Universes out of almost empty space

Abstract

Baby universes (inflationary or non--inflationary) are regions of spacetime that disconnect from the original ambient spacetime, which we take to be asymptotically flat spacetime. A particular kind of baby universe solution, involving string--like matter, is studied to show that it can be formed by ``investing'' an arbitrarily small amount of energy, i.e. it can appear from an almost flat space at the classical level. Since this possibility has not yet been clearly recognized in the literature, we then discuss in detail its properties, relevance and possible generalizations.