A Grothendieck universe is known in set theory as the set Vκ for a (strongly) inaccessible cardinal κ. They are exactly the same thing. Thus, the existence of a Grothendieck universe is exactly equivalent to the existence of one inaccessible cardinal. These cardinals and the corresponding universes have been studied in set theory for over a century.
The Grothendieck Universe axiom (AU) is the assertion that every set is an element of a universe in this sense. Thus, it is equivalent to the assertion that the inaccessible cardinals are unbounded in the cardinals. In other words, that there is a proper class of inaccessible cardinals. This is the axiom you sought, which is exactly equivalent to AU. In this sense, the axiom AU is a statement in set theory, having nothing necessarily to do with category theory.
The large cardinal axioms are fruitfully measured in strength not just by direct implication, but also by their consistency strength. One large cardinal property LC1 is stronger than another LC2 in consistency strength if the consistency of ZFC with an LC1 large cardinal implies the consistency of ZFC with an LC2 large cardinal.
Measured in this way, the AU axiom has a stronger consistency strength than the existence of any finite or accessible number of inaccessible cardinals, and so one might think it rather strong. But actually, it is much weaker than the existence of a single Mahlo cardinal, the traditional next-step-up in the large cardinal hierarchy. The reason is that if κ is Mahlo, then κ is a limit of inaccessible cardinals, and so Vκ will satisfy ZFC plus the AU axiom. The difference between AU and Mahloness has to do with the thickness of the class of inaccessible cardinals. For example, strictly stronger than AU and weaker than a Mahlo cardinal is the assertion that the inaccessible cardinals form a stationary proper class, an assertion known as the Levy Scheme (which is provably equivconsistent with some other interesting axioms of set theory, such as the boldface Maximality Principle, which I have studied a lot). Even Mahlo cardinals are regarded as rather low in the large cardinal hierarchy, far below the weakly compact cardinals, Ramsey cardinals, measurable cardinals, strong cardinals and supercompact cardinals. In particular, if δ is any of these large cardinals, then δ is a limit of Mahlo cardinals, and certainly a limit of strongly inaccessible cardinals. So in particular, Vδ will be a model of the AU axiom.
Rather few of the large cardinal axioms imnply AU directly, since most of them remain true if one were to cut off the universe at a given inaccessible cardinal, a process that kills AU. Nevertheless, implicit beteween levels of the large caridnal hiearchy are the axioms of the same form as AU, which assert an unbounded class of the given cardinal. For example, one might want to have unboundedly many Mahlo cardinals, or unboundedly many measurable cardinals, and so on. And the consistency strength of these axioms is still below the consistency strength of a single supercompact cardinal. The hierarchy is extremely fine and intensely studied. For example, the assertion that there are unboundedly many strong cardinals is equiconsistent with the impossiblity to affect projective truth by forcing. The existence of a proper class of Woodin cardinals is particularly robust set-theoretically, and all of these axioms are far stronger than AU.
There are natural weakenings of AU that still allow for almost all if not all of what category theorists do with these universes. Namely, with the universes, it would seem to suffice for almost all category-theoretic purposes, if a given universe U were merely a model of ZFC, rather than Vκ for an inaccessible cardinal κ. The difference is that U is merely a model of the Power set axiom, rather than actually being closed under the true power sets (and similarly using Replacement in place of regularity). The weakening of AU I have in mind is the axiom that asserts that every set is an element of a transitive model of ZFC. This assertion is strictly weaker in consistency strength thatn even a single inaccessible cardinal. One can get much lower, if one weakens the concept of universe to just a fragment of ZFC. Then one could arrive at a version of AU that was actually provable in ZFC, but which could be used for most all of the applications in cateogory theory to my knowledge. In this sense, ZFC itself is a kind of large cardinal axiom relative to the weaker fragments of ZFC.
The usual relations to consider in the large cardinal hierarchy
are
- Direct implication: every A cardinal is also a B cardinal
- Consistency strength implication: if ZFC + there is an A cardinal
is consistent, then so is ZFC + there is a B cardinal.
Your concept, however, is focused on the least instance of the
large cardinal notion, and this is also studied.
