Spectral gap formation to kinetic equations with soft potentials in bounded domain

TL;DR

This paper proves the existence of a spectral gap for linearized kinetic equations with soft potentials in bounded domains with inflow boundary conditions, revealing exponential decay and new hypocoercivity insights.

Contribution

It introduces a novel weighted Hilbert space approach to establish spectral gap existence in bounded domains, overcoming previous spectral accumulation issues.

Findings

Spectral gap exists for soft potentials in bounded domains with inflow boundary.

Exponential decay of solutions is achieved, contrasting with sub-exponential decay in other settings.

New hypocoercivity mechanism is identified for kinetic equations with soft potentials.

Abstract

It has been unknown in kinetic theory whether the linearized Boltzmann or Landau equation with soft potentials admits a spectral gap in the spatially inhomogeneous setting. Most of existing works indicate a negative answer because the spectrum of two linearized self-adjoint collision operators is accumulated to the origin in case of soft interactions. In the paper we rather prove it in an affirmative way when the space domain is bounded with an inflow boundary condition. The key strategy is to introduce a new Hilbert space with an exponential weight function that involves the inner product of space and velocity variables and also has the strictly positive upper and lower bounds. The action of the transport operator on such space-velocity dependent weight function induces an extra non-degenerate relaxation dissipation in large velocity that can be employed to compensate the degenerate spectral gap and hence give the exponential decay for solutions in contrast with the sub-exponential decay in either the spatially homogeneous case or the case of torus domain. The result reveals a new insight of hypocoercivity for kinetic equations with soft potentials in the specified situation.