Flat-Directions in Grand Unification With $U(1)_R$ Symmetry
  It is shown that in SO(10) and SU(5) models having a $U(1)_R$ symmetry, the requirement of breaking the unified group to the Standard Model leads to flat directions in the scalar potential. These can lead to a ``cosmological modulus problem". This is relevant to grand unified models of inflation, where $U(1)_R$ symmetries are often used to insure the flatness of the inflaton potential. A way that the modulus problem might be avoided is discussed. 
S. M. Barr, Bumseok Kyae, Qaisar Shafi