BBN and CMB combined and separate constraints on new physics: measuring N_eff and probing its evolution in the early Universe

Big Bang Nucleosynthesis (BBN) and the cosmic microwave background (CMB) both probe the physics of the early Universe. BBN accounts for the cosmic origin of the lightest elements, e.g., ²H and ⁴He. Having precisely measured nuclear data as inputs, BBN abundance calculation depends on two cosmological parameters: the cosmic baryon-to-photon ratio η = n_b/n_γ and the effective number of neutrino species N_eff. BBN data analyses then use observed primordial abundances to determine η and N_eff in return. The CMB can also independently determine both parameters. Hence, the BBN+CMB joint constraint on N_eff provides a sharpened cosmological probe to new physics. Any departure from the Standard Model N_eff^SM would unveil nonstandard cosmology and likely BSM physics. Moreover, BBN+CMB constraints on new physics improve when newer precision observations are available. Latest developments of such joint analyses will be presented. In addition, both BBN and the CMB analyses independently reach levels of precision that can meaningfully probe changes in η and/or N_eff between these two epochs. This open a new window to study a broad variety of BSM models, including extra entropy and/or radiation injection between BBN and the CMB. Likelihoods of (Δη, ΔN_eff) from our latest research will be reported.