Speaker
Description
We present new Big Bang Nucleosynthesis (BBN) limits on the cosmic expansion rate or relativistic energy density, quantified via the number $N_\nu$ of equivalent neutrino species. We use the latest light element observations, neutron mean lifetime, and update our evaluation for the nuclear rates d + d → $^3$He + n and d + d → $^3$H + p. Combining this result with the independent constraints from the cosmic microwave background (CMB) yields tight limits on new physics that perturbs $N_\nu$ and the baryon-to-photon ratio $\eta$ prior to cosmic nucleosynthesis: a joint BBN+CMB analysis gives $N_\nu$ = 2.898 ± 0.141, resulting in $N_\nu$ < 3.180 at 2$\sigma$. The strength of the independent BBN and CMB constraints now opens a new window: we can search for limits on potential changes in $N_\nu$ and/or $\eta$ between the two epochs. The present data place strong constraints on the allowed changes in $N_\nu$ between BBN and CMB decoupling; for example, we find -0.708 $<N_\nu^{\rm CMB}\ $ - $\ N_\nu^{\rm BBN}< $ 0.328 in the case where $\eta$ and the primordial helium mass fraction $Y_p$ are unchanged between the two epochs; we also give limits on the allowed variations in $\eta$ or in ($\eta$, $N_\nu$) jointly. We discuss scenarios in which such changes could occur. Looking to the future, we forecast the tightened precision for $N_\nu$ arising from both CMB Stage 4 measurements as well as improvements in astronomical $^4$He measurements. We find that CMB-S4 combined with present BBN and light element observation precision can give $\sigma$($N_\nu$)$\simeq$ 0.03. Such future precision would reveal the expected effect of neutrino heating ($N_{\rm eff}$ - 3 = 0.044) of the CMB during BBN, and would be near the level to reveal any particle species ever in thermal equilibrium with the standard model. Improved $Y_p$ measurements can push this precision even further.
| Keyword-1 | Big Bang Nucleosynthesis |
|---|---|
| Keyword-2 | Cosmology |
| Keyword-3 | Astroparticle Physics |
