The cosmological redshift phenomenon can be described by the dark matter field fluid model, the results deduced from this model agree very well with the observations. The observed cosmological redshift of light depends on both the speed of the emitter and the distance between the emitter and the observer. If the emitter moves away from us, a redshift is observed. If the emitter moves towards us, whether a redshift, a blueshift or no shift is observed will depend on the speed vs. the distance. If the speed is in the range of c(exp[-beta*D]-1) < v < 0, a redshift is observed; if the speed equals c(exp[-beta*D]-1), no shift is observed; if the speed v less than c(exp[-beta*D]-1), a blueshift is observed. A redshift will be always observed in all directions for any celestial objects as long as their distance from us is large enough. Therefore, many more redshifts than blueshifts should be observed for galaxies and supernovae, etc in the sky. This conclusion agrees with current observations. The estimated value of the redshift constant beta of the dark matter field fluid is in the range of 10^(-3) ~ 10^(-5)/Mpc. A large redshift value from a distant celestial object may not necessarily indicate that it has a large receding speed. Based on the redshift effect of dark matter field fluid, it is concluded that at least in time average all photons have the same geometry (size and shape) in all inertial reference frames and do not have length contraction effect.
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