  Current observations of the cosmic microwave background could confirm an
increase in the radiation energy density after primordial nucleosynthesis but
before photon decoupling. We show that, if the gravitino problem is solved by a
light axino, dark (decoupled) radiation emerges naturally in this period
leading to a new upper bound on the reheating temperature T_R < 10^{11} GeV. In
turn, successful thermal leptogenesis might predict such an increase. The Large
Hadron Collider could endorse this opportunity. At the same time, axion and
axino can naturally form the observed dark matter.
