# What is a String Wall?

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?

• 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. – Mitchell Porter Mar 6 at 1:16
• 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. – Keith Knauber Mar 12 at 1:42

## 1 Answer

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?)

• 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. – Rick Mar 14 at 13:41