In broad terms, the large cardinal hierarchy is roughly linear,
with the stronger cardinals being stronger with respect to all
three of these relations. In most instances, we have that every A
cardinal (the stronger notion) is also a B cardinal, as well as a
limit of B cardinals, and so we get also the consistency
implication and the least A cardinal is strictly larger than the
least B cardinal.
However, there are some notable deviations from this. These
deviations come in two types.
First, there are the instances where a large cardinal concept A
has stronger consistency strength than B, but the least instance
of A is definitely less than the least instance of B. For example,
a superstrong cardinal has higher consistency strength
than a mere strong cardinal, since if $\kappa$ is superstrong, then $V_\kappa\models$ ZFC + there
is a proper class of strong cardinals, but the least superstrong
cardinal is definitely less than the least strong cardinal. This
is simply because superstrongness is witnessed by a single object,
and strong cardinals are $\Sigma_2$ reflecting, and therefore
reflect the least instance below.
There are numerous similar instances of this. Any time a large
cardinal notion is witnessed by a single object or is witnessed
inside some $V_\theta$ — and this would include weakly
compact, Ramsey, measurable, superstrong, almost huge, huge,
rank-to-rank and others — then the least instance of that cardinal will be
less than the least $\Sigma_2$-reflecting cardinal and indeed less
than the least $\Sigma_2$-correct cardinal. But $\Sigma_2$ correct
cardinals provably exist in ZFC, and therefore have very low
consistency strength.
So we have numerous interesting instances where your $<$ order
does not align with consistency strength:
- The least almost huge cardinal is strictly less than the least
strong cardinal.
- The least rank-to-rank cardinal is strictly less than the least
strongly unfoldable cardinal.
- The least $5$-huge cardinal is strictly less than the least
uplifting cardinal.
- There are hundreds of other similar examples. You can invent them yourself!
Meanwhile, second, there are examples of your $\perp$ situation,
where the size of the smallest instance is not yet settled. This
phenomenon is known as the "identity-crises" phenomenon, named by
Magidor when he proved that the least measurable can be the same
as the least strongly compact, or strictly less, depending on the
model of set theory. Many further instances of this are now known,
some of which appear in my paper:
This paper provides many instances of your $\perp$ situation, where the question of whether the least A cardinal is smaller than or the same size as the least B cardinal is not settled in ZFC.
Finally, let me qualify my remark that the large cardinal hierarchy is roughly linear. The hierarchy is indeed mainly linear, but one sometimes hears stronger assertions of linearity, as something that we know and which needs explanation, but I don't feel these knowledge claims are justified. Of course, the identity crises phenomenon provides instances of non-linearity in the direct implication hierarchy, and so when large cardinal set theorists assert that the large cardinal hierarchy is linear, they are speaking of the consistency strength order. So let me mention a few cases where we simply don't yet know linearity:
A supercompact cardinal versus a strongly compact plus an inaccessible above.
A supercompact cardinal versus a proper class of strongly compact cardinals.
A Laver-indestructible weakly compact cardinals versus a strongly compact cardinal.
A cardinal $\kappa$ that is $\kappa^+$-supercompact versus $\kappa$ is $\kappa^{++}$-strongly compact.
A PFA cardinal versus a strongly compact cardinal.
And many others.
My perspective is this. Because we have essentially no method for proving non-linearity in the consistency strength hierarchy, it is not surprising that we see only instances of linearity, and this may be a case of confirmation bias. But don't get me wrong: of course I agree that the consistency strength hierarchy is mainly linear in broad strokes.
Best Answer
There is another path that can be used to justify the existence of very large cardinals. Consider, for example, the abstract to Magidor's paper, "On the Role of Supercompact and Extendible Cardinals in Logic" (Israel Journal of Mathematics, Vol. 10, 1971, pp. 147-157) :
Here the theorems:
Note that the forward implication '$\Rightarrow$' of Theorems 1, 2, and 4 show that if the required large cardinal exists, the logic in question will have the required property. This suggests to me the following 'formalist' justification of the existence of large cardinals:
This also suggests the following research program:
Similarly, one can hypothesize the existence of certain Lowenheim-Skolem-Tarski numbers, as V$\ddot a$$\ddot a$n$\ddot a$nen shows in his paper, "Sort Logic and Foundations of Mathematics" ("Sort Logic...is a many-sorted extension of second-order logic."):