Editing Cosmic inflation (section)

===Few inhomogeneities remain===
Because the accelerating expansion of space stretches out any initial variations in density or temperature to very large length scales, an essential feature of inflation is that it smooths out [[homogeneity (physics)|inhomogeneities]] and [[anisotropy|anisotropies]], and reduces the [[shape of the universe|curvature of space]]. This pushes the Universe into a very simple state in which it is completely dominated by the [[inflaton]] field and the only significant inhomogeneities are tiny [[quantum fluctuation]]s. Inflation also dilutes exotic heavy particles, such as the [[magnetic monopole]]s predicted by many extensions to the [[Standard Model]] of [[particle physics]]. If the Universe was only hot enough to form such particles ''before'' a period of inflation, they would not be observed in nature, as they would be so rare that it is quite likely that there are none in the [[observable universe]]. Together, these effects are called the inflationary "no-hair theorem"<ref>{{harvp|Kolb|Turner|1988}}</ref> by analogy with the [[no hair theorem]] for [[black hole]]s.

The "no-hair" theorem works essentially because the cosmological horizon is no different from a black-hole horizon, except for not testable disagreements about what is on the other side. The interpretation of the no-hair theorem is that the Universe (observable and unobservable) expands by an enormous factor during inflation. In an expanding universe, [[energy density|energy densities]] generally fall, or get diluted, as the volume of the Universe increases. For example, the density of ordinary "cold" matter (dust) declines as the inverse of the volume: when linear dimensions double, the energy density declines by a factor of eight; the radiation energy density declines even more rapidly as the Universe expands since the wavelength of each [[photon]] is stretched ([[redshift]]ed), in addition to the photons being dispersed by the expansion. When linear dimensions are doubled, the energy density in radiation falls by a factor of sixteen (see [[Equation of state (cosmology)#Ultra-relativistic matter|the solution of the energy density continuity equation for an ultra-relativistic fluid]]). During inflation, the energy density in the inflaton field is roughly constant. However, the energy density in everything else, including inhomogeneities, curvature, anisotropies, exotic particles, and standard-model particles is falling, and through sufficient inflation these all become negligible. This leaves the Universe flat and symmetric, and (apart from the homogeneous inflaton field) mostly empty, at the moment inflation ends and reheating begins.{{efn|
Not only is inflation very effective at driving down the number density of magnetic monopoles, it is also effective at driving down the number density of every other type of particle, including photons.<ref name="Ryden2003">{{cite book |author=Barbara Sue Ryden |title=Introduction to cosmology |date=2003 |publisher=Addison-Wesley |isbn=978-0-8053-8912-8}}</ref>{{rp|style=ama|p= 202–207}}
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