Abstract
We study how inhomogeneities of the cosmological fluid fields backreact on the homogeneous part of energy density and how they modify the Friedmann equations. In general, backreaction requires to go beyond the pressureless ideal fluid approximation, and this can lead to a reduced growth of cosmological large-scale structure. Since observational evidence favors evolution close to the standard growing mode in the linear regime, we focus on two-component fluids in which the nonideal fluid is gravitationally coupled to cold dark matter and in which a standard growing mode persists. This is realized, e.g., for a baryonic fluid coupled to cold dark matter. We calculate the backreaction for this case and for a wide range of other two-fluid models. Here the effect is either suppressed because the nonideal matter properties are numerically too small, or because they lead to a too stringent UV cutoff of the integral over the power spectrum that determines backreaction. We discuss then matter field backreaction from a broader perspective and generalize the formalism such that also far-from-equilibrium scenarios relevant to late cosmological times and nonlinear scales can be addressed in the future.
Highlights
The evolution equations used in cosmology are usually obtained under the assumption of a spatially homogeneous and isotropic distribution of matter
Since observational evidence favors evolution close to the standard growing mode in the linear regime, we focus on two-component fluids in which the nonideal fluid is gravitationally coupled to cold dark matter and in which a standard growing mode persists
For baryons gravitationally coupled to cold dark matter, we had found in Sec
Summary
The evolution equations used in cosmology are usually obtained under the assumption of a spatially homogeneous and isotropic distribution of matter. Because Einstein’s gravitational field equations are nonlinear, this question amounts to asking whether the Einstein equations for spatially averaged fields are affected by socalled backreaction effects, i.e., averages of second and higher orders in spatial inhomogeneities. Arguments that such backreaction effects are always irrelevant for cosmology have been made [1] and have been contested [2,3], the discussion focusing mainly on inhomogeneities of the. [6] how such inhomogeneities give rise to backreaction effects in late-time cosmology where nonlinear terms in metric inhomogeneities and their temporal variations are negligible compared to those of inhomogeneities in the matter fields. We generalize the formalism of Ref. [6] to systems in which a Navier-Stokes fluid description cannot be taken for granted
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