This is a talk in the virtual CUNY Models of Peano Arithmetic seminar, on 2020 July 29. It is about joint work with Joel David Hamkins. Our paper can be found here.

[Slides]

Recall that if $M \subseteq N$ are models of set theory then $N$ end-extends $M$ if $N$ does not have new elements for sets in $M$. In this talk I will discuss a $\Sigma_1$-definable finite sequence which is universal for end extensions in the following sense. Consider a computably axiomatizable extension $\overline{\mathsf{ZF}}$ of $\mathsf{ZF}$. There is a $\Sigma_1$-definable finite sequence

$a_0, a_1, \ldots, a_n$

with the following properties.

• $\mathsf{ZF}$ proves that the sequence is finite.

• In any transitive model of $\overline{\mathsf{ZF}}$ the sequence is empty.

• If $M$ is a countable model of $\overline{\mathsf{ZF}}$ in which the sequence is $s$ and $t \in M$ is a finite sequence extending $s$ then there is an end-extension $N \models \overline{\mathsf{ZF}}$ of $M$ in which the sequence is exactly $t$.

• Indeed, for the previous statements it suffices that $M \models \mathsf{ZF}$ and end-extends a submodel $W \models \overline{\mathsf{ZF}}$ of height at least $(\omega_1^{\mathrm{L}})^M$.

This universal finite sequence can be used to determine the modal validities of end-extensional set-theoretic potentialism, namely to be exactly the modal theory $\mathsf{S4}$. The sequence can also be used to show that every countable model of set theory extends to a model satisfying the end-extensional maximality principle, asserting that any possibly necessary sentence is already true.

The $\Sigma_1$ universal finite sequence is a sister to the $\Sigma_2$ universal finite sequence for rank-extensions of Hamkins and Woodin, and both are cousins of Woodin’s universal algorithm for arithmetic. 