Speaker
Description
Big Bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) both probe physics of the early universe. BBN is the intersection of nuclear astrophysics and early-universe cosmology, explaining the cosmic origin of the lightest elements, such as $^4$He and deuterium. Having precisely measured nuclear data as input, BBN predictions depend on the cosmic baryon-to-photon ratio $\eta = \rm n_{b}/n_{\gamma}$ and the effective number of cosmic neutrino species $N_{\rm eff}$. BBN analysis has long used observed primordial abundances from astronomical observations to infer $\eta$ and $N_{\rm eff}$. Crucially, both parameters are also measured independently from the CMB. Thus, the concordance between BBN and CMB determinations of these two parameters not only provides a critical test of the standard cosmology but also hints at new physics.
BBN and CMB constraints on $N_{\rm eff}$ are key concerns in the quest for Beyond-the-Standard-Model (BSM) physics. Any deviation from the Standard Model prediction would point to nonstandard cosmology and likely new physics, as a complementary cosmological approach to terrestrial particle experiments. In this talk, we show latest developments of the joint BBN+CMB constraint on the cosmological neutrinos. Moreover, we can search for any changes in $\eta$ and/or $N_{\rm eff}$ between BBN and the CMB. This is a new probe: only recently BBN and the CMB independently reach levels of precision that can meaningfully reveal such changes. This open a new window to study a broad variety of BSM processes, including extra entropy and/or radiation injection between primordial nucleosynthesis and recombination.
