Local observed time and redshift in curved space-time

Using the observed time and spatial intervals defined originally by Einstei n and the observational frame in the vierbein formalism, we propose that in curved space-time, for a wave received in laboratories, the observed frequ ency is the changing rate of the phase of the wave relative to the local ob servable time scale and the momentum is the changing rate of the phase rela tive to the local observable spatial length scale. The case of Robertson-Wa lker universe is especially considered and the application to de Sitter uni verse results in a cosmological constant in perfect agreement with the obse rvational data.