Electron-positron plasma in GRBs and in cosmology

Electron-positron plasma is believed to play imporant role both in the early Universe and in sources of Gamma-Ray Bursts (GRBs). We focus on analogy and difference between physical conditions of electron-positron plasma in the early Universe and in sources of GRBs. We discuss a) dynamical differences, namely thermal acceleration of the outflow in GRB sources vs cosmological deceleration; b) nuclear composition differences as synthesis of light elements in the early Universe and possible destruction of heavy elements in GRB plasma; c) different physical conditions during last scattering of photons by electrons. Only during the acceleration phase of the optically thick electron-positron plasma comoving observer may find it similar to the early Universe. This similarity breaks down during the coasting phase. Reprocessing of nuclear abundances may likely take place in GRB sources. Heavy nuclear elements are then destroyed, resulting mainly in protons with small admixture of helium. Unlike the primordial plasma which recombines to form neutral hydrogen, and emits the Cosmic Microwave Background Radiation, GRB plasma does not cool down enough to recombine.

💡 Research Summary

The paper presents a systematic comparison between the electron‑positron plasma that existed in the early Universe and the plasma that forms in the relativistic outflows of gamma‑ray bursts (GRBs). Although both environments are initially characterized by extremely high temperatures (≫ MeV) and densities that sustain copious e⁺e⁻ pairs, the subsequent evolution diverges dramatically because of differences in dynamics, nuclear processing, and photon‑electron scattering conditions.

First, the dynamical regimes are opposite. In cosmology the plasma is embedded in an expanding Friedmann‑Lemaître‑Robertson‑Walker (FLRW) metric; the scale factor a(t) grows, the temperature falls as T ∝ a⁻¹, and the bulk flow decelerates under cosmic expansion. The plasma therefore transitions smoothly from a relativistic radiation‑dominated state to a non‑relativistic matter‑dominated one. By contrast, a GRB engine injects a compact, ultra‑dense fireball that is initially optically thick. Internal radiation pressure drives a rapid thermal acceleration, converting internal energy into bulk kinetic energy. The outflow reaches Lorentz factors of several hundred and then enters a coasting phase where the bulk velocity is essentially constant. During coasting the temperature remains far above the recombination threshold, so the plasma never experiences the deceleration that characterizes the early Universe.

Second, nuclear composition evolves differently. In the early Universe, Big‑Bang nucleosynthesis (BBN) proceeds while the temperature drops from ∼ 1 MeV to ∼ 0.1 MeV, producing primarily H‑1, He‑4, and trace amounts of D, He‑3, and Li‑7. The reaction network is governed by the neutron‑to‑proton ratio and the expansion rate, after which nuclear reactions freeze out. In a GRB fireball, any pre‑existing heavy nuclei are subjected to intense shock heating and a flood of high‑energy photons and neutrinos. Photodisintegration and spallation efficiently destroy nuclei heavier than helium, leaving a plasma dominated by free protons with a modest helium admixture. This “nuclear reprocessing” is a hallmark of GRB outflows and contrasts sharply with the primordial synthesis that enriches the Universe with light elements.

Third, the conditions at the last‑scattering surface differ fundamentally. In cosmology, when the temperature falls below ∼ 3000 K, electrons recombine with protons to form neutral hydrogen. The free‑electron density drops precipitously, allowing photons to decouple and travel freely; these photons constitute the Cosmic Microwave Background (CMB) observed today. In GRB outflows, even after the fireball becomes optically thin, the comoving temperature remains of order keV to MeV, far above the recombination threshold. Consequently, electrons never recombine, and photons continue to scatter off the residual e⁺e⁻ pairs until the fireball expands to very large radii. The resulting radiation field bears no resemblance to a black‑body CMB; instead it retains a non‑thermal spectrum shaped by ongoing Comptonization and pair annihilation.

The authors conclude that the superficial similarity—an initially hot, pair‑rich plasma—holds only during the early acceleration phase of a GRB fireball. Once the outflow reaches the coasting stage, the plasma’s thermodynamic trajectory diverges: it does not cool enough to permit recombination, and heavy nuclei are largely destroyed. These differences have observable consequences, influencing GRB prompt‑emission spectra, afterglow signatures, and the potential for nucleosynthetic yields from GRB environments. The study underscores that while electron‑positron plasmas are a common thread linking cosmology and high‑energy astrophysics, the surrounding physical context determines entirely distinct evolutionary pathways.