Uniformization and internal absoluteness

Measurability with respect to ideals is tightly connected with absoluteness principles for certain forcing notions. We study a uniformization principle that postulates the existence of a uniformizing function on a large set, relative to a given ideal. We prove that for all \sigma-ideals I such that the ideal forcing \mathbb {P}_I of Borel sets modulo I is proper, this uniformization principle is equivalent to an absoluteness principle for projective formulas with respect to \mathbb {P}_I that we call internal absoluteness . In addition, we show that it is equivalent to measurability with respect to I together with 1-step absoluteness for the poset \mathbb {P}_I. These equivalences are new even for Cohen and random forcing and they are, to the best of our knowledge, the first precise equivalences between regularity and absoluteness beyond the second level of the projective hierarchy.