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We study the possibility of detecting dark radiation (DR) produced by acombination of interactions with the thermal bath and ultra-light primordialblack hole (PBH) evaporation in the early universe. We show that the detectionprospects via cosmic microwave background (CMB) measurements of the effectiverelativistic degrees of freedom ${\rm \Delta N_{eff}}$ get enhanced in somepart of the parameter space compared to the purely non-thermal case where DR isproduced solely from PBH. On the other hand, for certain part of the parameterspace, DR which initially decouples from the bath followed by its productionfrom PBH evaporation, can re-enter the thermal bath leading to much tighterconstraints on the PBH parameter space. We also discuss the complementarydetection prospects via observation of stochastic gravitational wave (GW)sourced by PBH density perturbations. The complementary probes offered by CMBand GW observations keep the detection prospects of such light degrees offreedom very promising in spite of limited discovery prospects at particlephysics experiments.

Thermalised dark radiation in the presence of PBH: ${\rm \Delta N_{\rm eff}}$ and gravitational waves complementarity