Our question arises from wondering about the systems of natural numbers in models ZFC + Con(ZFC) and ZFC + $\neg$Con(ZFC). In thinking of the systems of natural numbers of these models, we came to the following remark and questions. If we take the ultrapower of a model of Peano arithmetic we get a model of Peano arithmetic with non-standard natural numbers like <1, 2, 3, ...> which have infinitely many natural numbers below them. My question is: if we take an ultrapower, $M$, of a model of ZFC, $V$, does $M$ have non-standard natural numbers? 



Models of the natural numbers in ultrapowers in the universe.
An ultrapower of a model $M$ of ZFC will have non-standard natural numbers if and only if either $M$ has non-standard natural numbers or the ultrafilter used in forming the ultrapower is countably incomplete. The natural numbers of the ultrapower of $M$ with respect to an ultrafilter $U$ are isomorphic to the ultrapower with respect to U of the natural numbers of $M$. (More generally, the operations of $U$-ultrapower and taking the subset of a model defined by a particular formula always commute up to canonical isomorphism.) The ultrapower will satisfy Con($PA$) if and only if $M$ does, by Los's theorem. 


The preceding was written with "ordinary" ultrapowers in mind. You could also form "internal" ultrapowers of a model $M$ of ZFC. You'd use an element $U$ of $M$ that satisfies in $M$ the formula "$U$ is an ultrafilter on a set $I$", and elements of the ultrapower would be represented by functions on $I$ in the sense of $M$ (i.e., elements $f$ of $M$ that satisfy in $M$ the formula "$f$ is a function on $I$"). Everything I wrote in the first paragraph would still be correct provided you interpret "countably complete" in the sense of $M$.


Finally, one can, in some situations, form ultrapowers where the ultrafilter is external to the model $M$ but the functions representing ultrapower elements are internal to $M$. Similar (but not identical) results hold for these also.


