Cosmological Non-Linearities as an Effective Fluid

TL;DR

This paper develops an effective fluid theory for large-scale cosmological dynamics, showing small-scale non-linearities have negligible backreaction and providing a new framework to understand non-linear effects in cosmology.

Contribution

It introduces a viscous fluid effective theory for long-wavelength cosmological perturbations, incorporating non-linearities and dissipation, with parameters matchable to simulations or observations.

Findings

Backreaction from small-scale non-linearities is negligible.

Virialized scales decouple from large-scale dynamics.

Effective pressure remains positive and insignificant for background evolution.

Abstract

The universe is smooth on large scales but very inhomogeneous on small scales. Why is the spacetime on large scales modeled to a good approximation by the Friedmann equations? Are we sure that small-scale non-linearities do not induce a large backreaction? Related to this, what is the effective theory that describes the universe on large scales? In this paper we make progress in addressing these questions. We show that the effective theory for the long-wavelength universe behaves as a viscous fluid coupled to gravity: integrating out short-wavelength perturbations renormalizes the homogeneous background and introduces dissipative dynamics into the evolution of long-wavelength perturbations. The effective fluid has small perturbations and is characterized by a few parameters like an equation of state, a sound speed and a viscosity parameter. These parameters can be matched to numerical simulations or fitted from observations. We find that the backreaction of small-scale non-linearities is very small, being suppressed by the large hierarchy between the scale of non-linearities and the horizon scale. The effective pressure of the fluid is always positive and much too small to significantly affect the background evolution. Moreover, we prove that virialized scales decouple completely from the large-scale dynamics, at all orders in the post-Newtonian expansion. We propose that our effective theory be used to formulate a well-defined and controlled alternative to conventional perturbation theory, and we discuss possible observational applications. Finally, our way of reformulating results in second-order perturbation theory in terms of a long-wavelength effective fluid provides the opportunity to understand non-linear effects in a simple and physically intuitive way.