Probing physics beyond the standard model: limits from BBN and the CMB independently and combined

We present new Big Bang Nucleosynthesis (BBN) limits on the cosmic expansion rate or relativistic energy density, quantified via number $N_\nu$ of equivalent neutrino species. use latest light element observations, neutron mean lifetime, and update our evaluation for nuclear rates $d+d \rightarrow He3 + n$ H3 p$. Combining this result with independent constraints from microwave background (CMB) yields tight physics that perturbs $\eta$ prior to nucleosynthesis: a joint BBN+CMB analysis gives $N_\nu = 2.898 \pm 0.141$, resulting in < 3.180$ at $2\sigma$. apply these wide variety scenarios including right-handed neutrinos, dark radiation, stochastic gravitational wave background. also search potential {\em changes} and/or baryon-to-photon ratio between two epochs. The data place strong allowed changes BBN CMB decoupling; example, we find $-0.708 N_\nu^{\rm CMB}-N_\nu^{\rm BBN} 0.328$ case where primordial helium mass fraction $Y_p$ are unchanged epochs; give variations $(\eta,N_\nu)$ jointly. Looking future, forecast tightened precision arising both Stage 4 measurements as well improvements astronomical \he4 measurements. CMB-S4 combined observation can $\sigma(N_\nu) \simeq 0.03$. Such future would reveal expected effect heating ($N_{\rm eff}-3=0.044$) during BBN, be near level any particle species ever thermal equilibrium standard model.