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 ΛCDM 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 ∼ 10 −6 M , results in a median density of ∼ 4×10 −3 in units of the mean density, whereas a 10 µeV axion with free-streaming mass ∼ 10 −12 M gives ∼ 3 × 10 −3 , and Warm Dark Matter with a (thermal relic) mass of 1 keV gives ∼ 8 × 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.

Consistency of Primordial Black Hole Dark Matter with LIGO/Virgo Merger Rates

“…For PBH masses somewhat smaller than a solar mass, these limits are significantly relaxed due to a sensitivity loss of LIGO/Virgo. Such small f free [assuming f M ðM PBH Þ ≈ 1] are close to what one finds in CDM simulations [47]. However, values for f free could be even smaller in PBH dark matter scenarios due to the much earlier onset of structure formation and the initial zero velocities of PBHs.…”

Dark Matter Haloes and Subhaloes

“…Even today at most ∼ 20% is expected to be unclustered in the CDM model [94]. A recent update of this work (applied also to WDM) puts the fraction even lower, at the percent level [95].…”

Universal structure of dark matter haloes over a mass range of 20 orders of magnitude

“…This low value is a consequence of repeatedly choosing to refine regions that avoid any massive nonlinear structure. It is still somewhat larger than the median z = 0 density of a universe with Planck cosmology dominated by a 100 GeV WIMP 12 . One may nevertheless question whether the haloes we have simulated can be considered representative of the general population of similar mass objects.…”

“…This is expected since most of the mass of the universe is contained in "high mass" haloes that have, by construction, been excluded from the higher simulation levels. 12 The fact that typical present-day haloes of very low mass (e.g. Earth mass) form from Lagrangian regions with atypically low (linear) overdensity on larger scales (e.g.…”