 According to the formerly reported 4-D spherical model of the universe, factors affecting the redshift are discussed.  In addition to the factor from the space expansion, two other factors derived from the light speed variation are proposed: 1) Energy density factor of the wave medium, and 2) Electromagnetic interaction factor caused by interaction of light with substances in the 3-D space.  Subsequently the direct correlation of the light propagated distance with the redshift is given. 

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... perimposed graphs of it on the observed data from the Supernova Cosmology Project exhibited an excellent fit for a case that the current radius (equal to our observed time) of the universe is between 0.7 and 0.8 of its maximum. 4-D Spherical Model of the Universe 1) -3)  The space of the universe is the area where energy is spread.  At the Big Bang, the energy distribution transformed to be spread with expansion in 3-D surface of a 4-D sphere by the original tracing time (t).  A vibration of the intrinsic space energy (Spacia) in the 3-D space vests additional energy, which is our observable energy as light / quantum particle.  The Time we observe passing constantly (T) is the radius dimension (x) of the 4-D sphere for the distribution of the space energy.  Light speed has been varying with space expansion due to changing energy density of the medium. There is a special frame stationary to the medium 3) .