https://hal-cea.archives-ouvertes.fr/cea-00979168Ballesteros, GuillermoGuillermoBallesterosUniversität Heidelberg [Heidelberg]Bellazzini, BrandoBrandoBellazziniDipartimento di Fisica e Astronomia "Galileo Galilei" - Universita degli Studi di PadovaIPHT - Institut de Physique Théorique - UMR CNRS 3681 - CEA - Commissariat à l'énergie atomique et aux énergies alternatives - Université Paris-Saclay - CNRS - Centre National de la Recherche ScientifiqueMercolli, LorenzoLorenzoMercolliDepartment of Astrophysical Sciences [Princeton] - Princeton University The effective field theory of multi-component fluidsHAL CCSD2014[PHYS.GRQC] Physics [physics]/General Relativity and Quantum Cosmology [gr-qc][PHYS.HPHE] Physics [physics]/High Energy Physics - Phenomenology [hep-ph][PHYS.HTHE] Physics [physics]/High Energy Physics - Theory [hep-th]De Laborderie, Emmanuelle2016-01-05 15:24:562022-07-12 16:40:202016-01-07 10:43:39enJournal articleshttps://hal-cea.archives-ouvertes.fr/cea-00979168/document10.1088/1475-7516/2014/05/007application/x-download1We study the effective Lagrangian, at leading order in derivatives, that describes the propagation of density and metric fluctuations in a fluid composed by an arbitrary number of interacting components. Our results can be applied to any situation in cosmology where different species have non-gravitational interactions. In time-dependent backgrounds, such as FLRW, the quadratic action can only be diagonalized at fixed time slices and flavour mixing is unavoidable as time flows. In spite of this, the fluid can be interpreted at the level of the linear equations of motion as an ensemble of individual interacting species. We show that interactions lead to anisotropic stresses that depend on the mixing terms in the quadratic action for perturbations. In addition to the standard entrainment among different components, we find a new operator in the effective action that behaves as a cosmological constant when it dominates the dynamics.