Dark radiation from modulated reheating

Abstract

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