Borel Summability of the Spatial Gradient Expansion in Linear Kinetic Theory

Abstract

The passage from kinetic theory to hydrodynamics proceeds through gradient expansions whose large-order behavior depends on the model and on whether the gradients are temporal or spatial. Known examples of divergent temporal gradient expansions include those arising in Bjorken flow in Müller-Israel-Stewart theory, where positive-axis Borel singularities obstruct ordinary Borel summation and require a transseries completion. By contrast, some spatial gradient expansions in holographic theories and relativistic kinetic theory converge within a finite radius. We show that the spatial Chapman-Enskog expansion of the linear one-dimensional BGK equation realizes a third possibility: its coefficients can be obtained in closed form at all orders, and the resulting series is factorially divergent yet Borel summable along the positive axis. The unambiguous Borel sum reconstructs the hydrodynamic dispersion branch up to its merger with the essential spectrum and provides its analytic continuation beyond that point. We trace the divergence of the spatial expansion to the unbounded velocity support of the Maxwellian equilibrium: replacing it with a compactly supported equilibrium removes the factorial growth and gives the series a nonzero radius of convergence.