🔭 astrophysics

Deficit of primordial Li7 and primordial black holes

This paper proposes that primordial black holes can resolve the cosmological lithium-7 problem by evaporating nucleons that convert excess primordial lithium-7 into unstable isotopes, which subsequently decay into helium-4 nuclei to match observed abundances.

Original authors: E. V. Arbuzova, A. D. Dolgov

Published 2026-06-03

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: E. V. Arbuzova, A. D. Dolgov

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "Missing Lithium" Mystery

Imagine the universe as a giant kitchen where the Big Bang was the chef cooking the first meal. This meal consisted mostly of Hydrogen and Helium, with tiny sprinkles of heavier elements like Lithium.

For decades, astronomers have been tasting this "meal" and comparing it to the recipe (theoretical physics). The recipe predicts exactly how much of each ingredient should exist.

The Good News: The amounts of Helium, Deuterium, and Helium-3 match the recipe perfectly.
The Bad News: The recipe predicts there should be three times more Lithium-7 than what we actually see in the universe. It's as if the chef baked a cake with three times the expected amount of chocolate chips, but when we eat it, we only find a few. This is known as the "Lithium Problem."

The Proposed Solution: Cosmic "Vacuum Cleaners"

The authors of this paper suggest a new way to explain why the Lithium is missing. They propose that the universe was once filled with tiny, invisible "vacuum cleaners" called Primordial Black Holes (PBHs).

These aren't the massive black holes at the centers of galaxies; these are microscopic ones that formed right at the beginning of time. As they "evaporated" (disappeared), they didn't just vanish; they spat out energy and particles.

The paper suggests two different ways these black holes could have cleaned up the extra Lithium, depending on how "hot" they were.

Scenario A: The "Hot" Black Holes (The Particle Shredder)

Imagine a Primordial Black Hole that is very hot (about 1 billion degrees).

Spitting out Neutrons: As this hot black hole evaporates, it spits out high-speed neutrons (tiny particles found in the center of atoms).
The Collision: These neutrons zoom through the early universe and crash into the extra Lithium-7 atoms.
The Transformation: When a neutron hits a Lithium-7 atom, it turns it into a heavier, unstable version (Lithium-8 or Beryllium-8).
The Explosion: These unstable atoms are like over-inflated balloons; they instantly pop and split apart into two Helium-4 atoms.

The Result: The extra Lithium is effectively "shredded" and turned into Helium. Since Helium is already abundant, no one notices the change, but the Lithium count drops to the correct, observed level.

The Catch: The authors had to check if this process would accidentally destroy the Helium we do need. They calculated that as long as there weren't too many of these black holes, the "shredding" would only target the Lithium and leave the Helium safe.

Scenario B: The "Cool" Black Holes (The Photon Eraser)

Now, imagine a Primordial Black Hole that is cooler (not hot enough to spit out neutrons).

Spitting out Light: Instead of particles, these black holes spit out high-energy light (photons/gamma rays).
The Interaction: These light particles bounce around and eventually hit the Lithium atoms.
The Breakup: The energy from the light is enough to break the Lithium apart, turning it into Helium, just like in the hot scenario.

The Result: The extra Lithium is destroyed by light instead of particles.

What Does This Mean for Us?

The paper concludes that if the universe was filled with just the right number of these evaporating black holes, they could have acted as a cosmic filter, removing the "excess" Lithium predicted by the recipe and leaving us with the amount we actually see today.

Side Effects:
The authors note that if this happened, it might leave a "fingerprint" on the universe today:

It might have slightly changed the temperature of the universe's background glow (the Cosmic Microwave Background) in specific ways.
It might have helped "re-ionize" the universe (turning neutral gas back into charged plasma) a bit later in history than we thought.

Summary

In short, the paper says: "We have too much Lithium in our theoretical recipe. Maybe tiny, ancient black holes acted like cosmic janitors, sweeping up the extra Lithium and turning it into Helium before we could count it."

This solves the mystery without changing the fundamental laws of physics, provided these tiny black holes existed in the specific numbers the authors calculated.

