In de Sitter space-time using Robertson coordinates the Hubble redshift can be interpreted as gradual decrease of photon frequency. How this decrease of frequency might occur was suggested previously (Browne, 1962). The radiation field for each Planck oscillator is quantized gravitationally as a field of gravitons of minute constant energy. Scattering of a gravitron from one field to another toward the equilibrium blackbody spectrum results in a redshift, , where distance dl is propagated in a medium with radiant energy density U and A is a constant. A cross section, previously suggested (Browne, 1976), provides a theoretical value for A. The law becomes the Hubble redshift dw/w = dl/R if U = Kpoc2, where po is the gravitational mass density required to close the universe at radius R and K is the ratio of inertial to gravitational mass (a dimensional constant with value unity). It is argues that zero-point radiation (vacuum fluctuations) have renormalized energy density Kpoc2.
In quasars U is large enough to yield anomalous redshifts comparable with the Hubble redshift of the sources, but the sources must be at large distances in order to have the large values of U.