Non-existence of Universal Orders in Many Cardinals

Abstract

Our theme is that not every interesting question in set theory is independent of $ZFC$. We give an example of a first order theory $T$ with countable $D(T)$ which cannot have a universal model at ${\aleph }_1$ without CH; we prove in $ZFC$ a covering theorem from the hypothesis of the existence of a universal model for some theory; and we prove --- again in ZFC --- that for a large class of cardinals there is no universal linear order (e.g. in every ${\aleph }_1<ł<2^{{\aleph }_0}$). In fact, what we show is that if there is a universal linear order at a regular $ł$ and its existence is not a result of a trivial cardinal arithmetical reason, then $ł$ ``resembles'' ${\aleph }_1$ --- a cardinal for which the consistency of having a universal order is known. As for singular cardinals, we show that for many singular cardinals, if they are not strong limits then they have no universal linear order. As a result of the non existence of a universal linear order, we show the non-existence of universal models for all theories possessing the strict order property (for example, ordered fields and groups, Boolean algebras, p-adic rings and fields, partial orders, models of PA and so on).