Regularized modified log-Sobolev inequalities and comparison of Markov chains

Abstract

In this work we develop a comparison procedure for the modified log-Sobolev inequality (MLSI) constants of two reversible Markov chains on a finite state space. Efficient comparison of the MLSI Dirichlet forms is a well-known obstacle in the theory of Markov chains. We approach this problem by introducing a regularized MLSI constant, which, under some assumptions, has the same order of magnitude as the usual MLSI constant yet is amenable for comparison and thus considerably simpler to estimate in certain cases. As an application of this general comparison procedure, we provide a sharp estimate of the MLSI constant of the switch chain on the set of simple bipartite regular graphs of size n with a fixed degree d. Our estimate implies that the total variation mixing time of the switch chain is of order ${\mathit{O}}_{\mathit{d}}(\mathit{n}log\mathit{n})$. The result is optimal up to a multiple depending on d and resolves a long-standing open problem. We expect that the MLSI comparison technique implemented in this paper will find further applications.