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This question has nothing to do with sound waves, physical strings or walls...

I am reading: https://arxiv.org/abs/1709.07091

It states

"In the post-inflationary PQ symmetry breaking scenario, axions can be produced from the decay of string-wall systems, and we must take account of such contributions quantitatively."

I am not sure what they mean by decay of a string wall system.

Can someone please explain this?

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    $\begingroup$ A network of "topological defects": 1-dimensional (cosmic string), 2-dimensional (domain wall). Lines attached to planes, with the lines and planes being places where the ground state of the quantum fields is different. I hope someone will give a proper answer. $\endgroup$ – Mitchell Porter Mar 6 at 1:16
  • $\begingroup$ Throw a string of cooked spaghetti on the backsplash. If it sticks, you know it's done. Douse in cold water immediately, so that it is slightly al dente. $\endgroup$ – Keith Knauber Mar 12 at 1:42
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From the same paper:

Since the axion fieldamust vary from $0$ to $2πv_{PQ}$ around the core of the string, strings are always attached by $N_{DW}$ domain walls. The subsequent evolution of such string-wall systems isdifferent according to the value of $N_{DW}$, which is determined from the QCD anomaly coefficientand hence depends on underlying particle physics models

So, "string-wall systems" are essentially formed by one or more domain walls attached to a cosmic string.

(Or is the question what are cosmic strings and domain walls?)

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  • $\begingroup$ I understand the cosmic string theory. ( hypothetical 1-dimensional topological defects which may have formed during a symmetry breaking phase transition in the early universe ) - what I do not understand is the domain wall part. $\endgroup$ – Rick Mar 14 at 13:41

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