Flattening Of Galaxies' Disc

1. Nov 20, 2008

81+

The discs of all large galaxies show a "flattening" of their discs where the material at the outer edge of the discs rotate at the same rate as the material near the center of the galaxy. The explanation for this is that the galaxy must be embedded in a halo of invisible "dark" matter which, I assume, is also thought to be rotating. The question is: why does this theory assume that all of the material in the "dark" matter disc rotates at the same rate from its outer edge to its center --- rather than having the material at the outer edge rotating more slowly than near the center as would be expected?

Frank

2. Nov 20, 2008

mgb_phys

I think you have it slightly backward.
We can directly measure the rotation rate of the galaxy at each distance from the centre. From the shape of this curve and the law of gravity - we can work out how much mass there must be at each radius.
If you add up the mass from counting the visible stars you don't get enough mass for this to work - so we assume there must be some mass other than the stars that we can't see.

3. Nov 20, 2008

turbo

Either that, or we must consider that the Newtonian inverse square model of gravity and/or its relativistic re-expression are inaccurate. I prefer a universe where observables (the ONLY thing we have in cosmology) can falsify our models. If our observations cannot falsify our cosmologies, our cosmologies are faith-based fancies.

Last edited: Nov 20, 2008
4. Nov 20, 2008

81+

I understand that we can measure the rotational velocity of all of the mass at a given radius from the center of the galaxy.

I do not understand what you mean (in your second sentence) by "How much mass there must be at each radius". Must be to accomplish what? How does increasing the mass at a given radius change the velocity of that mass? I guess I've forgotten the relationship between the mass at some radius and it's rotational velocity.

Frank

Frank

5. Nov 20, 2008

wolram

It is COLD DARK MATTER, whatever that is, AFAIK no one has ever experienced this stuff, it is a stuff (evoked) to explain what we think we see, i will gladly except this (stuff) if some one will tell me what it (is).

6. Nov 20, 2008

mgb_phys

More accuratetly it is the mass within a certain radius.
The orbital speed of an object depends on the total mass inside the orbit.
So you can work out the mass of the earth from the distance of the moon's orbit.
So knowing the orbital speed of a star at a certain distance from the centre you can work out the mass within that distance.
Assuming you believe the law of gravity of course - it could be that there is no dark matter but the law of gravity is slightly different at large distances.

7. Nov 21, 2008

81+

mgb phys, thanks. I've got it now. If I live long enough, I'll understand everything.

Frank

8. Nov 30, 2008

tommy dee

I think what you are trying to understand is why galaxies tend to flatten out. this is due to there being no friction in space, so stars at the centre of the galaxy can travel at the stable relative position to those on the rim; but the farther from galactic centre you are, the more able you are to break away from the gravitic pull of the galaxy. stars move away from galactic centre because they have the momentum, being a sum of energy and mass, to wriggle between all the other stars that affect their escape attempt in the process. the gravity of the galactic core being supposedly around 2% or 1/50th of the total of an observed galaxy is just enough to effect a chain of gravitic events to make a galaxy, that ends where the momentum and massergy of any star (or any item) reaches a 'break away point', when it drifts out from the galaxy as a forgotten sibling, and (using the weakening drag of the galactic gravity) accelerates into intra-galactic space, possibly taking some other masses and gas clouds along with it. there are bigger and smaller galaxies but the ratios are consistent as observed. gravity is not increased or erroded by space itself.
The 'pecking order' of a flattening galaxy is entirely interdependent on its star neighbour and hood. as the galaxy stabilizes, events that alter the pecking order internally disappear and only a super nova or external event will affect something new.
the black hole at the galactic centre will continue to expand in balance to the extent and limit of its 'inward pull' against the 'outward pull' of the other 49/50ths. eventually, the galaxy matures and becomes flat about the circumference of maximum gravity(equator) which makes the galaxy appear flat, except for a spherical bulge of the 1/50th of the galactic core.
eventually, either the galaxy can die, being stripped and erroded of its stars, or it can implode into a supermassive black hole, or accrete into another galaxy. gravitic infiuence defines the connection of any star to a galaxy, so if you are in the pattern, you are 'in' or if not, you are 'out' or a 'rogue star' from another galaxy passing through.
rogue stars or even two or three galaxies can merge; but they will be very much different to thier solus pecking orders, and collisions and double novas' are most likely for a long time, until the conjoined masses settle down.
as for dark matter holding it all together... atoms that have been 'burnt' in a super nova (98%) where the neucleus has lost its electrons, and been left a 'husk' of matter with no energy, are damaged and unable to exchange charges. quarks are unaffected by them as they have no force of attraction or repulsion., consequently, this dark matter is the ash of the fires of suns, and can be best observed where one would expect an event to occur which is impeded for no viewable reason, forgiving the tragedy of an error. our world is full of these 'dead' atoms. and so is space. they have no gravitic power because they have no charge. they are dead mass.