Speaker
Description
The axion was proposed as a solution to the strong CP problem and is currently one of the main candidates for cold dark matter. Superficially, recent field theory simulations disagree on whether the number of axion strings per Hubble patch is constant or log dependent, as well as what form that log dependence might take. This is of particular relevance since this parameter strongly impacts the abundance of axion dark matter. We use the Velocity-dependent One Scale Model to explore the evolution of axion string networks, aiming to compare the solutions obtained with different approaches: introducing a time-varying string tension at the microscopic or macroscopic scales, or using a suitable wiggly string extension of the VOS model. Since all global axion strings simulations have a relatively small dynamic range, an important distinction between realistic networks and simulations, which is reflected in our analysis, is the size of the log, which is about 10 times larger for cosmologically relevant networks than in simulations. Therefore we contrast 'large log' asymptotic solutions for the density and velocity of the network, with 'small log' ones relevant to the limited time-scale of simulations. This, together with the impact of the peculiar initial conditions on the latter (most simulations start with low densities but high velocities) turns out to be the key to the conundrum.
| Other topic / keywords: | Axion strings; field theory numerical simulations; VOS model |
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