A quantum fluctuation is a nonzero variance of an observable in a quantum state, even when its mean vanishes. A vacuum field mode has such zero-point variance. In an inflationary background, the expanding geometry transfers short-wavelength vacuum field fluctuations into primordial perturbations as their wavelengths cross the Hubble radius.
Quantum fluctuation refers to temporary changes in the amount of energy in a point in space, as predicted by the principles of quantum mechanics. This concept arises from the uncertainty principle articulated by Werner Heisenberg, which states that certain pairs of physical properties, like position and momentum, cannot be simultaneously known to arbitrary precision. Similarly, fluctuations in energy levels can occur, even in a vacuum.
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