Photon gas

A collection of photons treated statistically as a gas, typically in thermal equilibrium with a temperature-dependent blackbody spectrum. Its particle number can change because photons are created or absorbed.

Planck's law: A law giving the spectral distribution of electromagnetic radiation in thermal equilibrium at a specified temperature. It gives the equilibrium spectrum of a photon gas.
Electromagnetic mode: A permitted spatial pattern and frequency of an electromagnetic field in a specified system. Thermal photons populate these modes rather than behaving like classical gas molecules.
Cosmic microwave background: Relic electromagnetic radiation from the early universe, observed today as an almost uniform microwave background. It is an exceptionally precise natural example of a nearly thermal photon gas.
Ideal gas: A model of particles with negligible volume and interactions whose pressure and volume obey a simple equation of state. Unlike an ordinary ideal gas, a photon gas has no conserved particle number.
Bose–Einstein statistics: Quantum statistics governing indistinguishable bosons, including their occupation of energy states. Photons occupy radiation modes according to these statistics.
Photon energy: The energy carried by a photon, equal to Planck’s constant times its frequency. It sets the energy contribution of each occupied radiation mode.
Radiation-dominated universe: An early cosmological era when radiation contributed more energy density than matter. The photon gas governed the expansion and thermal history during this era.
Classical gas: A gas whose particles follow classical rather than quantum statistical mechanics. Classical statistics cannot reproduce the photon gas’s blackbody spectrum.
Black-body radiation: Electromagnetic radiation in thermal equilibrium with an ideal absorber and emitter. It is the characteristic radiation of a photon gas in equilibrium.
Temperature: A physical quantity that characterizes thermal state and determines equilibrium energy distributions. It fixes the photon gas’s spectral shape and energy density.

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