Gravitational Turbulence: the Small-Scale Limit of the Cold-Dark-Matter Power Spectrum

TL;DR

This paper reveals a universal small-scale $k^{-d}$ power spectrum scaling for cold dark matter in cosmology, explained through gravitational turbulence and phase-space analysis, supported by Vlasov-Poisson simulations.

Contribution

It introduces a non-perturbative phase-space framework explaining the universal small-scale power spectrum scaling in cold dark matter cosmology.

Findings

Derives the $k^{-d}$ scaling using phase-space integrals and saddle-point analysis.

Shows gravitational collapse induces turbulence that balances linear and non-linear time scales.

Vlasov-Poisson simulations confirm the theoretical predictions.

Abstract

The matter power spectrum, $P(k)$, is one of the fundamental quantities in the study of large-scale structure in cosmology. Here, we study its small-scale asymptotic limit, and show that for cold dark matter in $d$ spatial dimensions, $P(k)$ has a universal $k^{-d}$ asymptotic scaling with the wave-number $k$, for $k \gg k_{\rm nl}$, where $k_{\rm nl}^{-1}$ denotes the length scale at which non-linearities in gravitational interactions become important. We propose a theoretical explanation for this scaling, based on a non-perturbative analysis of the system's phase-space structure. Gravitational collapse is shown to drive a turbulent phase-space flow of the quadratic Casimir invariant, where the linear and non-linear time scales are balanced, and this balance dictates the $k$ dependence of the power spectrum. A parallel is drawn to Batchelor turbulence in hydrodynamics, where large scales mix smaller ones via tidal interactions. The $k^{-d}$ scaling is also derived by expressing $P(k)$ as a phase-space integral in the framework of kinetic field theory, which is analysed by the saddle-point method; the dominant critical points of this integral are precisely those where the time scales are balanced. The coldness of the dark-matter distribution function - its non-vanishing only on a $d$-dimensional sub-manifold of phase-space - underpins both approaches. The theory is accompanied by $1\mathrm{D}$ Vlasov-Poisson simulations, which confirm it.