Published online by Cambridge University Press: 01 February 2021
I investigate the relationships between three hierarchies of reflection principles for a forcing class
$\Gamma $
: the hierarchy of bounded forcing axioms, of
$\Sigma ^1_1$
-absoluteness, and of Aronszajn tree preservation principles. The latter principle at level
$\kappa $
says that whenever T is a tree of height
$\omega _1$
and width
$\kappa $
that does not have a branch of order type
$\omega _1$
, and whenever
${\mathord {\mathbb P}}$
is a forcing notion in
$\Gamma $
, then it is not the case that
${\mathord {\mathbb P}}$
forces that T has such a branch.
$\Sigma ^1_1$
-absoluteness serves as an intermediary between these principles and the bounded forcing axioms. A special case of the main result is that for forcing classes that don’t add reals, the three principles at level
$2^\omega $
are equivalent. Special attention is paid to certain subclasses of subcomplete forcing, since these are natural forcing classes that don’t add reals.