A new study has revealed the possibility of photon Bose-Einstein condensation within astrophysical environments, shedding light on the interaction dynamics between radiation and matter in the early universe.

Conducted by researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), the study was published in The Astrophysical Journal on February 26.

As one of the fundamental bosons, photons have long been a subject of scientific curiosity regarding their potential to undergo Bose-Einstein condensation, a phenomenon first observed in ultracold atomic gases. Since Einstein predicted Bose condensation in 1924, researchers have sought to answer the question of whether photons can form a condensed state.

In 1969, Soviet physicists first proposed an astrophysical context for photon condensation, considering photon scattering with cold electrons in the expanding universe. However, it was not until 2010 that scientists observed photon Bose-Einstein condensation at room temperature within an optical cavity.

In this study, the researchers introduced a modified Kompaneets equation, extending the classical Kompaneets formalism to scenarios in which high-energy photons are comparable to the electron thermal energy.

Using this framework, the researchers demonstrated that the interactions between high-energy photons and cold electron gas could accumulate photons in the low-energy regime, creating conditions conducive to Bose-Einstein condensation.

"Numerical simulations reveal a gradual accumulation of photons towards the low-energy end under photon-number conservation," said GUO Bingang from IMP, first author of this study. "This process results in an enhancement of the photon occupation number near zero energy, a signature of Bose-Einstein condensation." Entropy analysis confirms that this condensate corresponds to a thermodynamic equilibrium, representing the maximum entropy state of the system.

However, despite these theoretical indications, the study suggests that, in realistic cosmic plasma environments, processes that do not conserve photon number occur on timescales shorter than those required for condensation formation. These absorption mechanisms tend to suppress or prevent the formation of photon condensate in the observable universe. Thus, while the formation of photon Bose-Einstein Condensation remains theoretically feasible, its long-term sustainability in cosmic settings appears unlikely.

"Nonetheless, this work advances our understanding of radiation-matter interactions under extreme conditions, with implications for spectral distortions of the cosmic microwave background and the thermodynamics of primordial plasma," said Prof. CHEN Xurong from IMP, corresponding author of the study.

The study reveals photon condensation mechanisms from both kinetic and thermodynamic perspectives. The developed theoretical models and numerical methods provide a foundation for investigating Bose-Einstein condensation phenomena, with applications in cosmology, high-energy astrophysics, and laboratory plasma physics.

This work was supported by the National Key Research and Development Program of China.

Numerical simulations of the evolution of the photon distribution over time, based on the modified Kompaneets equation, are presented. (Image by IMP)