Cosmological Perturbations of Quantum Mechanical Origin: Are Nonvacuum Initial States Allowed?

We address the question of whether nonvacuum initial states for cosmologicalperturbations are allowed, or whether they are ruled out on the basis of presentexperimental and observational data. Our choice of a nonvacuum initial state isguided by the idea that the initial state could have a built-in characteristic scale.We find that a class of initial states can fit the data; however, the initial statesmust be close to the vacuum.