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Cosmological Non-Linearities as an Effective Fluid

Mixed citation behavior. Most common role is background (54%).

30 Pith papers citing it
Background 54% of classified citations
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.

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representative citing papers

Effective Field Theory of Large Scale Structure and Newtonian Motion Gauges

astro-ph.CO · 2026-05-06 · unverdicted · novelty 7.0 · 2 refs

A Newtonian Motion Gauge found via Einstein-Boltzmann solver maps linear dynamics with scale-dependent growth and GR corrections to Newtonian equations, enabling consistent nonlinear EFT calculations that are transformed back for accuracy in real and redshift space.

Phase-space perturbation theory for cosmic large-scale structure

astro-ph.CO · 2025-12-08 · unverdicted · novelty 7.0

A phase-space perturbation theory solves the Vlasov-Poisson system without truncating momentum cumulants, recovers standard results for cold initial conditions, and generates higher cumulants dynamically from slight initial velocity dispersion.

Post-collapse Lagrangian perturbation theory in three dimensions

astro-ph.CO · 2025-09-23 · unverdicted · novelty 7.0

The authors introduce post-collapse Lagrangian perturbation theory (PCPT) that asymptotically captures early post-shell-crossing dynamics in 3D by using high-order LPT until crossing and then 1D pancake results for gravitational backreaction.

Unitary and Analytic Renormalisation of Cosmological Correlators

hep-th · 2025-09-02 · unverdicted · novelty 7.0

Different dimensional regularization schemes agree with each other and with unitarity; new analytic eta regulators simplify the work and fix the imaginary part of one-loop coefficients by the logarithmic running of the real part under scale invariance and Bunch-Davies conditions.

Analytic compression of the effective field theory of the Lyman-alpha forest

astro-ph.CO · 2026-04-10 · unverdicted · novelty 7.0

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(The) Wiggles going non-linear

astro-ph.CO · 2026-05-29 · unverdicted · novelty 6.0 · 2 refs

Calibrates a one-parameter semi-analytic damping model for oscillatory primordial power spectrum features using N-body simulations and validates sub-percent accuracy via GPR emulation when modulation frequency is high enough.

Cosmological Averaging in Nonminimally Coupled Gravity

astro-ph.CO · 2026-02-17 · unverdicted · novelty 6.0

In f(R,T) = R + F(T) gravity, nonlinear F makes the averaged modified term differ from F at averaged T, invalidating the common unity-ratio assumption and giving dust nonzero proper pressure.

Parity Violation in Galaxy Shapes: Primordial Non-Gaussianity

astro-ph.CO · 2025-09-10 · conditional · novelty 6.0

The parity-odd intrinsic alignment power spectrum probes the collapsed limit of the parity-odd primordial trispectrum and can tighten constraints on parity-violating PNG when bias parameters are calibrated from N-body simulations.

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astro-ph.CO · 2026-04-28 · unverdicted · novelty 6.0 · 2 refs

A joint multi-tracer Fisher analysis of DESI and PFS cross-spectra calibrates bias parameters from data, improving forecasted constraints on fσ8 by 33%, neutrino mass by 80%, and Ωm by 49% compared to single-tracer baselines.

Balancing bias, baryons, and scale cuts in LSST 3x2pt analysis

astro-ph.CO · 2026-06-09 · unverdicted · novelty 5.0

A minimal bias model yields unbiased LambdaCDM constraints up to k_max=0.7 h/Mpc but biases neutrino mass estimates, while higher-order bias mimics baryonic suppression in LSST 3x2pt analyses using the new MGL pipeline.

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