# Continuous-time random-walk model for anomalous diffusion in expanding media

## TL;DR

This paper develops a fractional diffusion model for anomalous diffusion in expanding or contracting media, analyzing effects like localization and the influence of medium dynamics on particle transport.

## Contribution

It introduces a bifractional diffusion equation for CTRWs in expanding media and provides analytical solutions for specific cases like Lévy flights and subdiffusion.

## Key findings

- In expanding media, the propagator tends to a stationary profile at long times.
- In contracting media, particles can become localized due to subdiffusive effects.
- The 'Lévy horizon' quantifies the impact of medium expansion on Lévy flights.

## Abstract

Expanding media are typical in many different fields, e.g. in Biology and Cosmology. In general, a medium expansion (contraction) brings about dramatic changes in the behavior of diffusive transport properties. Here, we focus on such effects when the diffusion process is described by the Continuous Time Random Walk (CTRW) model. For the case where the jump length and the waiting time probability density functions (pdfs) are long-tailed, we derive a general bifractional diffusion equation which reduces to a normal diffusion equation in the appropriate limit. We then study some particular cases of interest, including L\'evy flights and subdiffusive CTRWs. In the former case, we find an analytical exact solution for the Green's function (propagator). When the expansion is sufficiently fast, the contribution of the diffusive transport becomes irrelevant at long times and the propagator tends to a stationary profile in the comoving reference frame. In contrast, for a contracting medium a competition between the spreading effect of diffusion and the concentrating effect of contraction arises. For a subdiffusive CTRW in an exponentially contracting medium, the latter effect prevails for sufficiently long times, and all the particles are eventually localized at a single point in physical space. This "Big Crunch" effect stems from inefficient particle spreading due to subdiffusion. We also derive a hierarchy of differential equations for the moments of the transport process described by the subdiffusive CTRW model. In the case of an exponential expansion, exact recurrence relations for the Laplace-transformed moments are obtained. Our results confirm the intuitive expectation that the medium expansion hinders the mixing of diffusive particles occupying separate regions. In the case of L\'evy flights, we quantify this effect by means of the so-called "L\'evy horizon".