Computable Structures in Generic Extensions

In this paper, we investigate connections between structures present in every generic extension of the universe V and computability theory. We introduce the notion of generic Muchnik reducibility that can be used to to compare the complexity of uncountable structures; we establish basic properties of this reducibility, and study it in the context of generic presentability, the existence of a copy of the structure in every extension by a given forcing. We show that every forcing notion making ω2 countable generically presents some countable structure with no copy in the ground model; and that every structure generically presentable by a forcing notion that does not make ω2 countable has a copy in the ground model. We also show that any countable structure A that is generically presentable by a forcing notion not collapsing ω1 has a countable copy in V , as does any structure B generically Muchnik reducible to a structure A of cardinality א1. The former positive result yields a new proof of Harrington’s result that counterexamples to Vaught’s conjecture have models of power א1 with Scott rank arbitrarily high below ω2. Finally, we show that a rigid structure with copies in all generic extensions by a given forcing has a copy already in the ground model.