A **cosmological defect** is a large-scale structure in the universe formed from a phase transition breaking symmetry, and can constribute to the energy density of the universe. Types of topological defect include **cosmic strings**, which have a constant linear energy density, and **domain walls**, which have a constant surface energy density. Typically, these cosmological defects will span the entire universe, and natural ones were almost invariably formed during the very early universe.

Since these cosmological defects are constant energy density, rather than constant energy, they will alter the density of the universe as it expands. Cosmic strings have a density dropoff of a^{-2}, domain walls have a density dropoff of a^{-1}, and more generally in an n-dimensional universe an m-dimensional cosmological defect will have its density drop off as a^{m-n}.

Smaller cosmological defects can also form, by curving one of the existing types. Cosmic strings can be curved into small **cosmic loops**, and domain walls can be curved into small **domain tubes** and **domain spheres**. These can act as lower-dimensional cosmological defects on the small scale; for example, a domain tube will dilute more quickly than a domain wall, with very narrow domain tubes acting like cosmic strings.

Advanced civilisations can alter the properties of space in order to artifically produce cosmological defects. For example, a domain sphere can act as an impenetrable force field.

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Phantom energy · Cosmological constant Cosmological defect Matter · Radiation · Ultralight |