Coherent state representation of quantum fluctuations in the early Universe

Abstract

Using the squeezed state formalism the coherent state representation of quantum fluctuations in an expanding universe is derived. It is shown that this provides an interesting alternative to the Wigner function as a phase space representation of quantum fluctuations. The quantum to classical transition of fluctuations is simply implemented by decohering the density matrix in this representation. The entropy of the decohered vacua is derived and is shown to agree with previous results in the high squeezing limit. It is shown that the decoherence process can significantly change the predictions derived using pure states. In particular, scale-invariant power spectra are not obtained for any decoherence process, though it can be obtained by decohering in the coherent state representation.