The mass of the graviton and the cosmological constant puzzle
  We propose an interpretation of the cosmological constant puzzle - i.e., the enormous value of the ratio $\rho_{Pl}/\rho_{vac} \approx 10^{120} $- in terms of the total number of gravitons in the observable universe based on a recently discovered relationship between $\Lambda$ and the mass $m_{g}$ of the graviton. 
M. Novello