As a model-independent generic feature of string moduli, we find that their derivative interactions make them decay into (pseudo) Nambu-Goldstone bosons (NGBs) without suppression by the mass of decay products. Combining this feature with the fact that moduli decay well after spontaneous electroweak symmetry breaking, we show that the decay into the longitudinal weak gauge bosons is generally dominant, recovering the standard hot big bang universe. Further, interestingly, as a natural consequence of non-suppression of axionic decay modes, an observable amount of dark radiation is expected in any extension of the Standard Model that includes a cosmologically stable NGB such as the QCD axion. In such extensions, the fermionic superpartner of the NGB can also contribute to the dark matter as a feebly interacting massive particle.
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