# How can the rotation of galaxies be explained?

• Raimund Kempe
In summary, the principle of spin conservation states that the total angular momentum of a system cannot be zero. This is why galaxies have a residual angular momentum even though the total angular momentum of the universe is not zero.

#### Raimund Kempe

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How can galaxies rotate coming from a singularity?

Raimund

Raimund Kempe said:
How did you find PF?: Google search

How can galaxies rotate coming from a singularity?

Raimund
You apparently have a misconception of what "singualrity" means and you think it means a point in space where things came from. Even in pop-science it is not stated that galaxies came from a singularity but it is stated (incorrectly) that the universe exploded from a singularity.

Singularity just means "the place where our math model breaks down and we know know WHAT is/was going on".

DaveE and davenn
Thanks for your response and the rectification, but the real question remains unanswered: How can galaxies' rotation be explained?

Raimund

Raimund Kempe said:
How can galaxies' rotation be explained?
It is explained by Conservation of Angular Momentum.

1. Take a volume of matter with random movement.
2. Gravity draws the matter into clumps.
3. The clumps retain the angular momentum of the original dust cloud, leaving a non-zero residual motion.
4. Paths that do not intersect survive longer than paths that collide, leading to the gas cloud planing out and rotating in a common direction.

russ_watters, sophiecentaur, davenn and 1 other person
Thanks for bringing in this important principle of spin conservation.
Yet we have two questions:
What does this principle mean for the whole universe with its billions of rotating galaxies?
And what does it mean for the state at the very beginning?

Raimund

Raimund Kempe said:
What does this principle mean for the whole universe with its billions of rotating galaxies?
nothing - the universe is not rotating - you seem to have reverted back to thinking that the universe "exploded" from a point in space.
And what does it mean for the state at the very beginning?
nothing - there was no rotation at the beginning - you seem to have reverted back to thinking that the universe "exploded" from a point in space and was somehow rotating then.

EDIT: think of a table on which there are a large number of spinning tops, some spinning one way, some the other way. Do you think that means that the table will rotate?

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The vector-sum of all galactic spins defines what could be called 'rotation'.
This vector-sum is not zero.

Raimund

Perhaps it was a bad analogy. Do you think the entire universe is spinning? Do you realize that that would imply an axis that it is spinning about, which would imply a preferred frame of reference, thus negating the Cosmological Principle (the basis of modern cosmology)?

sophiecentaur and DaveC426913
Raimund Kempe said: