Primordial black holes and wormholes from domain wall networks

Domain walls (DWs) are topological defects originating from phase transitions in the early Universe. In the presence of an energy imbalance between distinct vacua, enclosed DW cavities shrink until the entire network disappears. By studying the dynamics of thin-shell bubbles in general relativity, we demonstrate that closed DWs with sizes exceeding the cosmic horizon tend to annihilate later than the average. This delayed annihilation allows for the formation of large overdensities, which, upon entering the Hubble horizon, eventually collapse to form primordial black holes (PBHs). We rely on 3D percolation theory to calculate the number density of these late-annihilating DWs, enabling us to infer the abundance of PBHs. A key insight from our study is that DW networks with the potential to emit observable gravitational waves are also likely to yield detectable PBHs. Additionally, we study the production of wormholes connected to baby universes and conclude on the possibility to generate a multiverse.