* Astronomy

Speed limit found for sluggish dark matter

Now, for the first time, Cristian Armendáriz-Picón and Jayanth Neelakanta of Syracuse University in New York have calculated how fast unclumped dark matter particles would zip around randomly in space. The team looked at snapshots of how matter was distributed at different points in cosmic history. They examined the distribution at very small scales, given by spectral analysis of the earliest light in the universe, and at much larger scales, given by surveys of galaxy clusters. They calculated how fast dark matter must have been moving in the early universe to create the observed large-scale structure and then extrapolated how fast the particles would be moving today if they were not in clumps.
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Cosmic Giants Shed New Light on Dark Matter

The density of dark matter depends on the properties of the individual dark matter particles, just like the density of everyday materials depends on their components. CDM, the leading theory about dark matter to date, asserts that dark matter particles only interact with each other and with other matter via the force of gravity; they do not emit or absorb electromagnetic radiation and are difficult if not impossible to see. Therefore, the team chose to observe dark matter by using gravitational lensing, which detects its presence through its gravitational interactions with ordinary matter and radiation. According to Einstein's theory of relativity, light from a very distant bright source bends around a massive object, e.g., a cluster of galaxies, between the source object and the observer. It follows from this principle that the dark matter in cosmic giants like galaxy clusters alters the apparent shape and position of distant galaxies.
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Title: Why Comparable? A Multiverse Explanation of the Dark Matter-Baryon Coincidence
Authors: Raphael Bousso, Lawrence Hall

The densities of dark and baryonic matter are comparable: zeta = rho_D / rho_B ~ O(1). This is surprising because they are controlled by different combinations of low-energy physics parameters. Here we consider the probability distribution over \zeta in the landscape. We argue that the Why Comparable problem can be solved without detailed anthropic assumptions, and independently of the nature of dark matter. Overproduction of dark matter suppresses the probability like 1/(1+zeta), if the causal patch is used to regulate infinities. This suppression can counteract a prior distribution favouring large zeta, selecting zeta ~ O(1).
This effect not only explains the Why Comparable coincidence but also renders otherwise implausible models of dark matter viable. For the special case of axion dark matter, Wilczek and independently Freivogel have already noted that a 1/(1+zeta) suppression prevents overproduction of a GUT-scale QCD axion. If the dark matter is the LSP, the effect can explain the moderate fine-tuning of the weak scale in simple supersymmetric models.

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Title: Dark matter in galaxies: the dark matter particle mass is about 2 keV
Authors: H. J. de Vega, N. G. Sanchez

Warm dark matter (WDM) means DM particles with mass m in the keV scale. For large scales, for structures beyond 100 kpc, WDM and CDM yield identical results which agree with observations. For intermediate scales, WDM gives the correct abundance of substructures. Inside galaxy cores, below 100 pc, N-body classical physics simulations are incorrect for WDM because at such scales quantum effects are important for WDM. Quantum calculations (Thomas-Fermi approach) provide galaxy cores, galaxy masses, velocity dispersions and density profiles in agreement with the observations. All evidences point to a dark matter particle mass around 2 keV. Baryons, which represent 16% of DM, are expected to give a correction to pure WDM results. The detection of the DM particle depends upon the particle physics model. Sterile neutrinos with keV scale mass (the main WDM candidate) can be detected in beta decay for Tritium and Renium and in the electron capture in Holmiun. The sterile neutrino decay into X rays can be detected observing DM dominated galaxies and through the distortion of the black-body CMB spectrum. The effective number of neutrinos, N_{eff} measured by WMAP9 and Planck satellites is compatible with two Majorana sterile neutrinos with mass much smaller than the electron mass. One of them can be a WDM sterile neutrino. So far, not a single valid objection arose against WDM.

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