The median density of the Universe

Abstract

Despite the fact that the mean matter density of the universe has been measured to an accuracy of a few percent within the standard $\Lambda$CDM paradigm, its median density is not known even to order of magnitude. Typical points lie in low-density regions and are not part of a collapsed structure of any scale. Locally, the dark matter distribution is then simply a stretched version of that in the early universe. In this single-stream regime, the distribution of unsmoothed density is sensitive to the initial power spectrum on all scales, in particular on very small scales, and hence to the nature of the dark matter. It cannot be estimated reliably using conventional cosmological simulations because of the enormous dynamic range involved, but a suitable excursion set procedure can be used instead. For the Planck cosmological parameters, a 100 GeV WIMP, corresponding to a free-streaming mass $\sim 10^{-6}{M}_{\odot }$, results in a median density of $\sim 4\times 10^{-3}$ in units of the mean density, whereas a 10 $\mu$eV axion with free-streaming mass $\sim 10^{-12}{M}_{\odot }$ gives $\sim 3\times 10^{-3}$, and Warm Dark Matter with a (thermal relic) mass of 1 keV gives $\sim 8\times 10^{-2}$. In CDM (but not in WDM) universes, single-stream regions are predicted to be topologically isolated by the excursion set formalism. A test by direct N-Body simulations seems to confirm this prediction, although it is still subject to finite size and resolution effects. Unfortunately, it is unlikely that any of these properties is observable and so suitable for constraining the properties of dark matter.

17 pages, 17 figures, for additional material like movies, see http://wwwmpa.mpa-garching.mpg.de/paper/singlestream2017/index.html

References in corpus (9)

• The Excursion Set Theory of Halo Mass Functions, Halo Clustering, and Halo Growth
• The fine-grained phase-space structure of Cold Dark Matter halos
• Quantifying distortions of the Lagrangian dark-matter mesh in cosmology
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• Cosmological N-Body Simulations with a Large-Scale Tidal Field
• Measuring the Tidal Response of Structure Formation: Anisotropic Separate Universe Simulations using TreePM
• Simulating the Complexity of the Dark Matter Sheet I: Numerical Algorithms
• Large-Scale Velocity Dispersion and the Cosmic Web
• Cosmological Vlasov-Poisson equations for dark matter: Recent developments and connections to selected plasma problems
• The Schrödinger-Poisson method for Large-Scale Structure
• Cosmic Web Dissection in Fuzzy Dark Matter Cosmologies
• The Tessellation-Level-Tree: characterising the nested hierarchy of density peaks and their spatial distribution in cosmological N-body simulations
• Power spectrum of density fluctuations, halo abundances and clustering with primordial black holes
• Identifying Dark Matter Haloes by the Caustic Boundary
• Early evolution of spin direction in dark matter halos and the effect of the surrounding large-scale tidal field
• Fast and accurate collapse-time predictions for collisionless matter
• Starting Cosmological Simulations from the Big Bang
• Cosmology with Cosmic Voids
• Excursion Sets with a "Perfect" Collapse Model