Boltzmann Dynamics in K-essence Cosmology: Photon Propagation in an Emergent Spacetime

Abstract

Abstract Recent cosmological tensions, notably the Hubble and $S_{8}$ tensions, motivate extensions of the conventional FLRW framework in which additional dynamical fields can alter the effective spacetime experienced by matter and radiation. In K-essence cosmology, the scalar field induces an emergent FLRW geometry disformally related to the gravitational metric, leading to a \emph{tilted causal structure} in which the photon light cone differs from that of gravity. We develop a covariant Boltzmann formalism in a homogeneous K-essence background and derive the modified mass-shell condition, geodesic equations, and collision integrals for both massless and massive species. The photon distribution remains thermal in the emergent frame, while appearing geometrically rescaled in the gravitational frame. Microphysical Thomson/Compton scattering is unmodified; instead, the emergent geometry induces effective rest-frame energies and a geometrically rescaled scattering rate, conveniently encoded in a transport coefficient $\sigma_T^{(\mathrm{geom})}$. Focusing on the tightly coupled pre-recombination era, we obtain modified acoustic dynamics and, for a purely kinetic DBI-type model, find an effective interaction rate scaling $n_{e}\sigma_T^{(\mathrm{geom})}a\propto a^{-8}$ and a diffusion scale scaling $k_{D}^{-2}\propto a^{29/2}$, enhancing the sensitivity of small-scale anisotropies to the evolving geometry. Our results provide a self-consistent kinetic description of particle transport in the emergent K-essence spacetime, and they lay the groundwork for a full numerical Einstein-Boltzmann implementation to compute post-recombination free streaming and the CMB angular power spectrum. A detailed numerical analysis will be pursued in future work.