[FOM] Absoluteness of Clay prize problems

[John T. Baldwin]

I had lunch with a mathematician the other day who opined that the Clay 
problems were probably all provable
or refuatable in ZFC.    One way to argue this (without finding proofs) 
is to give a syntactic analysis showing that the varous statements
are absolute.  For instance,  I have heard that the Riemann Hypothesis 
is pi^0_1.  Can anyone provide references on specific
statements or even better a summary article on this subject.