Skip to main content
2 of 4
added 287 characters in body
Asaf Karagila
  • 39.8k
  • 8
  • 135
  • 283

Existence of Non-Borel sets in models of "All sets measurable"

We know that the consistency of ZFC+"Exists an inaccessible cardinal" implies the consistency of ZF+DC+"All sets are Lebesgue measurable"; and DC proves the existence of non-Borel sets.

J. Truss proved that repeating Solovay's construction by collapsing any limit cardinal to be $\aleph_1$ we obtain a model of ZF+"All sets are Lebesgue measurable", and in that model DC holds if and only if we collapsed an inaccessible. If we collapsed a singular cardinal then the resulting model has the property that all sets are Borel.

It begs the question, if we assume ZF+"All sets are Lebesgue measurable"+"There exists a non-Borel set", can we conclude that there is an inner model with an inaccessible cardinal?


Some clarifications:

  1. When I say Borel sets, I mean an element of the $\sigma$-algebra generated by the open sets.
  2. When I say Lebesgue sets, I mean an element of the $\sigma$-algebra generated by completing the Borel $\sigma$-algebra with respect to the null ideal.
Asaf Karagila
  • 39.8k
  • 8
  • 135
  • 283