- #31
ohwilleke
Gold Member
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Angular momentum is not conserved in galaxies anyway. If it has more angular momentum than it can gravitationally constrain then it flings some of the stars on the rim off into deep space. As new objects passing through space enter a galaxy, they may be assimilated and transfer their momentum angular and otherwise to the galaxy. A galaxy is not a closed system.
We clearly do not have enough data, because we have competing theories, whose differences are not conclusively resolved by that data.
MOND is just one modified gravity theory. There are many other theories in this class, some of which are yet to be discovered.
Gravity of some kind. from DM, in a modified gravity theory, or otherwise, constrains particles that would otherwise fly away from a spiral galaxy to stay within it at a constant angular rotation speed. This is well explained by both classes of theories. These theories create torque by taking stars with linear kinetic energy that pass into the plane of a spiral galaxy and using gravity to force that passing object into a circular movement around the galactic core which is the center of gravity. It is no different than the spin created if a speed skater grabs a rope connected to a pole on the ice rink and starts twisting around the pole rather than going straight. The way that gravity can create torque is elementary. What is remarkable is that the strength of the gravitational pull, in either DM theories or modified gravity theories (NOT JUST MOND!) is more than we would naively expect from the luminous matter alone.
In any situation where a galaxy is adding mass, or could shed mass, it is not a closed system and the conservation of total angular momentum does not apply.
As I explained, in some modified gravity theories, the direction of merger of two spiral galaxies is not random because the strength of the gravitational field is different on the axis of the galaxy and in its plane of rotation. This does explain who spiral galaxies avoid most destructive mergers and also explains how galaxies can grow from the edges feeding off smaller galaxies and clumps of star in their disk plane at the edges. Problem solved.
We clearly do not have enough data, because we have competing theories, whose differences are not conclusively resolved by that data.
MOND is just one modified gravity theory. There are many other theories in this class, some of which are yet to be discovered.
Gravity of some kind. from DM, in a modified gravity theory, or otherwise, constrains particles that would otherwise fly away from a spiral galaxy to stay within it at a constant angular rotation speed. This is well explained by both classes of theories. These theories create torque by taking stars with linear kinetic energy that pass into the plane of a spiral galaxy and using gravity to force that passing object into a circular movement around the galactic core which is the center of gravity. It is no different than the spin created if a speed skater grabs a rope connected to a pole on the ice rink and starts twisting around the pole rather than going straight. The way that gravity can create torque is elementary. What is remarkable is that the strength of the gravitational pull, in either DM theories or modified gravity theories (NOT JUST MOND!) is more than we would naively expect from the luminous matter alone.
In any situation where a galaxy is adding mass, or could shed mass, it is not a closed system and the conservation of total angular momentum does not apply.
As I explained, in some modified gravity theories, the direction of merger of two spiral galaxies is not random because the strength of the gravitational field is different on the axis of the galaxy and in its plane of rotation. This does explain who spiral galaxies avoid most destructive mergers and also explains how galaxies can grow from the edges feeding off smaller galaxies and clumps of star in their disk plane at the edges. Problem solved.