We investigate the reheating process by the gluon pair productions through QCD trace anomaly. Energy densities of inflaton efficiently transferred into thermal radiation. We identify the conformal zero mode as inflaton The potential energy of inflaton is dark energy.
We argue dark energy decays rapidly by gluon pair emission during reheating and even after the big bang. The reheating temperature is determined by the decay width ¥sqrt{M_p ¥Gamma} as 10^6 GeV. As the Universe cools below the hadronic scale, dark energy density is almost frozen. The energy density of dark energy still decreases by emitting two photons through QED trace anomaly. We can estimate the magnitude of dark energy from the QED decay width ¥sqrt{M_p ¥Delta} ¥sim eV. We have come a long way to give the upper bound on the present magnitude of dark energy as (10^{-2.5} eV)^4.