Cosmological Birth of Electron-Positron Pairs: Light Creating Antimatter ✛ ✎ ✦
Context
The origin of positrons is an astrophysical mystery. The cosmic photon background (CPB), which includes the cosmic microwave background discovered by Penzias and Wilson, along with other components, is a relic of the Big Bang. It was thought that electron-positron pair production is efficient only in dense radiation fields, but the vast volumes due to cosmological expansion (discovered by Hubble) make photon-photon interactions within the CPB an inevitable source of pairs.
Methods
The CPB was decomposed into 28 gray bodies following Planck's law. The evolution of their amplitudes with redshift was modeled based on the luminosity functions of quasars, supernovae, active galactic nuclei, and star-forming regions. The pair production rate was obtained by integrating the gamma-gamma reaction cross section over energy and angles, accounting for the expansion of the Universe via the Hubble parameter. Energy losses of pairs in the intergalactic medium included ionization, plasma, and inverse Compton scattering.
Results
The pair production rate increases from 2×10⁻³⁶ cm⁻³ s⁻¹ in the local Universe to a peak of 1.8×10⁻³¹ cm⁻³ s⁻¹ at z≈2.7, following the history of star formation. In total, up to z=10, ~10⁵⁴ positrons are born per second. The injection spectrum is a power law with index −1 and a cutoff at ~1 TeV. The secondary Compton emission on the CMB contributes 10–20% of the cosmic gamma-ray background (CGB) in the 1–100 MeV range, reducing the gap between models and observations without invoking dark matter.
Implications
This process sets a minimum background for any study of the gamma-ray background and reduces the need for exotic sources like annihilating dark matter particles. Uncertainties arise from poor knowledge of the CPB in the ultraviolet and MeV band.
Future development
Refining CPB measurements, especially the CMB and ultraviolet background, together with detailed models of quasars and supernovae, will reduce errors. Future gamma-ray telescopes will be able to isolate this component directly.
Impact
This work impacts cosmology, cosmic-ray physics, and gamma-ray astronomy.
Next steps
The next step is modeling with an inhomogeneous intergalactic medium and accounting for the back-reaction on the expansion.
Key open problems
Linked to the problem of the origin of galactic positrons and the nature of the cosmic gamma-ray background, as well as the physics of the early Universe after the Big Bang.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The paper calculates the rate of electron-positron pair production from γ-γ interactions of the cosmic photon background with itself at z ≤ 10. The CMB was modeled as a sum of 28 gray bodies, evolving according to the luminosity functions of quasars, supernovae, and other sources. The resulting rate is integrated accounting for the expansion of the Universe, and the secondary Compton emission on the CMB explains 10–20% of the cosmic gamma-ray background in the 1–100 MeV range.
Where its strength is
The work establishes an inevitable astrophysical background of pair production that requires no exotic sources, and shows how the evolution of the CMB naturally reproduces the peak at z≈2.7, following the star formation history.
Where it could go next
- It may be interesting to compare the obtained contribution to the gamma-ray background with alternative mechanisms of non-thermal emission in the galactic center, where cosmic-ray collisions with starlight also produce a power-law spectrum, rather than annihilation.based on: Gamma Glow at the Galactic Center: Not Annihilation, but Collisions arXiv:2605.08010
- Closely related is the study of gamma-ray emission from young stellar jets, where particle acceleration in shocks produces a signal in a similar range; comparing it with your pair contribution could clarify on which scales each process dominates.based on: Young Stars: Gamma Rays from Jets arXiv:2606.19445
Worth trying
- Test what fraction of the gamma-ray background from pairs can be isolated toward the galactic center, using the spatial distribution of the CMB and comparing with observations.
- Try adding to the model the contribution from cosmic-ray collisions with starlight (following the approach of 2605.08010) to jointly fit the observed background.
Why this may matter
If pair production on the CMB indeed sets the minimal gamma-ray background, this reduces the need for dark matter to explain the gamma-ray excess and refines the background conditions for all future surveys in the MeV range.
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The closest work in the archive sits at 0.66 by meaning — the usual level for our corpus.
Despite the negligible local rate (10³⁵ e⁺/s per galaxy), the cosmological 'fog' from the past provides a significant contribution to today's gamma-ray background.
Key numbers
- local pair emission: 2.0×10-36 cm^{-3} s^{-1}
- peak emission at z=2.7: 1.8×10-31 cm^{-3} s^{-1}
- total positron production rate up to z=10: ~1054 e+ s^{-1}
- CMB temperature: 2.725 K
- Hubble constant: 67.66 km/s/Mpc