Diffusion and first-passage characteristics on a dynamically evolving support

TL;DR

This paper analyzes diffusion on a dynamically evolving support, deriving analytical solutions for different expansion/contraction scenarios, revealing how space dynamics influence diffusion and first-passage properties.

Contribution

It introduces a generalized diffusion equation for evolving spaces and provides analytical solutions for algebraic and exponential scale transformations.

Findings

Exponential expansion dominates diffusion at all times.

Algebraic contraction slows diffusion, creating an effective diffusion constant.

Exponential contraction leads to a stationary state with no diffusion.

Abstract

We propose a generalized diffusion equation for a flat Euclidean space subjected to a continuous infinitesimal scale transform. For the special cases of an algebraic or exponential expansion/contraction, governed by time-dependent scale factors $a(t)\sim t^\lambda$ and $a(t)\sim \exp(\mu t)$, the partial differential equation is solved analytically and the asymptotic scaling behavior, as well as the dynamical exponents, are derived. Whereas in the algebraic case the two processes (diffusion and expansion) compete and a crossover is observed, we find that for exponential dynamics the expansion dominates on all time scales. For the case of contracting spaces, an algebraic evolution slows down the overall dynamics, reflected in terms of a new effective diffusion constant, whereas an exponential contraction neutralizes the diffusive behavior entirely and leads to a stationary state. Furthermore, we derive various first-passage properties and describe four qualitatively different regimes of (strong) recurrent/transient behavior depending on the scale factor exponent.