# Gabriel Goldberg

ggoldberg@berkeley.edu

I'm an NSF postdoc at UC Berkeley working in set theory, interested in large cardinals, inner models, and infinite combinatorics. Here's a link to my book, The Ultrapower Axiom, adapted from my 2019 thesis completed at Harvard University under Hugh Woodin.

## Teaching

• Math 236 (Lecturer). Metamathematics of set theory. UC Berkeley. Fall 2021. Syllabus.
• Math 113 (Lecturer). Abstract Algebra. UC Berkeley. Spring 2021.
• Math 99r (Instructor). Tutorial on Infinite Combinatorics. Harvard. Fall 2018.
• Math 99r (Instructor). Tutorial on Forcing. Harvard. Spring 2018.
• CS 20 (Teaching Fellow). Discrete mathematics. Harvard. Spring 2015.
• CS 121 (Teaching Fellow). Theory of computation. Harvard. Fall 2013.
• CSCI E-250 (Head Teaching Fellow). Introduction to Computer Science II. Harvard. Spring 2013.

## Books

• The Ultrapower Axiom. De Gruyter. 2022.

The inner model problem for supercompact cardinals, one of the central open problems in modern set theory, asks whether there is a canonical model of set theory with a supercompact cardinal. The problem is closely related to the more precise question of the equiconsistency of strongly compact cardinals and supercompact cardinals. This dissertation approaches these two problems abstractly by introducing a principle called the Ultrapower Axiom which is expected to hold in all known canonical models of set theory. By investigating the consequences of the Ultrapower Axiom under the hypothesis that there is a supercompact cardinal, we provide evidence that the inner model problem can be solved. Moreover, we establish that under the Ultrapower Axiom, strong compactness and supercompactness are essentially equivalent.

## Papers and Preprints

• Usuba's extendible cardinal. Preprint. 2021.

Answering a question of Usuba, we show that an extendible cardinal can be preserved by a set forcing that is not a small forcing.

• Reinhardt cardinals in inner models. Preprint. 2021.

A cardinal is weakly Reinhardt if it is the critical point of an elementary embedding from the universe of sets into a model that contains the double powerset of every ordinal. This note establishes the equiconsistency of a proper class of weakly Reinhardt cardinals with a proper class of Reinhardt cardinals in the context of second-order set theory without the Axiom of Choice.

• Measurable cardinals and choiceless axioms. Submitted, Kunen Memorial Volume, APAL. 2021.

We prove that if there is an elementary embedding from the universe to itself, then there is a proper class of measurable successor cardinals.

• Strongly compact cardinals and ordinal definability. Submitted, Journal of Mathematical Logic. 2021.

This paper explores several topics related to Woodin's HOD conjecture. We improve the large cardinal hypothesis of Woodin's HOD dichotomy theorem from an extendible cardinal to a strongly compact cardinal. We show that assuming there is a strongly compact cardinal and the HOD hypothesis holds, there is no elementary embedding from HOD to HOD, settling a question of Woodin. We show that the HOD hypothesis is equivalent to a uniqueness property of elementary embeddings of levels of the cumulative hierarchy. We prove that the HOD hypothesis holds if and only if every regular cardinal above the first strongly compact cardinal carries an ordinal definable omega-Jonsson algebra. We show that if the HOD hypothesis holds and HOD satisfies the Ultrapower Axiom, then every supercompact cardinal is supercompact in HOD.

• The uniqueness of elementary embeddings. Submitted, Journal of Symbolic Logic. 2021.

Much of the theory of large cardinals beyond a measurable cardinal concerns the structure of elementary embeddings of the universe of sets into inner models. This paper seeks to answer the question of whether the inner model uniquely determines the elementary embedding.

• On products of ultrafilters. Preprint. 2021.

Assuming the Generalized Continuum Hypothesis, this paper answers the question: when is the tensor product of two ultrafilters equal to their Cartesian product? It is necessary and sufficient that their Cartesian product is an ultrafilter; that the two ultrafilters commute in the tensor product; that for all cardinals λ, one of the ultrafilters is both λ-indecomposable and λ+-indecomposable; that the ultrapower embedding associated to each ultrafilter restricts to a definable embedding of the ultrapower of the universe associated to the other.

