On Sun, 26 Sep 1999, Robbie wrote:

> I read an answer to a previous question on this subject on your fine web
> service.
> Am I right in thinking that the expansion of matter from an exploding point
> in a soon-after Big Bang universe would suggest that the rotation axes of of
> clumping matter should all be perpendicular to the direction of expansion,
> and that none should be parallel to it?
> How does this change to the seeming randomness of galactic axis orientation
> seen today?

Hello,

You might here be thinking of a scenario like the following: Imagine that
I have a handful of marbles, and I drop them onto the floor. Even though
none of the marbles were spinning, they will all roll away from the spot
where I dropped them, and their rotation will be perpendicular to the
direction they are "expanding" away from me.

This analogy for the Big Bang doesn't work for two reasons that are not at
all intuitive and show the complexity of the Universe in which we live.

First, the expansion of the Universe does not mean that there is a
direction in which the Universe is expanding. There is no "center" of the
Universe from which all the galaxies came from. Rather, ALL space is
expanding in all directions. Imagine that the Universe is an infinitely
large rubber sheet, and something is causing the rubber sheet to stretch.
All of us living on the rubber sheet would notice that it is stretching,
but there is no center to the stretching - everything is getting further
away from everything else.

Secondly, in the analogy of the marbles, the marbles start spinning
because they hit the floor, and friction between the marbles and the floor
sets them to spinning. In space, though, there is no friction to set
things spinning. So any rotation that we see has to original - the clouds
of gas that created our galaxy (and the clouds that created our solar
system) had to have some initial spin (or angular momentum, as we prefer
to call it). Otherwise, the galaxy would not rotate.

But if the Big Bang did not have any angular momentum, where did the spin
come from? This is still uncertain, but it probably has to do with
turbulence within gas. Due to turbulence, fluids and gases which have no
angular momentum can form pockets that are rotating. This is easy enough
to see. Go to any water (whether in a cup, sink, lake, ocean, etc.) and
pull your hand through the water. Little eddies and whirlpools form behind
your hand, even though the water and your hand are not spinning! The same
probably happened in the early universe. As clouds of gas collided, tiny
amounts of spin were caused by turbulence.

Another way of "creating" angular momentum that also very likely occured
in the early univers is caused by two clouds of gas passing each other.
Although these clouds are initially travelling on straight lines, the
gravity of each cloud will pull of the other cloud, causing it to curve
from its path. If the gravity is strong enough that the clouds orbit will
orbit each other and merge (this also requires some form of turbulence to
get rid of some extra energy), then the merged system will have some spin
to it.

Once a cloud of gas starts spinning, it will continue to do so. And the
spin does not need to be very fast - as gravity causes the cloud to
collapse, the rate of spin will increase, just like an ice skater pulling
in their arms will spin faster. In fact, one of the big problems in
astronomy deals with the fact that most clouds that form stars have far
more angular momentum than the resulting stars do - if the stars do not
somehow get rid of this excess spin, then they would have to spin at
nearly the speed of light! How this shedding of angular momentum occurs
is still not clear.

Since both turbulence and the collision of clouds of gas are random
processes, then the orientation of the spin axes should be random - and it
is observed to be so.

Thanks for writing!

Sincerely,
Kurtis Williams

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