Abstract: (arXiv)
The matter power spectrum, P(k)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 dd spatial dimensions, P(k)P(k) has a universal kdk^{-d} asymptotic scaling with the wave-number kk, for kknlk \gg k_{\rm nl}, where knl1k_{\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 kk dependence of the power spectrum. A parallel is drawn to Batchelor turbulence in hydrodynamics, where large scales mix smaller ones via tidal interactions. The kdk^{-d} scaling is also derived by expressing P(k)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 dd-dimensional sub-manifold of phase-space -- underpins both approaches. The theory is accompanied by 1D1\mathrm{D} Vlasov--Poisson simulations, which confirm it.
Note:
  • Submitted, comments welcome