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A domain wall is a type of topological soliton that occurs whenever a discrete symmetry is spontaneously broken. Domain walls also sometimes called kinks in analogy with closely related kink solution of the sine-Gordon model. Unstable domain walls can also appear if spontaneously broken discrete symmetry is approximate and there is the metastable vacuum.
A domain (hyper volume) is extended in three spatial dimensions and one time dimension. A domain wall is the boundary between two neighboring domains. Thus a domain wall is extended in two spatial dimensions and one time dimension.
Important examples are:
- Domain wall (magnetism), an interface separating magnetic domains
- Domain wall (optics), for domain walls in optics
- Domain wall (string theory), a theoretical 2-dimensional singularity
Besides these important cases similar solitons appear in wide spectrum of the models. Here are other examples:
- Spontaneous breaking of discrete symmetries at early cosmological epochs can produce domain wall. Formation of domain wall network influence on the late stages of cosmological inflation and the cosmic microwave background radiation. Observations impose severe constraints on the existence of stable domain walls. Those constraints should be accounted for by the models of the beyond Standard Model physics. Unstable cosmic domain walls decay also should produce potentially observable radiation.
- There exist a class of the braneworld models where the brane is assumed to be a domain wall formed by interacting extra-dimensional fields. The matter is localized due to the interaction with this configuration and can leave it at sufficiently high energies. The jargon term for this domain wall is "thick brane" in contrast to the "thin brane" of the models where it is described as delta-potential or simply as some ideal surface with matter fields on it.
See also 
- V. A. Rubakov and M. E. Shaposhnikov,Do we live inside a domain wall?, Physics Letters B 125 (1983) 136–138.
- V. Dzhunushaliev, V. Folomeev, M. Minamitsuji, Thick brane solutions, Rept.Prog.Phys. 73 (2010).
Further reading 
- Vachaspaty, Tanmay (2006). Kinks and Domain Walls: An Introduction to Classical and Quantum Solitons. Cambridge University Press.
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