How are bubble universes described in inflationary multiverse models?

In inflationary cosmology, bubble universes are described as expanding regions within a common background of space-time that is still undergoing inflation.1

In models of eternal inflation, the transition from an inflationary state to ordinary expansion occurs at different times and in different locations. This process generates an arbitrarily large or infinite number of post-inflationary regions, which are conceptualized as bubbles. These bubbles inhabit the same overarching space-time, and because they are part of a connected system, it is theoretically possible for neighboring bubbles to interact.1

Some researchers have suggested that if these bubbles collide, the resulting impact could leave a detectable "bruise" in the form of a circular disturbance on the cosmic microwave background radiation. While these models are based on well-understood physics, they remain a subject of significant scientific debate regarding their empirical testability and philosophical implications.1