Conformal frames and the validity of Birkhoff's theorem
  Birkhoff's theorem is one of the most important statements of Einstein's general relativity, which generally can not be extended to modified theories of gravity. Here we study the validity of the theorem in scalar-tensor theories using a perturbative approach, and compare the results in the so-called Einstein and Jordan frames. The implications of the results question the physical equivalence between both frames, at least in perturbations. 
Salvatore Capozziello and Diego S\'aez-G\'omez
gr-qc hep-th