Gravitational Turbulence: the Small-Scale Limit of the Cold-Dark-Matter Power Spectrum
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Abstract: (arXiv)
The matter power spectrum, , 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 spatial dimensions, has a universal asymptotic scaling with the wave-number , for , where 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 dependence of the power spectrum. A parallel is drawn to Batchelor turbulence in hydrodynamics, where large scales mix smaller ones via tidal interactions. The scaling is also derived by expressing 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 -dimensional sub-manifold of phase-space -- underpins both approaches. The theory is accompanied by Vlasov--Poisson simulations, which confirm it.Note:
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