The Standard Model predicts an effective number of relativistic neutrino species $N_{eff} \approx 3.046$ (due to non-instantaneous decoupling and QED corrections). While Planck CMB data is consistent with this standard value ($2.99 \pm 0.17$), combining it with local measurements ($H_0$, BAO) sometimes pulls the preferred value higher ($N_{eff} \sim 3.3-3.4$), hinting at the presence of "dark radiation" (e.g., sterile neutrinos, axions, or light relics). Conversely, BBN constraints often favor slightly lower values. Any significant deviation from 3.046 would be a "smoking gun" for new physics or a non-standard thermal history in the early universe (Planck Collaboration 2018; Verde et al. 2013).