SCALAR FIELD FLUCTUATIONS IN PRE-BIG-BANG COSMOLOGIES

Jordan-Brans-Dicke theories with a linearized potential for the scalar field are investigated in the framework of the stochastic approach, T he fluctuations of this field are examined and their backreaction on t he classical background is described. We compute the mode functions an d analyse the time evolution of the variance of the stochastic ensembl e corresponding to the full quantum scalar field in the pre-big-bang r egime. We compute fluctuations of the term discriminating between the two branches of solutions present in the theory. We find, both analyti cally and upon direct integration of the stochastic equations of motio n, that the dispersion of these fluctuations grows to achieve the magn itude of the term separating the two classical solutions. This means t hat the ensembles representing classical solutions which belong to dif ferent branches do overlap, which may provide at the level of field th eory the quantum mechanical realization of the transition among soluti ons belonging to different classical branches. (C) 1997 Elsevier Scien ce B.V.