Abstract
The effects of particle creation and vacuum polarization in an external gravitational field offer one possibility to attack certain problems of classical cosmology such as the occurrence of particle horizons. We calculate the vacuum expectation value of the stress-energy tensor of 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 approximation, it proves possible to represent the result as an explicit functional of the scale factor. Its properties are discussed and the conformal anomaly is correctly reproduced. The density of created particles is also calculated. The energy density is probed for the particular class of degree-type scale factors which is relevant in Friedmann cosmology. Except for the square root expansion law, it is found to depend sensitively on the curvature coupling coefficient. The new contributions can become large compared to the previously known conformal contributions and may significantly influence the initial gravitational field.
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