Evaporating Primordial Black Holes and the Mystery of Lithium-7 Deficit ✦
A mechanism is proposed to reduce the theoretically predicted overabundance of lithium-7 using baryons evaporated from primordial black holes. It is shown that neutron capture by lithium-7 nuclei leads to the formation of unstable lithium-8 or beryllium-8 nuclei, which rapidly decay into two helium-4 nuclei. As a result, the fraction of lithium-7 relative to baryon density can be lowered to observed values. This scenario connects the physics of primordial black holes with the solution to the longstanding cosmological problem of light element abundance.
Context
The standard Big Bang scenario brilliantly describes the formation of light elements—hydrogen, deuterium, helium-3 and helium-4—whose predicted abundances match observations perfectly at a baryon-to-photon ratio η≈6×10⁻¹⁰. However, lithium-7 breaks this harmony: its calculated abundance is about three times higher than measured in the atmospheres of ancient stars (primordial nucleosynthesis yields ⁷Li/H≈5×10⁻¹⁰, while the observed value is ~1.6×10⁻¹⁰). This discrepancy, known as the "lithium problem," could not be explained by nuclear physics or modifications to cosmological parameters without destroying the agreement for other elements.
Methods
The researchers considered a population of primordial black holes (PBHs) born in the early universe from the collapse of overdense regions. According to the Hawking mechanism, a black hole radiates like a black body with a temperature inversely proportional to its mass. For "hot" PBHs with a temperature of ~1 GeV (mass around 10¹³ g), evaporation actively produces nucleons—neutrons and protons. These neutrons, entering the already cooled cosmic plasma at ~1.8 eV (in the era just before hydrogen recombination), can be captured by ⁷Li nuclei, triggering reaction chains ⁷Li + n → ⁸Li + γ or ⁷Li + n → ⁸Be + γ. The resulting ⁸Li and ⁸Be are unstable and within fractions of a second decay into two ⁴He nuclei. Numerical modeling, based on known neutron capture cross sections for lithium and PBH lifetimes, showed that at a reasonable black hole density, the amount of destroyed lithium is sufficient to compensate for the excess.
Results
Key result: at an allowable PBH density that does not disturb the abundance of helium-4 and deuterium, the lithium-7 concentration is suppressed by a factor of e ≈ 2.7. This almost exactly eliminates the discrepancy between theory and observations. Importantly, antibaryons, also emitted during evaporation, efficiently annihilate with the baryon background: calculations give a suppression factor of e⁻¹⁰, so they do not have time to destroy the precious relic nuclei of ⁴He and D. Additionally, "cold" PBHs (temperature ≤ 100 MeV) that emit photons instead of nucleons were considered; such photons, scattering off electrons, can contribute to the reionization of the cosmic microwave background and distort its spectrum at low frequencies.
Implications
The proposed mechanism for the first time naturally connects black hole physics with the chemical evolution of the early universe. It does not require introducing new particles or exotic physics beyond the Standard Model and general relativity; it merely assumes the existence of a population of PBHs with suitable masses. This strengthens the status of PBHs as a universal tool for solving cosmological puzzles — from dark matter to sources of gravitational waves. Moreover, the result highlights that processes near the event horizon can leave observable imprints on the large-scale structure of the cosmos.
Future development
The topic could develop in several directions. First, more precise measurements of the ⁷Li + n reaction cross section in the astrophysical energy range are needed. Second, the predicted distortions in the cosmic microwave background (both spectral and anisotropies) could be tested by new observational missions, such as future CMB-S4 class satellites. Third, the "cold" black hole scenario deserves detailed kinetic modeling that accounts for photon transport and thermalization in the expanding plasma, to reliably calculate the contribution to reionization and constrain PBH parameters.
Impact
This work will impact cosmology, particle astrophysics, and observational astronomy. It stimulates the search for correlations between anomalies in light element abundances and signals from primordial black holes, including gravitational waves and gamma-ray bursts.
Next steps
Immediate next steps include analyzing data from the James Webb Space Telescope and ground-based surveys for spectral distortions in the cosmic microwave background, as well as laboratory experiments to refine nuclear cross sections for lithium.
Key open problems
The research intimately connects the lithium problem with the dark matter mystery (if PBHs make up part of it) and the nature of singularities in quantum gravity. It also raises the question of how primordial black holes are distributed in mass and what their total density is — one of the key unresolved questions in modern physics.
🎯 If primordial black holes were the main component of dark matter, then every second within the volume of the Milky Way, a few such objects would evaporate, creating miniature "fireworks" of neutrons and gamma rays.
Key numbers
- Lithium-7 deficit: Observed concentration is ~3 times less than theoretical
- Hydrogen recombination temperature: 0.27 eV
- Lifetime of a black hole with mass 10^13 g: 1.6×10^11 seconds (~5000 years)
- Baryon-to-photon ratio η: 6×10^{-10}
- Suppression efficiency of antibaryons: Factor e^{-10}, rendering them harmless