Abstract
We study the contribution to the QCD axion dark matter abundance that is produced by string defects during the so-called scaling regime. Clear evidence of scaling violations is found, the most conservative extrapolation of which strongly suggests a large number of axions from strings. In this regime, nonlinearities at around the QCD scale are shown to play an important role in determining the final abundance. The overall result is a lower bound on the QCD axion mass in the post-inflationary scenario that is substantially stronger than the naive one from misalignment.
Highlights
We study the effects of these nonlinear dynamics in some detail
When the PQ symmetry is broken a network of axion strings forms [14,15,16] and this rapidly approaches an attractor solution [17,18,19,20] during the subsequent evolution of the Universe
While we only use the data from the physical simulations to extract the relevant parameters, the results obtained with the fat trick make some features of the attractor solution more manifest and our interpretation of the string dynamics more robust
Summary
Besides solving the strong CP problem [1] the QCD axion [2, 3] may explain the observed cold dark matter of the Universe [4,5,6]. Before the axion gets its mass at around the QCD crossover only string defects are present Their dynamics are governed by the so-called scaling solution — an attractor of the evolution on which the properties of the string network are supposed to have simple scaling laws in terms of the relevant scales of the system. This phenomenon can be understood as an instance of self-organized criticality [8]: the expansion of the Universe keeps increasing the number of strings per Hubble patch until the string density crosses the critical point when the configuration becomes unstable. In Appendix G we comment on the compatibility of our results with the existing literature
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