thread we note that
force '3' has been ignored.
I can't tell if the models on the page you point to consider those forces or
not.

Robert


  #4  
Old February 22nd 09, 02:39 PM posted to sci.astro,sci.physics,sci.physics.relativity
john190209
external usenet poster
 
Posts: 11
Default Questions on the enigmatic rotational curve of spiral galaxies

On Feb 22, 7:40 am, "Robert Karl Stonjek"
wrote:
"Ian Parker" wrote in message

...
On 22 Feb, 12:11, "Robert Karl Stonjek"
wrote:
Snip

I do not have the skills to test these ideas but I assume that either:
It has already been considered and shown to be flawed/viable or
members of this list can estimate the viability of the idea using a
simplified model (as I have suggested above).


http://www.google.co.uk/search?hl=en...alactic+rotati...

All galaxies seem to contain a supermassive black hole. This seems to
be required from the point of view of stability. A number of
simulations have been performed.

http://www.mpa-garching.mpg.de/galform/

This is a website for one of the foremost groups.

- Ian Parker

Thanks, Ian,
The simulations are a little to big but the paper search is more
interesting. For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J
the authors assume an instant propagation of gravity and the gravitational
pull of the galactic arm itself appears to be left out altogether.

My quick skim of several results leads me to conclude that the Kepler model
is the first approximation and this fits well with the inner region of a
galaxy eg 1pc for the Milky Way (the supermassive black hole accounts for
the motion of the inner objects). But the bar model may be a better
approximation for the outer region ie starting with evenly spaced massive
objects across, say 20kpc (comparable to the Milky Way) and then consider
the initial conditions of a static and rotating bar and see what happens.

The simplified model is the bread and butter of physics modelling, but the
sphere or disc may not be the best simplified model in this case. The bar
may be more instructive for the outer region.

Here is my objection clearly outlined in the abstract of another paper:
"Galaxies as rotating systems involve a balance of gravitational attraction
and centripetal force. When the central mass is dominant the dynamics,
referred to as Keplerian, are that the orbital velocities are proportional
to the inverse square root of the path radius. A plot of velocity vs. path
radius is a Rotation Curve. For galaxies viewed as the disk edge we see one
end moving toward us relative to the center and the other moving away. The
rotational velocities are measured from the variation in redshift along the
galactic diameter. Rotation curves so obtained are not the expected
Keplerian inverse square root; rather, [beyond the galactic core] they are
flat. In a solid sphere all parts move at rotational velocities directly
proportional to radius. Since the observed flat curves lie between the
Keplerian inverse square root of radius and the solid's direct proportion to
radius, a mass discrepancy is inferred -- that unseen matter is dispersed
throughout the galaxy, a halo of "dark matter" that causes the curve form
exhibited -- thus the "dark matter" hypothesis. The mass discrepancy only
appears where the acceleration is less than 10-8 cm/sec2. Modeling gives an
alternative hypothesis: Modification of Newtonian Dynamics or MOND, that
gravity and or inertia are modified when g or a is less than 10-8 cm/sec2.
No justification has been developed except correlation with the mass
discrepancies. An alternative explanation is presented -- the general
exponential decay of the overall universe, which has been analyzed and
developed in several papers. The universal decay accounts for the mass
discrepancies because the effect of the decay is to make the rotation curves
appear to deviate from the form expected in a Keplerian galactic disk
although the actual rotational behavior does not."http://www.citeulike.org/group/48/article/70704

Referring back to my illustration at the root of this thread we note that
force '3' has been ignored.
I can't tell if the models on the page you point to consider those forces or
not.

Robert



May I add a thought?

The Milky Way is about a hundred thousand light years across.
Let's take this as about average.

Now, if I am looking at a similar galaxy out
in space, won't I be seeing the front edge as
it was positioned at Time X in the same image
as the back edge as it was positioned at
Time X minus 100,000 years?

So why are galaxies all so
symmetrical? Shouldn't they all be
skewed in a way that reflects their
movement and our position and the difference
between when light arrives to us from
their closest and farthest parts relative to us?

john
  #5  
Old February 22nd 09, 04:27 PM posted to sci.astro,sci.physics,sci.physics.relativity
Ian Parker
external usenet poster
 
Posts: 2,554
Default Questions on the enigmatic rotational curve of spiral galaxies

On 22 Feb, 13:40, "Robert Karl Stonjek"
wrote:


Thanks, Ian,
The simulations are a little to big but the paper search is more
interesting. *For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J
the authors assume an instant propagation of gravity and the gravitational
pull of the galactic arm itself appears to be left out altogether.

There is always the question about the validity of assumptions.
Instantanious travekl for gravity can be defended on the basis that
the errors in that approximation are of the order of rotation
velocities relative to c.

Now the Sun is travelling at 600km/s c = 300,000km/s. Hence the
instantaneous travel of gravity can be justified. The velocities of
the stars relative to each other are lower still.

What is in fact more serious is assumptions about "dark matter".


