Casimir effect in nonstationary regions with Friedmann metric

A study is made of the two-dimensional problem of finding the energy-momentum tensor of a scalar massless field with symmetric nonstationary boundary conditions in the case of a nonstationary Friedmann metric. A combined method is proposed for regularizing the energy-momentum tensor – point splitting and Abel–Plana summation. A method is proposed for determining the wave functions and energy-momentum tensor when boundary conditions are specified on characteristics. The solution of these problems in two-dimensional spacetime makes it possible to obtain explicit expressions for the energy-momentum tensor.