Papers › Universal scaling laws and density slope for dark matter haloes
Universal scaling laws and density slope for dark matter haloes
Zhijie Xu
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Small scale challenges suggest some missing pieces in our understanding of dark matter. A cascade theory for dark matter is proposed to provide extra insights, similar to the cascade phenomenon in hydrodynamic turbulence. The kinetic energy is cascaded in dark matter from small to large scales involves a constant rate εᵤ (≈-4.6×10⁻⁷m²/s³). Confirmed by N-body simulations, energy cascade leads to a two-thirds law for kinetic energy vᵣ² on scale r such that vᵣ² ∝(εᵤ r)^(2/3). A four-thirds law can be established for mean halo density ρₛ enclosed in the scale radius rₛ such that ρₛ ∝εᵤ^(2/3)G⁻¹rₛ^(-4/3), which was confirmed by galaxy rotation curves. Critical properties of dark matter might be obtained by identifying key constants on relevant scales. The largest halo scale rₗ can be determined by -u₀³/εᵤ, where u₀ is the velocity dispersion. The smallest scale r_η is dependent on the nature of dark matter. For collisionless dark matter, r_η ∝(-Għ/εᵤ) ^(1/3)≈10⁻¹³m is found along with the mass scale m_X∝(-εᵤħ⁵G⁻⁴)^(1/9)≈10¹²GeV, where ħ is the Planck constant. An uncertainty principle for momentum and acceleration fluctuations is also postulated. For self-interacting dark matter, r_η ∝εᵤ² G⁻³(σ/m)³, where σ/m is the cross-section of interaction. On halo scale, the energy cascade leads to an asymptotic density slope γ=-4/3 for fully virialized haloes with a vanishing radial flow, which might explain the nearly universal halo density. Based on the continuity equation, halo density is analytically shown to be closely dependent on the radial flow and mass accretion, such that simulated haloes can have different limiting slopes.
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