• On cardinal preserving embeddings. Preprint. 2021.

We prove the consistency of the theory ZFC + there is a strongly compact cardinal from the existence of a cardinal preserving embedding from the universe into an inner model. The proof almost shows that under SCH, every cardinal preserving embedding preserves the cofinality and continuum functions.

• Periodicity in the cumulative hierarchy (with F. Schlutzenberg). JEMS. 2021.

Assuming choiceless large cardinal axioms, we show that for sufficiently large ordinals λ, the structure of Vλ depends on the parity of λ.

• Even ordinals and the Kunen inconsistency. Preprint. 2020.

This paper contributes to the theory of large cardinals beyond the Kunen inconsistency, or choiceless large cardinal axioms, in the context where the Axiom of Choice is not assumed. The first part of the paper investigates a periodicity phenomenon: assuming choiceless large cardinal axioms, the properties of the cumulative hierarchy turn out to alternate between even and odd ranks. The second part of the paper explores the structure of ultrafilters under choiceless large cardinal axioms, exploiting the fact that these axioms imply a weak form of the author's Ultrapower Axiom. The third and final part of the paper examines the consistency strength of choiceless large cardinals, including a proof that assuming DC, the existence of an elementary embedding from Vλ+3 to Vλ+3 implies the consistency of ZFC + I0. By a recent result of Schlutzenberg, an elementary embedding from Vλ+2 to Vλ+2 does not suffice.

• Some combinatorial properties of Ultimate L and V. Preprint. 2020.

This paper establishes a number of constraints on the structure of large cardinals under strong compactness assumptions. These constraints coincide with those imposed by the Ultrapower Axiom, a principle that is expected to hold in Woodin's hypothesized Ultimate L, providing some evidence for the Ultimate L Conjecture. We show that every regular cardinal above the first strongly compact that carries an indecomposable ultrafilter is measurable, answering a question of Silver for large enough cardinals. We show that any successor almost strongly compact cardinal of uncountable cofinality is strongly compact, making progress on a question of Boney, Unger, and Brooke-Taylor. We show that if there is a proper class of strongly compact cardinals then there is no nontrivial cardinal preserving elementary embedding from the universe of sets into an inner model, answering a question of Caicedo granting large cardinals. Finally, we show that if κ​ is strongly compact, then V is a set forcing extension of the inner model κ​-HOD consisting of sets that are hereditarily ordinal definable from a κ​-complete ultrafilter over an ordinal; κ​-HOD seems to be the first nontrivial example of a ground of V whose definition does not involve forcing.

• Strong compactness and the Ultrapower Axiom I. Accepted, Journal of Mathematical Logic.

The relationship between the large cardinal notions of strong compactness and supercompactness cannot be determined under the standard ZFC axioms of set theory. Under a hypothesis called the Ultrapower Axiom, we prove that the notions are equivalent except for a class of counterexamples identified by Menas. This is evidence that strongly compact and supercompact cardinals are equiconsistent.

• Rank-into-rank embeddings and Steel's conjecture. Journal of Symbolic Logic. 2021.

This paper establishes a conjecture of Steel regarding the structure of elementary embeddings from a level of the cumulative hierarchy into itself. Steel’s question is related to the Mitchell order on these embeddings, which has been studied by Laver, Neeman, Steel, and others. Although this order is known to be illfounded, Steel conjectured that it has certain large wellfounded suborders, which is what we establish. The proof relies on a simple and general analysis of the much broader class of extender embeddings and a variant of the Mitchell order called the internal relation.

• The Ultrapower Axiom and the GCH. Journal of Mathematical Logic. 2021.

We prove that the Generalized Continuum Hypothesis holds above a supercompact cardinal assuming the Ultrapower Axiom, an abstract comparison principle motivated by inner model theory at the level of supercompact cardinals.

• The Ketonen order. Journal of Symbolic Logic. 2020.

