The minisuperspace in
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which r ...
, especially in theories of
quantum gravity
Quantum gravity (QG) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics; it deals with environments in which neither gravitational nor quantum effects can be ignored, such as in the vi ...
, is an approximation of the otherwise infinite-dimensional
phase space
In dynamical system theory, a phase space is a space in which all possible states of a system are represented, with each possible state corresponding to one unique point in the phase space. For mechanical systems, the phase space usually ...
of a
field theory.
The phase space is reduced by considering the largest
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
modes to be of the order of the size of the universe when studying
cosmological models and removing all larger modes.
The validity of this approximation holds as long as the
adiabatic approximation
In thermodynamics, an adiabatic process (Greek: ''adiábatos'', "impassable") is a type of thermodynamic process that occurs without transferring heat or mass between the thermodynamic system and its environment. Unlike an isothermal process, ...
holds.
An example would be to only consider the scale factor and Hubble constant for a
Friedman–Robertson–Walker model in minisuperspace model the small true vacuum bubble which is nearly spherical with one single parameter of the scalar factor a is described as minisuperspace. It plays a significant role in the explanation of the origin of universe as a bubble in quantum cosmological theory.
References
Quantum gravity
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