General Relativity and Gravitation, Vol. 27, No. 3, 1995 Time Asymmetry in Semiclassical Cosmology Mario Castagnino, 1,2,3 Edgar Gunzig, 1 and Fernando Lombardo 1,2 Received September 13, I99~ An apparently innocuous property of nature is the universal alignment of the arrow of time for all physical systems, from the subatomic world to the overall cosmological dynamical evolution. This seems to point towards an explanation of irreversibflity as the manifestation of a uni- vernal intrinsic property, shared by physical systems of arbitrary scales. A major step towards an understanding of this property was taken in quantum mechanics. This required no alteration of the theory itself, but an extension of its mathematical formalism. A natural attitude is there- fore to adapt this procedure to cosmological considerations. The present essay is precisely devoted to the description of the peculiarities of this interpretation of the irreversible dynamical behaviour of the cosmological history. 1. INTRODUCTION The aim of this work is to propose a new avenue of approach to a longstand- ing paradoxical situation which has plagued the overall area of physics for ages: the systematic breakdown of time symmetry despite the fact that dy- namical evolutions are controlled by perfectly time-symmetric equation~. The apogee of this perplexing feature is undoubtedly represented by the time arrow of our cosmological history. The latter is driven indeed by 1 Servicede Chimie-Physique, UniversitdLibre de Bruxelles~C a m p u s Plaine 231, 1050 Bruxelles, Belgium 2 Departamento de Ffsica, t~acultadde Ciencias Exactas y Naturales Universidad de Buenos Aires, Ciudad Universitaria,1428 Buenos Aires, Argentina 3 Instituto de Astronom/a y Ffsicadel Espacio, Casillade Correo 67, Sucursal 28, 1428 Buenos Aires, Argentina 257 0001-7701/95/0300-0257507.50/0 {~) 1995 P|enum Publlsh]ng Corporation