Probing Physics Beyond the Standard Model: Limits from BBN and the CMB Independently and Combined

Understand

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 $d+d \rightarrow He3 + n$ and $d+d \rightarrow H3 + p$.
• Combining this result with the independent constraints from the cosmic microwave background (CMB) yields tight limits on new physics that perturbs $N_\nu$ and $\eta$ prior to cosmic nucleosynthesis: a joint BBN+CMB analysis gives $N_\nu = 2.898 \pm 0.141$, resulting in $N_\nu < 3.180$ at $2\sigma$.
• We apply these limits to a wide variety of new physics scenarios including right-handed neutrinos, dark radiation, and a stochastic gravitational wave background.