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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From the quora site, Stephen Perrenod stated that "temperature will be cut in half, each 11 billion years or so into the future, i.e. 11 billion years or now it will be around 1.4 K." Is that true? How did he know that? any data or observation for that? – Ron Ald Apr 02 '16 at 09:13
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@RonAld: I haven't read that post, but I would guess he is saying that the Hubble time is about 11 billion years i.e. in 11 billion years the value of $a(t)$ will have doubled and the temperature will have halved. I'm a bit surprised at this because the accelerated expansion caused by dark energy means the Hubble time is tending to a constant value around 20% greater than the current value of 14 billion years. – John Rennie Apr 02 '16 at 09:19