Approach to scaling in axion string networks. (arXiv:2102.07723v1 [astro-ph.CO])
<a href="http://arxiv.org/find/astro-ph/1/au:+Hindmarsh_M/0/1/0/all/0/1">Mark Hindmarsh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lizarraga_J/0/1/0/all/0/1">Joanes Lizarraga</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lopez_Eiguren_A/0/1/0/all/0/1">Asier Lopez-Eiguren</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Urrestilla_J/0/1/0/all/0/1">Jon Urrestilla</a>
We study the approach to scaling in axion string networks in the radiation
era, 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. The string length has also been commonly expressed in terms of a
dimensionless string length density $zeta$, proportional to the number of
Hubble lengths of string per Hubble volume. In simulations with initial
conditions far from the fixed point $zeta$ is still evolving after half a
light-crossing time, which has been interpreted in the literature as a
long-term logarithmic growth. We show that all our simulations, even those
starting far from the fixed point, are accounted for by a VOS model with an
asymptote of $zeta_*=1.20pm0.09$ (calculated from the string length in the
cosmic rest frame) and $v_* = 0.609pm 0.014$.
We study the approach to scaling in axion string networks in the radiation
era, 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. The string length has also been commonly expressed in terms of a
dimensionless string length density $zeta$, proportional to the number of
Hubble lengths of string per Hubble volume. In simulations with initial
conditions far from the fixed point $zeta$ is still evolving after half a
light-crossing time, which has been interpreted in the literature as a
long-term logarithmic growth. We show that all our simulations, even those
starting far from the fixed point, are accounted for by a VOS model with an
asymptote of $zeta_*=1.20pm0.09$ (calculated from the string length in the
cosmic rest frame) and $v_* = 0.609pm 0.014$.
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