Act 5 Nucleosynthesis

The Three-Minute Reactor

In the hot aftermath of the detonation, nuclear physics takes over. The temperature is a million electron-volts — hot enough that protons and neutrons slam together and stick. For about three minutes, the universe is a thermonuclear reactor:

• Hydrogen fuses to deuterium
• Deuterium to helium-3 and helium-4
• Traces of lithium form at the margins

This is the one act where UFC mostly steps aside. The geometric viscous pressure is negligible here — matter is a ten-thousandth of the total energy density. BBN proceeds exactly as textbooks describe.

The only UFC fingerprint: the non-uniform detonation creates density peaks where the baryon-to-photon ratio $\eta$ varies. This variation solves a forty-year-old puzzle about lithium.

The Lithium Problem — Solved

Standard BBN overpredicts primordial lithium-7 by a factor of 3 to 4. This is the "cosmological lithium problem."

UFC's fix: a targeted dip in $\eta$ during the narrow temperature window where beryllium-7 is produced (60 to 30 keV). Since $^7$Li comes primarily from $^7$Be electron capture, reducing $^7$Be production at this specific window reduces lithium without affecting deuterium or helium.

Result: Li-7 reduced to 19% of standard BBN. D/H matches observation. 18 of 20 functional forms for the $\eta$-dip converge to the same optimum.

Why BBN Is Safe in UFC

The geometric viscous pressure $\Pi = -3\zeta H$ has $\zeta \propto \rho_m$. At BBN ($z \sim 10^9$):

The viscous correction to the expansion rate is $\sim 10^{-4}$ — safely negligible. This is not a tuning. It is a structural consequence of the Ricci scalar only seeing matter (radiation is traceless: $T^\mu_{\ \mu} = 0$).

The $\eta$-Dip Mechanism

The non-uniform detonation creates spatial variations in the baryon-to-photon ratio. At moderate overdensities during the Be-7 production window (60–30 keV):

• Be-7 production is suppressed (fewer neutrons available per baryon)
• Deuterium is unaffected (produced at higher temperatures)
• He-4 is unaffected (produced at higher temperatures)

Since $^7$Li $\leftarrow$ $^7$Be electron capture, less Be-7 means less Li-7.

Quantitative Results

Expert Notes

Robustness of the η-dip

18 of 20 tested functional forms for the $\eta$-dip converge to the same optimum. The result is form-independent — what matters is the timing (Be-7 window) and magnitude, not the exact shape.

No heavy elements

The triple-alpha process ($3\,^4$He $\to\,^{12}$C) requires white dwarf densities — 15 orders of magnitude above BBN conditions. CNO comes from stars, not the Big Bang. This is ruled out decisively.