  We show that the modulated reheating mechanism can naturally account for dark
radiation, whose existence is hinted by recent observations of the cosmic
microwave background radiation and the primordial Helium abundance. In this
mechanism, the inflaton decay rate depends on a light modulus which acquires
almost scale-invariant quantum fluctuations during inflation. We find that the
light modulus is generically produced by the inflaton decay and therefore a
prime candidate for the dark radiation. Interestingly, an almost
scale-invariant power spectrum predicted in the modulated reheating mechanism
gives a better fit to the observation in the presence of the extra radiation.
We discuss the production mechanism of the light modulus in detail taking
account of its associated isocurvature fluctuations. We also consider a case
where the modulus becomes the dominant component of dark matter.
