Dual symmetry and the vacuum energy
  In this work we present a new hidden symmetry in gravity for the scale factor in the FRW model, for $k=0$. This exact symmetry vanishes the cosmological constant. We interpret this hidden symmetry as a dual symmetry in the sense that appears in the string theory. 
V.I. Tkach, J. Socorro, J.J. Rosales and J.A. Nieto