Conformal-symmetry breaking and cosmological particle creation in $\lambda {\phi }^4$ theory

We show that creation of field quanta can occur even in conformally trivial situations, contrary to popular belief. This conformal-symmetry breaking owes its existence to the so-called conformal anomaly in the field stress tensor which, in the case of interacting fields in curved spacetime, contains state-dependent, nongeometrical, and other terms which are associated with particle generation. The effect is explicitly demonstrated for massless $\lambda {\phi }^4$ theory in a Robertson-Walker spacetime.