We study a partial order on countably complete ultrafilters introduced by Ketonen as a generalization of the Mitchell order. The following are our main results: the order is wellfounded; its linearity is equivalent to the Ultrapower Axiom, a principle introduced in the author’s dissertation; finally, assuming the Ultrapower Axiom, the Ketonen order coincides with Lipschitz reducibility in the sense of generalized descriptive set theory.

• The Rudin-Frolík order. Preprint. 2018.

We study the structure of the Rudin-Frolík order on countably complete ultrafilters under the assumption that this order is directed. This assumption, called the Ultrapower Axiom, holds in all known canonical inner models. It turns out that assuming the Ultrapower Axiom, one can prove much more about the Rudin-Frolík order than is possible in ZFC alone. Our main theorem is that under the Ultrapower Axiom, a countably complete ultrafilter has at most finitely many predecessors in the Rudin-Frolík order. In other words, any wellfounded ultrapower (of the universe) is the ultrapower of at most finitely many ultrapowers.

• The linearity of the Mitchell order. Journal of Mathematical Logic. 2018.

We show from an abstract comparison principle (the Ultrapower Axiom) that the Mitchell order is linear on sufficiently strong ultrafilters: normal ultrafilters, Dodd solid ultrafilters, and assuming GCH, generalized normal ultrafilters. This gives a conditional answer to the well-known question of whether a 2κ-supercompact cardinal κ must carry more than one normal measure of order 0. Conditioned on a very plausible iteration hypothesis, the answer is no, since the Ultrapower Axiom holds in the canonical inner models at the finite levels of supercompactness.

## Invited Talks

• Large cardinals, maximality principles, and the multiverse. Helsinki Logic Seminar. 2021.

We discuss some mathematical results indicating a tension between large cardinal axioms and maximality principles such as forcing axioms and the Axiom of Choice. Topics will include the failure of the Ground Axiom in natural models, the optimality of Usuba's theorem, the analogy between large cardinal axioms and determinacy principles, and the role of forcing in theory selection.

• The Jackson analysis and the strongest hypotheses. XVI International Luminy Workshop in Set Theory. 2021.

In his work on the projective ordinals, Jackson initiated a detailed analysis of the structure of the cardinal numbers in the inner model L(R). To complete this analysis seems to require the development of a global theory of measures on ordinals in L(R). We present some results in this direction which are inspired by the unexplained analogy between the Axiom of Determinacy and the strongest large cardinal hypotheses.

• Measurable cardinals and choiceless axioms. Harvard Set Theory Seminar. 2021.

I'll prove that if there is a Reinhardt cardinal, there is a proper class of regular cardinals and discuss further results on measurable cardinals.

• Predictions of UA. ASL Regional Meeting. 2021.

The program to build canonical models for large cardinal axioms has hit an impasse below the level of supercompact cardinals. The Ultrapower Axiom (UA), a simple combinatorial principle that is expected to hold in all canonical models, enables one to develop a detailed structure theory for supercompact cardinals, apparently providing the first glimpse of the canonical models at this level. Assuming they exist, canonical models with supercompact cardinals are expected to exert a strong influence on the structure of the universe of sets itself, motivating the prediction that certain consequences of UA are actually provable outright in ZFC or from large cardinal axioms. This talk surveys some attempts to confirm this prediction.

Sample results: any two elementary embeddings from the universe of sets to the same inner model agree on the ordinals (a conjecture of Woodin); any regular cardinal above the first strongly compact that carries a uniform indecomposable ultrafilter is measurable (a question of Silver); assuming a proper class of strongly compact cardinals, there is no elementary embedding from the universe into a cardinal correct inner model (a conjecture of Caicedo); if κ is extendible, the ultrapower of the universe by a κ+-complete ultrafilter admits no non-trivial self-embeddings (a question of van Name); the Mitchell order is wellfounded on elementary embeddings from Vλ to Vλ with critical points bounded strictly below λ (a conjecture of Steel). Applications to countably incomplete ultrafilters, set-theoretic geology, the HOD conjecture, and the theory of large cardinals beyond choice will also be discussed.

