The geology of inner mantles
This is a talk in the virtual CUNY set theory seminar, on 2020 May 29.
An inner model is a ground if V is a set forcing extension of it. The intersection of the grounds is the mantle, an inner model of ZFC which enjoys many nice properties. Fuchs, Hamkins, and Reitz showed that the mantle is highly malleable. Namely, they showed that every model of set theory is the mantle of a bigger, better universe of sets. This then raises the possibility of iterating the definition of the mantle—the mantle, the mantle of the mantle, and so on, taking intersections at limit stages—to obtain even deeper inner models. Let’s call the inner models in this sequence the inner mantles.
In this talk I will present some results about the sequence of inner mantles, answering some questions of Fuchs, Hamkins, and Reitz. Specifically, I will present the following results, analogues of classic results about the sequence of iterated HODs.

(Joint with Reitz) Consider a model of set theory and consider an ordinal eta in that model. Then this model has a class forcing extension whose etath inner mantle is the model we started out with, where the sequence of inner mantles does not stabilize before eta.

It is consistent that the omegath inner mantle is an inner model of ZF + ¬AC.

It is consistent that the omegath inner mantle is not a definable class, and indeed fails to satisfy Collection.