They aren't exactly bubbles expanding at the speed of light (necessarily). They are more like: when you get a particle interaction whose outcome is determined b

They aren't exactly bubbles expanding at the speed of light (necessarily). They are more like: when you get a particle interaction whose outcome is determined by a fact which has more than one answer, you get more than one outcome. And that applies recursively as those particles whizz off and interact with anything else. It's possible though unlikely that they could be expanding no faster than the speed of a walking scientist - if nothing else in the universe is yet causally determined by the outcome.