Technical Summary: Deficit of Primordial 7Li and Primordial Black Holes

Problem Statement
The paper addresses the long-standing "lithium problem" within the framework of Big Bang Nucleosynthesis (BBN). While the canonical cosmological model, constrained by the baryon-to-photon ratio ( $\eta \approx 6 \times 10^{-10}$ ) derived from Cosmic Microwave Background (CMB) fluctuations, successfully predicts the observed abundances of $^4$ He, deuterium, and $^3$ He, it significantly overpredicts the abundance of primordial $^7$ Li. Theoretical predictions exceed observational data by a factor of approximately three. Existing reviews indicate that previous attempts to resolve this discrepancy have not yet yielded a fully satisfactory solution.

Methodology
The authors propose a mechanism where the excessive abundance of $^7$ Li is reduced through interactions with particles generated by the evaporation of Primordial Black Holes (PBHs). The study analyzes two distinct scenarios based on the temperature (and consequently mass) of the PBHs:

"Hot" Black Holes ( $T_{BH} \sim 1$ GeV):
- These PBHs (mass $\sim 10^{13}$ g) evaporate during an era when the cosmic plasma is still ionized but has cooled sufficiently ( $T_{cosm} \approx 1.8$ eV) that existing nuclei are stable against thermal destruction.
- The evaporation process produces baryons and anti-baryons. The authors calculate the thermalization time for these energetic baryons, demonstrating they quickly equilibrate with the primeval plasma.
- The mechanism relies on the capture of neutrons by $^7$ Li via the reaction $^7\text{Li} + n \rightarrow {}^8\text{Li} + \gamma$ (or $^7\text{Li} + n \rightarrow {}^8\text{Be} + \gamma$ ).
- The resulting unstable isotopes ( $^8$ Li or $^8$ Be) rapidly decay into a pair of $^4$ He nuclei, effectively removing $^7$ Li from the abundance count.
- The authors rigorously constrain the PBH number density ( $n_{BH}$ ) by ensuring that the anti-baryons produced do not destroy the observed $^4$ He abundance (limiting helium fission to $<10\%$ ).
"Cool" Black Holes ( $T_{BH} \le 100$ MeV):
- These heavier PBHs ( $M_{BH} \approx 10^{14}$ g) do not produce baryons. Instead, they emit electrons, positrons, photons, and neutrinos.
- The energetic photons produced are thermalized via scattering on cold background electrons, reducing their energy below the binding energy of $^4$ He, thus preserving helium.
- However, these photons are posited to be capable of reducing the number densities of lithium and beryllium to observed levels, though the specific reaction pathways are less detailed than in the "hot" scenario.

Key Results

Hot Scenario Feasibility: By calculating the flux of neutrons produced by a population of evaporating PBHs, the authors demonstrate that the destruction rate of $^7$ $^{7}$ Li can be sufficient to bridge the gap between theory and observation.
- Using a cross-section for $^7$ Li + $n$ of $\approx 2 \times 10^{-25} \text{ cm}^2$ and a cosmological time scale of $\tau_{cosm} \approx 1.54 \times 10^{11}$ seconds, the authors show that a baryon density of $n_B \lesssim 10^3 \text{ cm}^{-3}$ (derived from constrained PBH densities) is sufficient to reduce the $^7$ Li fraction to the observed value of $\sim 1.6 \times 10^{-10}$ .
- The constraint on PBH density ( $n_{BH} \lesssim 2 \times 10^{-24} \text{ cm}^{-3}$ ) ensures that the destruction of $^4$ He remains within observational limits ( $<10\%$ uncertainty).
Cool Scenario Potential: The paper suggests that cooler PBHs could also achieve the necessary lithium reduction via photon interactions, provided the photons are thermalized to energies that spare $^4$ He but affect lithium/beryllium.

Significance and Claims
The paper claims to offer an alternative mechanism to explain the observed deficit of primordial $^7$ Li without altering the canonical baryon asymmetry of the universe. The authors assert that a population of evaporating primordial black holes can successfully solve the lithium problem through the transformation of $^7$ Li into $^8$ Li/ $^8$ Be and subsequent decay into $^4$ He.

The authors modestly note that while the mechanism solves the lithium abundance issue, it introduces potential observable consequences:

CMB Distortions: The process may lead to distortions in the CMB angular fluctuations at low frequencies.
Reionization: In the "cool" black hole scenario, the interaction of energetic gamma quanta with neutral atoms could contribute to the reionization of the universe at redshifts $z_{reion} \lesssim 20$ .

The work does not propose new experimental setups but rather utilizes existing theoretical frameworks (Hawking radiation, BBN cross-sections) to propose a cosmological solution to a specific observational discrepancy.