I recall a pulp scifi book about some scientists who set up some sort of containment field and created a sort of mini big bang in the tank.It turned out to be a sort of analog to our universe, but due to its small size it evolved so fast they could watch it. Galaxies formed in it, and star systems within the galaxies. Life occurred throughout, and the various denizens grew to explore their cosmos. Meanwhile the scientists could watch with super power microscopes and follow the developments. The minis developed interstellar travel and eventually became aware of the limits of their universe - the walls of the experimental chamber. Blah blah blah, they build ships to breach that barrier to find out what was outside their universe. Eventually these microscopic beings succeeded and their ships broke through into our universe. As they flew through the ships expanded into our size things, and teh book ended with a what have we done thought.

It was a nice enough story. It might have been one side of an Ace double novel. Don't recall title or author.