• Strong compactness and the ω-club filter. UIC Logic Seminar. 2021.

I'll discuss some ideas relating strong compactness, club filters, and inner models, with two applications: first, a generalization of Woodin's HOD dichotomy that applies to any inner model with access to the ω-club filter on each ordinal of uncountable cofinality, and second, an analysis of strong compactness in the HODs of models satisfying either the Axiom of Determinacy or choiceless large cardinal assumptions.

• The HOD Conjecture and the structure of elementary embeddings. CUNY Set Theory Seminar. 2021.

Woodin's HOD conjecture asserts that in the context of very large cardinals, the inner model HOD closely approximates the universe of sets in the same way Gödel's constructible universe does assuming 0# does not exist. The subject of these two talks is the relationship between Woodin's conjecture and certain constraints on the structure of elementary embeddings of the universe of sets. For example, in the second talk, we will prove that any two elementary embeddings of the universe of sets into the same inner model agree on HOD, while if a local version of this theorem held, then the HOD conjecture would follow.

• Ordinal definability and the structure of large cardinals. CMU Logic Seminar. 2021.

Roughly speaking, Woodin's HOD conjecture asserts that assuming the existence of very large cardinals, arbitrary sets can be closely approximated by definable ones. This talk outlines an approach to the conjecture based on an analysis of the uniqueness properties of ultrafilters and elementary embeddings, which has a number of applications: for example, a proof of a variant of the HOD conjecture for sets definable from ultrafilters, a proof of Woodin's HOD dichotomy theorem from a single strongly compact cardinal, and a proof that past an extendible cardinal, elementary embeddings of the universe of sets are uniquely determined by their codomains.

• Embeddings of HOD. UC Irvine Logic Seminar. 2021.

Jensen's covering lemma states that either every uncountable set of ordinals is covered by a constructible set of ordinals of the same size or else there is an elementary embedding from the constructible universe to itself. This talk takes up the question of whether there could be an analog of this theorem with constructibility replaced by ordinal definability. For example, we answer a question posed by Woodin: assuming the HOD conjecture and a strongly compact cardinal, there is no nontrivial elementary embedding from HOD to HOD.

• Ultrafilters and definability Cornell Logic Seminar. 2020.

The Ultrapower Axiom is a structural principle for ultrafilters expected to hold in the canonical inner model for a supercompact cardinal if it exists. The existence of such a model should also imply the HOD Conjecture, which asserts that under large cardinal hypotheses, HOD correctly computes successors of singular cardinals. The proof that the HOD Conjecture follows from UA motivates the study of definability from ultrafilters under large cardinal assumptions. Combined with techniques from set theoretic geology, this investigation leads to some new structural properties of large cardinals related to both UA and the HOD Conjecture. For example, while it is consistent that there is an inner model M ​​that admits 22κ​​-many elementary embeddings j : V → M​​ where κ​​ is the least measurable cardinal, we show that if there is an extendible cardinal δ​​ and a proper class of strongly compact cardinals, then any inner model M​​ admits at most one elementary embedding j : V → M​​ with critical point greater than δ​​.

• Even ordinals and the Kunen inconsistency. Oxford Set Theory Seminar. 2020.

The Burali-Forti paradox suggests that the transfinite cardinals “go on forever,” surpassing any conceivable bound one might try to place on them. The traditional Zermelo-Frankel axioms for set theory fall into a hierarchy of axiomatic systems formulated by reasserting this intuition in increasingly elaborate ways: the large cardinal hierarchy. Or so the story goes. A serious problem for this already naive account of large cardinal set theory is the Kunen inconsistency theorem, which seems to impose an upper bound on the extent of the large cardinal hierarchy itself. If one drops the Axiom of Choice, Kunen’s proof breaks down and a new hierarchy of choiceless large cardinal axioms emerges. These axioms, if consistent, represent a challenge for those “maximalist” foundational stances that take for granted both large cardinal axioms and the Axiom of Choice. This talk concerns some recent advances in our understanding of the weakest of the choiceless large cardinal axioms and the prospect, as yet unrealized, of establishing their consistency and reconciling them with the Axiom of Choice.

