Jul 5 – 10, 2021
Europe/Rome timezone

Approach to scaling in axion string networks

Jul 5, 2021, 6:20 PM
Talk in the parallel session From cosmic strings to superstrings From Cosmic Strings to Superstrings


Asier Lopez Eiguren (Tufts University)


In the QCD axion dark matter scenario with post-inflationary Peccei-Quinn symmetry breaking, the number density of axions, and hence the dark matter density, depends on the length of string per unit volume at cosmic time $t$, by convention written $\zeta/t^2$. The expectation has been that the dimensionless parameter $\zeta$ tends to a constant $\zeta_0$, a feature of a string network known as scaling. It has recently been claimed that in larger numerical simulations $\zeta$ shows a logarithmic increase with time. This case would result in a large enhancement of the string density at the QCD transition, and a substantial revision to the axion mass required for the axion to constitute all of the dark matter. With a set of new simulations of global strings we compare the standard scaling (constant-$\zeta$) model to the logarithmic growth. We also study the approach to scaling, through measuring the root-mean-square velocity $v$ as well as the scaled mean string separation $x$. We find good evidence for a fixed point in the phase-space analysis in the variables $(x,v)$, providing a strong indication that standard scaling is taking place. We show that the approach to scaling can be well described by a two parameter velocity-one-scale (VOS) model, and show that the values of the parameters are insensitive to the initial state of the network. We conclude that the apparent corrections to $\zeta$ are artifacts of the initial conditions, rather than a property of the scaling network.

Primary authors

Joanes Lizarraga (University of the Basque Country) Prof. Mark Hindmarsh (University of Helsinki) Dr Jon Urrestilla (University of the Basque Country) Asier Lopez Eiguren (Tufts University)

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