The purpose of this paper is to outline some recent progress in descriptive inner model theory, a branch of set theory which studies descriptive set theoretic and inner model theoretic objects using tools from both areas. There are several interlaced problems that lie on the border of these two areas of set theory, but one that has been rather central for almost two decades is the conjecture known as the Mouse Set Conjecture (MSC). One particular motivation for resolving MSC is that it provides grounds for solving the inner model problem which dates back to 1960s. There have been some new partial results on MSC and the methods used to prove the new instances suggest a general program for solving the full conjecture. It is then our goal to communicate the ideas of this program to the community at large.The program of constructing canonical inner models for large cardinals has been a source of increasingly sophisticated ideas leading to a beautiful theory of canonical models of fragments * 2000 Mathematics Subject Classifications: 03E15, 03E45, 03E60. † of set theory known as mice. It reads as follows:The inner model program. Construct canonical inner models with large cardinals.It is preferred that the constructions producing such inner models are applicable in many different situations and are independent of the structure of the universe. Such universal constructions are expected to produce models that are "canonical": for instance, the set of reals of such models must be definable. Also, it is expected that when the universe itself is complicated, for instance, Proper Forcing Axiom (PFA) holds or that it has large cardinals, then the inner models produced via such universal constructions have large cardinals. To test the universality of the constructions, then, we apply them in various situations such as under large cardinals, PFA or under other combinatorial statements known to have large cardinal strength, and see if the resulting models indeed have significant large cardinals. Determining the consistency strength of PFA, which is the forward direction of the PFA Conjecture stated below, has been one of the main applications of the inner model theory and many partial results have been obtained (for instance, see Theorem 3.32 and Theorem 3.33). The reverse direction of the PFA Conjecture is a well-known theorem due to Baumgartner.Conjecture 0.1 (The PFA Conjecture) The following theories are equiconsistent.1. ZFC+PFA.
ZFC+"there is a supercompact cardinal".Recently, in [54], Viale and Weiss showed that if one forces PFA via a proper forcing then one needs to start with a supercompact cardinal. This result is a strong evidence that the conjecture must be true.One approach to the inner model program has been via descriptive set theory. Section 3.8 contains some details of such an approach. Remark 3.34 summarizes this approach in more technical terms then what follows. The idea is to use the canonical structure of models of determinacy to squeeze large cardinal strength out of stronger and stronger theories from t...