- Ian Parker
  #6  
Old February 22nd 09, 07:48 PM posted to sci.astro,sci.physics,sci.physics.relativity
Greg Neill[_6_]
external usenet poster
 
Posts: 605
Default Questions on the enigmatic rotational curve of spiral galaxies

Sam Wormley wrote:
john190209 wrote:


The Milky Way is about a hundred thousand light years across.
Let's take this as about average.

Now, if I am looking at a similar galaxy out
in space, won't I be seeing the front edge as
it was positioned at Time X in the same image
as the back edge as it was positioned at
Time X minus 100,000 years?

So why are galaxies all so
symmetrical? Shouldn't they all be
skewed in a way that reflects their
movement and our position and the difference
between when light arrives to us from
their closest and farthest parts relative to us?

john


I don't think M31 looks all that symmetrical.

But getting to your argument. If you assume that the a given
galaxy looks about the same as it did a quarter of a revolution
earlier, why wouldn't you expect to observer relative symmetry
even when looking back further in time for the backside?


Also, if we assume a rotation rate for the galaxy consistent
with the Solar System's orbit about the galactic center, that is,
about 240 million years, then the 100,000 year light crossing time
amounts to a rotational offset of only about 0.15 degrees from
one side to the other.


  #7  
Old February 22nd 09, 07:52 PM posted to sci.astro,sci.physics,sci.physics.relativity
Eric Gisse
external usenet poster
 
Posts: 1,465
Default Questions on the enigmatic rotational curve of spiral galaxies

On Feb 22, 7:27*am, Ian Parker wrote:
On 22 Feb, 13:40, "Robert Karl Stonjek"
wrote:



Thanks, Ian,
The simulations are a little to big but the paper search is more
interesting. *For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J
the authors assume an instant propagation of gravity and the gravitational
pull of the galactic arm itself appears to be left out altogether.


There is always the question about the validity of assumptions.
Instantanious travekl for gravity can be defended on the basis that
the errors in that approximation are of the order of rotation
velocities relative to c.

Now the Sun is travelling at 600km/s c = 300,000km/s. Hence the
instantaneous travel of gravity can be justified. The velocities of
the stars relative to each other are lower still.


Uh, no. Gravitational effects travel exclusively at c in GR, and this
has been shown to be consistent with reality in observation.


What is in fact more serious is assumptions about "dark matter".

* - Ian Parker


  #8  
Old February 22nd 09, 07:54 PM posted to sci.astro,sci.physics,sci.physics.relativity
[email protected]
external usenet poster
 
Posts: 41
Default Questions on the enigmatic rotational curve of spiral galaxies

On Feb 22, 7:11*am, "Robert Karl Stonjek"
wrote:
I just want to add a thought experiment to help us visualise what is being
suggested/asked.

Consider a number of stars arrayed in a line across space, say a few light
years in length.

Assuming the initial condition is one of a stationary motion


"stationary motion"?

AHAHAHAHAHAHAHAHA

Tom Davidson
Richmond, VA
  #9  
Old February 22nd 09, 08:34 PM posted to sci.astro,sci.physics,sci.physics.relativity
Ian Parker
external usenet poster
 
Posts: 2,554
Default Questions on the enigmatic rotational curve of spiral galaxies

On 22 Feb, 19:52, Eric Gisse wrote:
On Feb 22, 7:27*am, Ian Parker wrote:





On 22 Feb, 13:40, "Robert Karl Stonjek"
wrote:


Thanks, Ian,
The simulations are a little to big but the paper search is more
interesting. *For instance in The method of Galactic Rotationhttp://adsabs.harvard.edu/abs/1996A&AS..118...59J
the authors assume an instant propagation of gravity and the gravitational
pull of the galactic arm itself appears to be left out altogether.


There is always the question about the validity of assumptions.
Instantanious travekl for gravity can be defended on the basis that
the errors in that approximation are of the order of rotation
velocities relative to c.


Now the Sun is travelling at 600km/s c = 300,000km/s. Hence the
instantaneous travel of gravity can be justified. The velocities of
the stars relative to each other are lower still.


Uh, no. Gravitational effects travel exclusively at c in GR, and this
has been shown to be consistent with reality in observation.

I never said they did not. What I DID say was that the error involved
is proportional to relative velocities/c. (assuming travel at c). If a
lot of calculation is involved you may assume instantaneous travel for
orbital velocities of 600km/s and relative velocities lower still.


- Ian Parker
  #10  
Old February 23rd 09, 12:03 AM posted to sci.astro,sci.physics,sci.physics.relativity
Robert Karl Stonjek
external usenet poster
 
Posts: 196
Default Questions on the enigmatic rotational curve of spiral galaxies


wrote in message
...
On Feb 22, 7:11 am, "Robert Karl Stonjek"
wrote:
I just want to add a thought experiment to help us visualise what is being
suggested/asked.

Consider a number of stars arrayed in a line across space, say a few light
years in length.

Assuming the initial condition is one of a stationary motion


"stationary motion"?

AHAHAHAHAHAHAHAHA

Tom Davidson
Richmond, VA

RKS:
OK, I was thinking of writing either 'stationary' or 'no motion' and ended
up with an insane hybrid.

Thanks for pointing out the error (I think...)

Robert


 




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