PARTICLE CREATION AND VACUUM POLARIZATION OF A NONCONFORMAL SCALAR FIELD NEAR THE ISOTROPIC COSMOLOGICAL SINGULARITY

The effects of particle creation and vacuum polarization in an externa l gravitational field offer one possibility to attack certain problems of classical cosmology such as the occurrence of particle horizons. W e calculate the vacuum expectation value of the stress-energy tensor o f a non-conformal scalar field in a form that is suitable for studying the back-reaction effects in cosmology. The gravitational background metric is of Robertson-Walker type. By exploiting the early-time appro ximation, it proves possible to represent the result as an explicit fu nctional of the scale factor. Its properties are discussed and the con formal anomaly is correctly reproduced. The density of created particl es is also calculated. The energy density is probed for the particular class of degree-type scale factors which is relevant in Friedmann cos mology. Except for the square root expansion law, it is found to depen d sensitively on the curvature coupling coefficient. The new contribut ions can become large compared to the previously known conformal contr ibutions and may significantly influence the initial gravitational fie ld.