Dobado González, AntonioLópez Maroto, Antonio2023-06-202023-06-201999-11-150556-282110.1103/PhysRevD.60.104045https://hdl.handle.net/20.500.14352/58720©1999 The American Physical Society. A.L.M acknowledges support from SEUID-Royal Society. This work has been partially supported by Ministerio de Educación y Ciencia (Spain) CICYT (AEN96-1634).In this paper we show how the nonlocal effective action for gravity, obtained after integrating out the matter fields, can be used to compute particle production and spectra for different space-time metrics. Applying this technique to several examples, we End that the perturbative calculation of the effective action up to second order in curvatures yields exactly the same results for the total number of particles as the Bogolyubov transformations method, in the case of massless scaler fields propagating in a Robertson-Walker space-time. Using an adiabatic approximation we also obtain the corresponding spectra and compare the results with the traditional WKB approximation. [S0556-2821(99)02920-3].engjournal articlehttp://prd.aps.org/abstract/PRD/v60/i10/e104045http://prd.aps.orgopen access53Covariant Perturbation-TheoryLoop Effective ActionEarly UniverseInflationAnisotropyFieldFísica (Física)22 Física