* Astronomy

Blobrana · L

RE: Dark matter

Blobrana · L

Sun's dark matter trap

The Sun could be the best place to look for dark matter - the invisible 'stuff' that is thought to make up about 83% of the matter in the Universe.
That's what new Oxford University research reported in a recent Physical Review Letters suggests.
The work looks at the possibility that dark matter is much lighter than the WIMP particles most dark matter hunters are looking for. Such 'heavy' particles are also their own antiparticles, so that when a WIMP meets a WIMP they annihilate each other, making it puzzling that there's still so much dark matter around.
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Title: Gamma-ray Signal from Earth-mass Dark Matter Microhalos
Authors: Tomoaki Ishiyama, Junichiro Makino, Toshikazu Ebisuzaki

Earth-mass dark matter microhalos with size of ~ 100 AUs are the first structures formed in the universe, if we consider neutralino as the dark matter candidate. Early studies suggested that a noticeable fraction of microhalos born in early universe have survived up to present time and they might be observed as the dominant sources of the annihilation signal. On the other hand, others claimed that small-scale structure have a negligible impact on dark matter detectability. Here, we report the results of ultra-high-resolution simulation of the formation and evolution of these microhalos. We found that microhalos have the central density cusp of the form
ho \propto r^{-1.5}, much steeper than the cusp of larger dark halos. The very central regions of these microhalos survive the encounters with stars down to the radius of a few kpcs from the galactic center. The nearest microhalos at distance of ~ 0.1 pc, might be visible as point sources (radius less than 1'), with proper motion of ~ 0.2 degree per year. Subhalos are also observable by boosts due to microhalos. Also, we might be able to use the millisecond pulsar timing measurements by PPTA to detect microhalos.

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Telescope offers clues on dark matter

Secrets of the Universe are to be revealed as a new telescope equipped with the worlds most powerful digital camera begins its observations of the night sky.
The Pan-STARRS sky survey telescope, known as PS1, will enable scientists to better understand the mysteries of dark matter and dark energy, the material that is thought to account for much of the mass of the Universe but has never been proven to exist.
Astronomers from the Universities of Durham, Edinburgh and Queens University Belfast together with researchers from around the world are using the telescope to scan the skies from dusk to dawn each night.
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Title: Dark Matter from Split Seesaw
Authors: Alexander Kusenko, Fuminobu Takahashi, Tsutomu T. Yanagida

The seesaw mechanism in models with extra dimension is shown to be generically consistent with a broad range of Majorana masses. If the scales of the seesaw parameters are split, with two right-handed neutrinos at a high scale and one at a keV scale, it can explain the matter-antimatter asymmetry of the universe, as well as dark matter. The dark matter candidate, a sterile right-handed neutrino with mass of several keV, can explain the observed pulsar velocities and the recent data from Chandra X-ray telescope, which suggest the existence of a 5 keV sterile right-handed neutrino.

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Title: MiXDM: Cosmic Ray Signals from Multiple States of Dark Matter
Authors: Ilias Cholis, Neal Weiner (CCPP, NYU)
(27 Nov 2009)

Recent data from cosmic ray experiments such as PAMELA, Fermi, ATIC and PPB-BETS all suggest the need for a new primary source of electrons and positrons at high (>~100 GeV) energies. Many proposals have been put forth to explain these data, usually relying on a single particle to annihilate or decay to produce e+e-. In this paper, we consider models with multiple species of WIMPs with significantly different masses. We show if such dark matter candidates chi_i annihilate into light bosons, they naturally produce equal annihilation rates, even as the available numbers of pairs for annihilation n_chi_i² differ by orders of magnitude. We argue that a consequence of these models can be to add additional signal naturally at lower (~100 GeV) versus higher (~ TeV) energies, changing the expected spectrum and even adding bumps at lower energies, which may alleviate some of the tension in the required annihilation rates between PAMELA and Fermi. These spectral changes may yield observable consequences in the microwave Haze signal observed at the upcoming Planck satellite. Such a model can connect to other observable signals such as DAMA and INTEGRAL by having the lighter (heavier) state be a pseudo-Dirac fermion with splitting 100 keV (1 MeV). We show that variations in the halo velocity dispersion can alleviate constraints from final state radiation in the galactic centre and galactic ridge. If the lighter WIMP has a large self-interaction cross section, the light-WIMP halo might collapse, dramatically altering expectations for direct and indirect detection signatures.

