Ultrafilters over Successor Cardinals and the Tukey Order

Abstract

We study ultrafilters on regular uncountable cardinals, with a primary focus on $ω_1$, and particularly in relation to the Tukey order on directed sets. Results include the independence from ZFC of the assertion that every uniform ultrafilter over $ω_1$ is Tukey-equivalent to $[2^{\aleph_1}]^{<ω}$, and for each cardinal $κ$ of uncountable cofinality, a new construction of a uniform ultrafilter over $κ$ which extends the club filter and is Tukey-equivalent to $[2^κ]^{<ω}$. We also analyze Todorcevic's ultrafilter $\mathcal{U}(T)$ under PFA, proving that it is Tukey-equivalent to $[2^{\aleph_1}]^{<ω}$ and that it is minimal in the Rudin-Keisler order with respect to being a uniform ultrafilter over $ω_1$. We prove that, unlike PFA, $\text{MA}_{ω_1}$ is consistent with the existence of a coherent Aronszajn tree $T$ for which $\mathcal{U}(T)$ extends the club filter. A number of other results are obtained concerning the Tukey order on uniform ultrafilters and on uncountable directed systems.