  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.
