photonbaryon

Photonbaryon refers to the photon–baryon fluid, the tightly coupled mixture of photons and baryons (protons and neutrons) that dominated the radiation–matter content of the early Universe before recombination. It is not a fundamental particle, but an effective description of the coupled plasma in which Thomson scattering keeps photons and baryons moving together on short timescales.

In the early universe, photons and free electrons exchange momentum efficiently via Thomson scattering, tying the photon gas to the baryon component. The resulting fluid supports acoustic (pressure) waves driven by photon pressure and gravitational compression. The effective sound speed is c_s^2 = 1/[3(1+R)], where R ≡ 3ρ_b/(4ρ_γ). Baryons add inertia, lowering the sound speed and enhancing density fluctuations during compression, which shifts the acoustic peaks in the cosmic microwave background (CMB) and in the matter distribution.

Damping mechanisms such as photon diffusion (Silk damping) erase small-scale perturbations, imprinting a characteristic cutoff in the power spectra. The evolution of the photon–baryon fluid is described by coupled perturbation equations and is solved by cosmological codes (e.g., CLASS, CAMB) to predict CMB anisotropies and large-scale structure.

Decoupling occurs when the Universe cools enough for electrons and protons to combine into neutral hydrogen, dramatically increasing the photons’ mean free path. Photons then free-stream, forming the cosmic microwave background, while baryons continue to evolve under gravity, gradually forming the observed large-scale structure. The photon–baryon fluid thus provides a central framework for understanding early-universe dynamics and the imprint of primordial perturbations.