Entropy of Microwave Background Radiation in Observable Universe
  We show that the cosmological constant at late time places a bound on the entropy of microwave background radiation deposited in the future event horizon of a given observer, $S\leq S_{\Lambda_0}^{3/4}$. This bound is independent of the energy scale of reheating and the FRW evolution after reheating. We also discuss why the entropy of microwave background in our observable universe has its present value. 
Yun-Song Piao