Large Cardinals – $\aleph_1$ Without the Axiom of Choice

axiom-of-choicelarge-cardinalslo.logicset-theory

Assuming the axiom of choice it is very easy to see that $\aleph_1$ is a regular Joe of a successor cardinal. It is not very large in any way except the fact that it is the first uncountable cardinal.

If however we begin with a model of ZFC+Inaccessible, we can construct models of ZF in which $\aleph_1$ is somewhat inaccessible in the sense that $\aleph_1\nleq 2^{\aleph_0}$ If, on the other hand, we start with a model of ZF whic has this property then there exists an inner model with an inaccessible cardinal.

It can be that $\aleph_1$ is a measurable cardinal, you can even have that every subset of $\omega_1$ contains a club, or is non-stationary; and it is possible for $\aleph_1$ to have the tree property (I only know of models by Apter in which all successor cardinals have the tree property; but that would require a proper class of very large cardinals).

In general we say that $\aleph_1$ is P-large for a large cardinal property P, if it is consistent with ZF that $\aleph_1$ has property P, and from such model we can produce a model of ZFC+$\kappa>\aleph_0$ has property P.

Question: Is there a limit on how P-large can $\aleph_1$ be? (e.g. P can be tree property/$\kappa$-complete ultrafilter/supercompact measures/etc.) and are there properties P such that for $\aleph_1$ to have them we require more than ZFC+P?

Best Answer

-----edited to include corrections, thanks Joel and Tanmay-----

From a model of "ZFC + $large(\kappa)$" you can get a model of "ZF + $large(\kappa)$ + $\kappa=\omega_1$" if $large(\ )$ is a large cardinal property that is preserved under small forcing and can be written in the form

"for every set of ordinals $X$, there is a set $Y$ such that $\phi(X,Y)$ holds", for some upwards absolute formula $\phi$,

so for example weakly compact, Ramsey, measurable. The model you'd use is basically Jech's model for making $\omega_1$ measurable in "$\omega_1$ can be measurable".

For details see the comments below, Tanmay's answer and my thesis "Symmetric Models, Singular Cardinal Patterns, and Indiscernibles" Chapter 1, section 3.3. These large cardinals are there called "preserved under symmetric forcing".

For the other way around (a limit on how large can $large(\ )$ be), as Trevor Wilson said, measurable in ZF gives measurable in ZFC, and I guess similar arguments would work for large cardinal properties that are "preserved under symmetric forcing". I guess this is not a very good limit though and I can't come up with/remember something better right now. If I do I'll come back to answer.

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