Covariant linear response theory for a photon gas in curved spacetime

Abstract: To overcome the failure of the conventional non-relativistic description of statistics in strong gravitational fields, we develop a covariant theory for a photon gas in curved spacetime, starting from the Boltzmann equation and employing the relaxation time approximation. The transport coefficients are computed up to second order, which are not a massless limit of the massive-particle results due to the fundamentally different phase-space geometry of null particles. At second order, the gravitational field renders the coefficients tensor-valued and induces transverse fluxes, analogous to the Hall effect. Using the first-order results, we derive relativistic generalizations of Fick's and Fourier's laws and obtain the radiation diffusion equation for spherically symmetric accretion disks. This provides a covariant description of radiative transport in strong gravitational fields for high-energy astrophysical applications.