How could some inflationary multiverse models be empirically testable?

In the context of cosmology, inflationary multiverse models are considered empirically testable because they are based on well-understood physical processes and inhabit a common space-time, which may allow for detectable interactions between neighboring universes.1

The most prominent method for testing these models involves searching for evidence of "bubble collisions." In the inflationary multiverse, different universes are envisioned as expanding bubbles within a common background. If two such bubbles were to collide, the impact could leave a distinct, circular "bruise" or disturbance on the cosmic microwave background radiation.1

While some critics argue that multiverse theories are speculative or philosophical rather than scientific, proponents maintain that models arising as a side effect of well-motivated theories—such as inflation and string theory—are legitimate subjects of scientific inquiry, particularly when they admit the possibility of empirical verification through observable effects.1