How could collisions between bubble universes leave signs in the cosmic microwave background?

In the context of inflationary cosmology, the multiverse is often modeled as a collection of expanding bubbles within a common background. When these bubbles collide, the impact can leave a distinct physical imprint on the cosmic microwave background (CMB) radiation.1

Specifically, such a collision is theorized to create a "bruise" on the CMB, which would manifest as a circular disturbance or pattern in the radiation. This potential signature is a subject of interest because it offers a rare, albeit speculative, method for empirically testing models of the multiverse that arise from inflation.1

While the anthropic principle is often invoked to explain why physical constants—such as the cosmological constant—are compatible with life in our specific bubble, the search for these circular CMB signatures represents an attempt to move beyond purely philosophical arguments toward observable evidence for the existence of other universes.1