🚀 go-pugleaf

baez@ucrmath.ucr.edu (john baez) writes...
>>                                           The region of space which is
>>inside a galaxy satisfies the same local einstein equation as the above
>>recollapsing example, so why do galaxies live so long?
> 
>'Cause they're smaller.  I'm sure one can work out the math, but
>it's bound to amount to just that.  For example, one can have
>a fist-sized ball of iron, which will essentially never collapse
>(quantum fluctuations aside), but a galaxy-sized ball of iron
>would collapse mighty fast.

I'm surprised at you John, you forgot the atomic repusion! :)

I get that a fist-sized ball of superdust weighing in around a kilogram
collapses in about an hour. I think that will be about the same for an
observer in the center of your iron galaxy. But I'm depending on my
already disproven intuition. :)

I also think the size has an effect, along with the empty 'outside'. I.e.
there will be tidal forces which will disturb my intuition. There are no
tidal forces in the closed cosmology. I would then demand that the scale
at which I approximate the star density as a fluid is small enough that
the tidal forces are small. But a given region gets stretched out so
that they become large nevertheless. I haven't decided yet whether this solves
my problem.

Dave Ring
dwr2560@zeus.tamu.edu