• The uniqueness of elementary embeddings. Barcelona Set Theory Seminar. 2020.

This talk will explore the connection between Woodin's HOD Conjecture and certain uniqueness properties of elementary embeddings of models of set theory, which will lead to some consequences of the failure of the HOD Conjecture reminiscent of the consequences of choiceless large cardinal axioms.

• Ultrapowers and the approximation property. CUNY Set Theory Seminar. 2020.

Countably complete ultrafilters are the combinatorial manifestation of strong large cardinal axioms, but many of their basic properties are undecidable no matter the large cardinal axioms one is willing to adopt. The Ultrapower Axiom (UA) is a set theoretic principle that permits the development of a much clearer picture of countably complete ultrafilters and, consequently, the large cardinals from which they derive. It is not known whether UA is (relatively) consistent with very large cardinals, but assuming there is a canonical inner model with a supercompact cardinal, the answer should be yes: this inner model should satisfy UA and yet inherit all large cardinals present in the universe of sets. The predicted resemblance between the large cardinal structure of this model and that of the universe itself is so extreme as to suggest that certain consequences of UA must in fact be provable outright from large cardinal axioms. While the inner model theory of supercompact cardinals remains a major open problem, this talk will describe a technique that already permits a number of consequences of UA to be replicated from large cardinals alone. Still, the technique rests on the existence of inner models that absorb large cardinals, but instead of building canonical inner models, one takes ultrapowers.

• Structure theorems for strongly compact cardinals. Oberwolfach Webinar on Set Theory. 2020.

Expanding on ideas due to Hamkins in the context of forcing and Woodin in the context of inner model theory, we prove several theorems which suggest that the large cardinal structure of the universe of sets above a strongly compact cardinal is more tractable than the ubiquity of independence results at this level would suggest.

• Comparison principles and very large cardinals. Berkeley Logic Colloquium. 2019.

I outlined the consequences of the Ultrapower Axiom for the theory of supercompactness and strong compactness.

• The Ultrapower Axiom. European Set Theory, Plenary Talk. 2019.

I outlined the consequences of the Ultrapower Axiom for the theory of supercompactness and strong compactness.

• The Ground Axiom, the Ultrapower Axiom, and Ultimate L. UC Irvine Logic Seminar. 2019.

I tried to argue for the Ground Axiom on the basis of Usuba's Theorem and the Ultimate L Conjecture on the basis of the Ultrapower Axiom.

• Two generalizations of the Ultrapower Axiom. Harvard Logic Colloquium. 2019.

The Ultrapower Axiom (UA) is a set theoretic principle motivated by inner model theory. Under UA, it is possible to develop certain parts of large cardinal theory that are intractable under the standard ZFC axioms. Is UA consistent with all large cardinal axioms? Since UA is necessarily compatible with any large cardinal axiom that admits a canonical inner model theory, this question is likely to be intertwined with the longstanding open problem of constructing canonical inner models with very large cardinals. In this talk, we present two natural generalizations of UA that are not motivated by inner model theory. The goal is to refute these principles from large cardinal axioms. We view this as a way of identifying interesting complex structure at key levels of the large cardinal hierarchy. It also serves to corroborate the intuition that the (apparent) consistency of UA with supercompact cardinals is evidence that inner model theory extends to this level.

• Supercompact cardinals and the Mitchell order, I and II. Inner Model Theory Conference. 2017-2018.

In 2017, I presented a proof that the Ultrapower Axiom implies GCH above a supercompact cardinal. In 2018, I sketched a proof that the Ultrapower Axiom implies the equivalence of strong compactness and supercompactness (modulo a class of counterexamples identified by Menas in the 1970s).

## Education

• Harvard University. 2015 - 2019
• Harvard College. 2010 - 2014
• AB in Mathematics. Minor in Computer Science.
• Summa cum laude. Phi Beta Kappa.