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In quantum gravity, the phase space is infinite dimensional as we are dealing with a field theory. An approximation which is sometimes taken is to only consider the largest wavelength modes of the order of the size of the universe when studying cosmological models. This is the minisuperspace approximation. The validity of this approximation holds as long as the adiabatic approximation 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.