Temperature of the whole universe is about 2.7K base on the wmap satellite. I know that temperature has to decrease to time and I have difficult time finding references for rate of change of temperature of the universe over time. Is there any equation or Argument or observation or graph?
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The temperature is inversely proportional to the scale factor. If you're interested in the gory details see Cooling in the Universe by Sohrab Rahvar.
So your question reduces to how the scale factor $a(t)$ has changed with time. There isn't a simple answer to this because there is no simple analytic function for $a(t)$. It has to be computed numerically using the experimental data on densities e.g. from the WMAP experiment. I do this in How does the Hubble parameter change with the age of the universe?, or search the site for scale factor for various related answers.
John Rennie
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