for conformal scalar fields in a Bianchi I universe. (See also Lukash and Starobinsky 1974). That was before the basic issues of particle creation processes were fully understood (e.g., ambiguities in the choice of the vacuum in curved space, see, Fulling 1973) and the regularization techniques were perfected. But they managed to show the viability of such processes. It took several years (1974-77) before the common ground between the different ways of regularization (e.g., adiabatic, point-splitting, dimensional, zeta-function) was reached and a firmer foundation for this new theory of semiclassical gravity was established. Hawking’s discovery (1974) of black hole radiance, and the confirmed ‘legality’ of trace anomaly (Capper and Duff 1975, Deser, Duff and Isham 1976) certainly invigorated this endeavour. The theoretical importance of the subject and the infusion of talents from general relativity, field theory, and particle physics have made this field an important component of contemporary theoretical physics (Birrell and Davies 1982). It is, as we shall see, also an intermediate step (the low energy or adiabatic limit) towards a theory of quantum gravity and quantum cosmology.

Vigorous calculation of the backreaction of particle creation with regularized energy momentum sources began with the work of Hu and Parker 