Effective perfect fluids in cosmology

Abstract

We describe the cosmological dynamics of perfect fluids within the framework of effective field theories. The effective action is a derivative expansion whose terms are selected by the symmetry requirements on the relevant long-distance degrees of freedom, which are identified with comoving coordinates. The perfect fluid is defined by requiring invariance of the action under internal volume-preserving diffeomorphisms and general covariance. At lowest order in derivatives, the dynamics is encoded in a single function of the entropy density that characterizes the properties of the fluid, such as the equation of state and the speed of sound. This framework allows a neat simultaneous description of fluid and metric perturbations. Longitudinal fluid perturbations are closely related to the adiabatic modes, while the transverse modes mix with vector metric perturbations as a consequence of vorticity conservation. This formalism features a large flexibility which can be of practical use for higher order perturbation theory and cosmological parameter estimation.

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• The effective theory of fluids at NLO and implications for dark energy
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• The effective field theory of multi-component fluids
• Material models of dark energy
• Intrinsic Entropy Perturbations from the Dark Sector
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• Generalized unimodular gravity as a form of k-essence
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• Dynamical diffeomorphisms
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• Effective field theory of coupled dark energy and dark matter
• Lensing convergence and anisotropic dark energy in galaxy redshift surveys
• Quantum Mechanics from General Relativity and the Quantum Friedmann Equation
• Cosmology with a Non-minimally Coupled Dark Matter Fluid I. Background Evolution
• Accelerated expansion in the effective field theory of a radiation dominated universe