Claim: Big Bang Nucleosynthesis Accurately Predicts Light Element Abundances
It predicts three of four. For lithium-7, the prediction is wrong by a factor of 3.5. This discrepancy has been unresolved for thirty years.
Big Bang Nucleosynthesis is presented as one of the three pillars of Big Bang cosmology, alongside the CMB and the Hubble expansion. BBN predicts the abundances of the light elements — hydrogen, deuterium, helium-3, helium-4, and lithium-7 — produced in the first few minutes after the Big Bang. For most of these elements, the predictions are accurate.
For lithium-7, the prediction is wrong by a factor of 3.5. BBN predicts 5.6 × 10⁻¹⁰ lithium-7 atoms per hydrogen atom. Observation finds 1.6 × 10⁻¹⁰. The prediction is 3.5 times too high. This discrepancy is called the cosmological lithium problem. It has been recognised in the literature since the 1990s. It has never been resolved.
Why BBN Should Get Lithium Right
BBN models the universe as a hot, dense plasma in the first few minutes after the Big Bang, cooling through a narrow temperature window in which nuclear reactions produce light elements. The model is computationally precise. The reaction rates are laboratory-measured. The neutron-to-proton ratio at freeze-out, which determines the helium abundance, is well-constrained by the measured baryon density from CMB observations.
Given these inputs, BBN produces a specific prediction for lithium-7. The baryon density is not a free parameter — it is fixed by independent CMB measurements. The reaction rates are not free parameters — they are measured in accelerators. The prediction of 5.6 × 10⁻¹⁰ is robust within the model.
The observation is 1.6 × 10⁻¹⁰. The discrepancy is 3.5 times. This is not a measurement uncertainty. It is a factor of 3.5.
The Proposed Solutions
The standard model has proposed several explanations for the lithium problem. None has gained consensus.
Stellar depletion: Lithium-7 is destroyed in stellar interiors at temperatures above 2.5 million K. Perhaps the observed abundance has been depleted from its primordial value by stellar processing. The difficulty is that the depletion would need to be almost exactly a factor of 3.5 in every old metal-poor star ever measured, regardless of stellar mass, age, or environment. No physical mechanism has been identified that produces this precise uniform depletion universally. The so-called Spite plateau — the observation that old metal-poor stars all have the same lithium abundance — was originally cited as evidence that the observed abundance is primordial and undepleted. Now it is cited as evidence that depletion has been uniform. The interpretation reversed when the discrepancy became apparent.
New nuclear physics: Perhaps a reaction rate relevant to lithium-7 production or destruction is incorrectly measured. Multiple experimental campaigns have re-measured the relevant rates. No significant error has been found.
New physics: Various proposals involving supersymmetric particles, late-decaying particles, or inhomogeneous BBN have been made. None has been confirmed experimentally.
The Oxygen Analogy
The standard model's argument for primordial lithium commits a logical error that is clearer when applied to a different element. Atmospheric oxygen is 21% of the Earth's atmosphere. Every organism that breathes consumes it. Should we conclude that oxygen is primordial — produced before life existed — and that its observed abundance is a depleted remnant of its original value?
Obviously not. We understand that oxygen is continuously replenished by photosynthesis. The observed abundance is a dynamic equilibrium between production and consumption, not a frozen primordial remnant.
The same logic applies to lithium-7. The destruction reaction Li-7 + p → He-4 + He-4 (Q = 17.35 MeV) proceeds throughout stellar lifetimes. The production of lithium-7 occurs in specific astrophysical environments including the surfaces of red giants and AGB stars. The observed abundance is a dynamic equilibrium between ongoing production and destruction — not a frozen value from 13.8 billion years ago.
The BFUT Resolution
In BFUT, all element abundances reflect steady-state nucleosynthetic equilibria. The equilibrium condition for lithium-7 is d[Li-7]/dt = R_production - k_dest × [Li-7] × [p] = 0, giving [Li-7]_eq = R_production / (k_dest × [p]). The equilibrium abundance is set by the ratio of production to destruction rates in the current stellar environment. The observed value of 1.6 × 10⁻¹⁰ is this equilibrium. BBN's frozen snapshot of 5.6 × 10⁻¹⁰ is not a primordial value being depleted. It is the answer to the wrong question.