Speaker
Description
Big Bang Nucleosynthesis (BBN) provides a precision laboratory for the MeV-era Universe, turning light-element abundances and the radiation content into stringent tests of new physics.
I will review the key BBN ingredients and emphasize how departures from standard expansion and neutron–proton conversion translate into constraints on exotic energy injection, and the impact of hadronic decay products injected at the relevant epoch: mesons and (anti-)nucleons can drive efficient nonstandard $n \leftrightarrow p$ conversion and distort the freeze-out proton–neutron ratio, thereby shifting the primordial helium abundance $Y_\mathrm{P}$.
Firstly, I discuss MeV–GeV heavy neutral leptons (HNLs).
Using recent cosmological analyses that consistently track both the background evolution and meson-driven $n \leftrightarrow p$ conversion, I show that introducing sizable “dark” decay channels does not generically weaken BBN bounds.
Instead, dark decays typically increase the dark-sector energy density around BBN, enhancing deviations in $Y_P$ and $\Delta N_\mathrm{eff}$, and can even strengthen exclusions in regions targeted by laboratory experiments.
Secondly, I present updated BBN constraints on primordial black holes (PBHs) evaporating before nucleosynthesis.
A careful treatment of hadronization and meson/nucleon effects implies that observable BBN sensitivity requires $m_\mathrm{PBH}\gtrsim 10^9$ g, with limits on the initial PBH fraction $\beta \sim 10^{-17} – 10^{-19}$ for $m_\mathrm{PBH} \in [10^9, 10^{10}]~\mathrm{g}$.
In future work, we will continue to leverage BBN as a precision probe to explore a broader range of dark-matter scenarios and other manifestations of new physics.