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Fresh hint of dark matter seen in neutrino search

Flashes of X-rays from crowded galaxy clusters could be the long-awaited sign that we have found particles of dark matter - the elusive substance thought to make up the bulk of all matter in the universe.
If the results stand up, dark matter would consist of ghostly particles called "sterile" neutrinos. These tantalising particles would be the first kind found beyond the standard set known to science.
 
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Title: The shape of dark matter subhalos in the Aquarius simulations
Author: Carlos Vera-Ciro, Laura V. Sales, Amina Helmi, Julio F. Navarro

We analyse the Aquarius simulations to characterize the shape of dark matter halos with peak circular velocity in the range 8<Vmax<200 km/s, and perform a convergence study using the various Aquarius resolution levels. For the converged objects, we determine the principal axis (a<b<c) of the normalised inertia tensor as a function of radius. We find that the triaxiality of field halos is an increasing function of halo mass, so that the smallest halos in our sample are ~40-50% rounder than Milky Way-like objects at the radius where the circular velocity peaks, rmax. We find that the distribution of subhalo axis ratios is consistent with that of field halos of comparable Vmax. Inner and outer contours within each object are well aligned, with the major axis preferentially pointing in the radial direction for subhalos closest to the center of their host halo. We also analyze the dynamical structure of subhalos likely to host luminous satellites comparable to the classical dwarf spheroidals in the Local Group. These halos have axis ratios that increase with radius, and which are mildly triaxial with <b/a>~0.75 and <c/a>~0.60 at r~1 kpc. Their velocity ellipsoid become strongly tangentially biased in the outskirts as a consequence of tidal stripping.

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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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In search for dark matter components, IU physicists edge closer by watching radiation shifts

Nuclear magnetic resonance -- that phenomenon where nuclei of certain atoms, when in a magnetic field, take in and give off measurable amounts of electromagnetic radiation -- is everywhere.
A team of physicists at IU Bloomington that has been hunting for nuclear magnetic resonance frequency shifts as part of a search for new particles in nature that are very weakly coupled to matter, has never been closer to confirming whether the predicted particles do exist.

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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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Hints of lightweight dark matter get even stronger

The leading theoretical candidates for dark matter are weakly interacting massive particles (WIMPs). It's thought these particles annihilate when they meet, producing a shower of radiation, including gamma rays. Launched in 2008, NASA's Fermi space telescope has been scanning for excess gamma rays emanating from the centre of our galaxy, where dark matter should be concentrated.
Last year scientists ruled out a possible Fermi signal at 130 gigaelectronvolts (GeV) as dark matter's smoking gun. But there is another: In 2010, physicist Dan Hooper at Fermilab in Batavia, Illinois, and colleagues reported a possible hint from the space telescope of dark matter particles with a mass of about 10 GeV.

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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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Galaxies déformées : publication d'un nouveau relevé géant du ciel

Des chercheurs français, notamment du laboratoire d'Astrophysique AIM (CEA, Université Paris Diderot, CNRS), publient un nouveau relevé géant du ciel montrant avec précision la déformation de galaxies lointaines dans notre Univers. Obtenu à partir des observations du télescope Canada-France-Hawaii (CFHT), ce relevé, baptisé CFHTLenS, a nécessité 5 ans de travail et a permis d'observer un ensemble de plus de 4 millions de galaxies lointaines. Les mesures ont été réalisées sur le plus grand volume d'Univers jamais sondé. Avec ces travaux, les chercheurs ont pu mesurer l'impact de la matière noire sur la structuration et la géométrie de l'Univers. A terme, et grâce aux futures données de la mission spatiale Euclid, il sera possible d'établir une cartographie extrêmement précise de l'ensemble du ciel extra-galactique jusqu'à une profondeur de 10 milliards d'années, rassemblant ainsi les images de milliards de galaxies. Ces résultats font l'objet d'une publication le 11 avril dans l'édition papier de la revue Monthly Notices of the Royal Astronomical Society.
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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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Shining light on elusive dark matter

The antimatter hunter AMS-02 on the International Space Station is searching for the missing pieces of our Universe. The projects first results published today are hinting at a new phenomenon and revealing more about the invisible 'dark matter'.
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