Microwave background constraints on extended inflation voids

Extended inflation ends via the nucleation of bubbles of true vacuum, and in principle the earliest bubbles to nucleate can be expanded by the last stages of inflation to astrophysically interesting sizes, giving large voids empty of any matter. Several authors have suggested a link with the observed bubbly large-scale structure in the Universe. We examine in detail the constraints on the bubble distribution from the microwave background observations, particularly COBE, for different types of dark matter.