In cosmology, decoupling is a period in the development of the universe when different types of particles fall out of thermal equilibrium with each other.
This occurs as a result of the expansion of the universe, as their interaction rates decrease up to this critical point.
The two verified instances of decoupling since the Big Bang which are most often discussed are photon decoupling and neutrino decoupling, as these led to the cosmic microwave background and cosmic neutrino background, respectively.
In physical cosmology, fractal cosmology is a set of minority cosmological theories which state that the distribution of matter in the Universe, or the structure of the universe itself, is a fractal across a wide range of scales.
More generally, it relates to the usage or appearance of fractals in the study of the universe and matter.
A central issue in this field is the fractal dimension of the universe or of matter distribution within it, when measured at very large or very small scales.
