BIG BANG NUCLEOSYNTHESIS (BBN)
PARAMETERS
Baryon-to-photon ratio η (×10⁻¹⁰):
6.1
Neutron lifetime τₙ (s):
878
N_eff (extra neutrino species):
3.0
Animate BBN Timeline
KEY REACTIONS
p + n → D + γ
D + D → ³He + n
D + D → T + p
D + p → ³He + γ
³He + n → T + p
T + D → ⁴He + n
³He + ⁴He → ⁷Be + γ
⁷Be + n → ⁷Li + p
OBSERVED ABUNDANCES
⁴He: Y_p ≈ 0.247 (mass frac.)
D/H ≈ 2.5×10⁻⁵
³He/H ≈ 1.0×10⁻⁵
⁷Li/H ≈ 5×10⁻¹⁰ (CMB)
"Lithium problem": BBN
predicts 3× too much ⁷Li
TIMELINE
t=0.01s: kT≈10 MeV, n/p=1
t=1s: kT≈1 MeV, freeze-out
n/p → 1/7 (after β-decay)
t=3min: T drops, D stable
t=20min: reactions stop
²⁵% ⁴He, ⁷⁵% ¹H locked in