Domain walls separate regions of different physical phases in the early universe. A localized source — a monopole, a quark, a D-brane — crossing such a wall might seem like a transient event. The source passes through; the wall heals. Nothing permanent happens.

The slingshot effect says otherwise. When a source crosses from an unconfined phase to a confined one, it becomes trapped by a stretched structure — a string or flux tube of appropriate co-dimensionality — that tethers it to the domain wall. The crossing is not a passage but an arrest. The source is captured, and the capture is violent enough to produce observable cosmological signatures.

Three signatures emerge from a single mechanism. First, the confinement process generates gravitational waves during the capture event. Second, the stretched structures can produce Kaluza-Klein gravitons — massive gravitational excitations from extra dimensions — that serve as dark matter candidates. Third, if the structures are massive enough, they can collapse into primordial black holes in mass ranges that current and future gravitational wave detectors can probe.

The unifying claim is that gravitational waves, dark matter, and primordial black holes — usually treated as separate phenomena requiring separate explanations — can arise from the same underlying process. The slingshot effect is generic: it occurs whenever localized sources encounter domain walls, which is a common feature of phase transitions in the early universe. The specifics (which signatures dominate, at what masses and frequencies) depend on the details of the phase transition, but the mechanism itself is universal.

One event, three observables. The domain wall crossing that looked transient was the most productive moment in the early universe.