mmm in the pushing Universe it has to be pushed to the surface or they wont have it... and like temp being almost a misrepresentation of what is really going on ....using the word bubble may give the old human grey matter a little different view as to what they may think is going on.. I could build pictures of a watermellon busting from that one. So it seems a case of matter within moving more rapidly in a given area, opps a 2d imagery in a 3d application (or 4?) lets say region, than within the rest of the object, hence the perhaps misleading illusion of bubble being suggested, and once this matter starts moving it moves to one end, presumably as this escape route offers less resistence ... to escape... but blasting all to bits in the escape attempt..er escape finally is right cause the result could not be called a mere attempt.
So what bits are moving so fast and why do they want to escape...I wonder are they being pushed..just kidding I think
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alex