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Title: Multicomponent Dark Matter in Supersymmetric Hidden Sector Extensions
Authors: Daniel Feldman, Zuowei Liu, Pran Nath, Gregory Peim
(Version v2)

Most analyses of dark matter within supersymmetry assume the entire cold dark matter arising only from weakly interacting neutralinos. We study a new class of models consisting of U(1)^n hidden sector extensions of the MSSM that includes several stable particles, both fermionic and bosonic, which can be interpreted as constituents of dark matter. In one such class of models, dark matter is made up of both a Majorana dark matter particle, i.e., a neutralino, and a Dirac fermion with the current relic density of dark matter as given by WMAP being composed of the relic density of the two species. These models can explain the PAMELA positron data and are consistent with the anti-proton flux data, as well as the photon data from FERMI-LAT. Further, it is shown that such models can also simultaneously produce spin independent cross sections which can be probed in CDMS-II, XENON-100 and other ongoing dark matter experiments. The implications of the models at the LHC and at the NLC are also briefly discussed.

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Title: Direct measurement of dark matter halo ellipticity from two-dimensional lensing shear maps of 25 massive clusters
Authors: Masamune Oguri, Masahiro Takada, Nobuhiro Okabe, Graham P. Smith

We present new measurements of dark matter distributions in 25 X-ray luminous clusters by making a full use of the two-dimensional (2D) weak lensing signals obtained from high-quality Subaru/Suprime-Cam imaging data. Our approach to directly compare the measured lensing shear pattern with elliptical model predictions allows us to extract new information on the mass distributions of individual clusters, such as the halo ellipticity and mass centroid. We find that these parameters on the cluster shape are little degenerate with cluster mass and concentration parameters. By combining the 2D fitting results for a subsample of 18 clusters, the elliptical shape of dark matter haloes is detected at 7\sigma significance level. The mean ellipticity is found to be e = 0.46 ±0.04 (1\sigma), which is in excellent agreement with the standard collisionless CDM model prediction. The mass centroid can be constrained with a typical accuracy of ~20" (~50 kpc/h) in radius for each cluster with some significant outliers, enabling to assess one of the most important systematic errors inherent in the stacked cluster weak lensing technique, the mass centroid uncertainty. In addition, the shape of the dark mass distribution is found to be only weakly correlated with that of the member galaxy distribution. We carefully examine possible sources of systematic errors in our measurements, finding none of them to be significant. Our results demonstrate the power of high-quality imaging data for exploring the detailed spatial distribution of dark matter.

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Research Illuminates the Shape of Dark Matter's Distribution

The nature of dark matter is still unknown and is currently a central problem in modern astronomy and physics. Dark matter is dark in a couple of ways. It is undetectable to visible light and has escaped detection at all electromagnetic wavelengths. Because it is invisible, its existence has to be inferred from its gravitational effect on other celestial objects as well as from theoretical models. Indirect evidence has established its relative abundance in our universe-probably five times greater than visible matter-in addition to its significance for understanding galaxy formation. For example, a considerable amount of dark matter probably sustains the structure of galaxies, because the gravitational force of visible matter cannot bind its member stars. The scientific challenge is how to study the nature of dark matter. Astronomers seek ways to use their observations to solve this puzzle.
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A dark matter lecture by physicist Peter Fisher at the American Museum of Natural History in New York City drew a sellout crowd April 12 in a theatre that seats more than 400, with museum staff turning away disappointed comers and at least one gentleman trying to talk his way in as if he were working to get past the velvet rope at a nightclub.
Fisher, of the Massachusetts Institute of Technology, gave his sizable audience a tidy roundup of what we know about dark matter and what we hope to find out in the coming years.
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