* Astronomy

Blobrana · L

RE: Dark matter

Blobrana · L

A team of astronomers used data from the COSMOS (Cosmic Evolution Survey) project to reveal previously unseen features in the mass distribution of galaxies, thus indicating that galaxies can have split personalities.
The astronomers used data from the COSMOS project to explore the connections between the growth of dark matter halos and the growth of galaxies within these dark matter halos.
They found previously unseen features in the mass distribution of galaxies and its evolution with time and interpreted these features as corresponding to the known sub-populations of galaxies in the local universe.
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Blobrana · L

The giant halo of dark matter that surrounds our galaxy is shaped like a flattened beach ball, researchers say.
It is the first definitive measure of the scope of the dark matter that makes up the majority of galaxies' masses.
The shape of this "dark matter halo" was inferred from the path of debris left behind as the Sagittarius dwarf galaxy slowly orbits the Milky Way.
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Blobrana · L

An international team of physicists believe they may have detected two particles of the elusive substance for the first time at the bottom of a mine shaft.
Should the findings be confirmed it will have an Earth-shattering effect on our understanding of the make up of the cosmos. It will also prove once and for all the existence of the substance first hypothesised 80 years ago.
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Blobrana · L

Dark matter candidate found

Blobrana · L

Title: The INTEGRAL/SPI 511 keV Signal from Hidden Valleys in Type Ia and Core Collapse Supernova Explosions
Authors: Hooman Davoudiasl, Gilad Perez

We examine under what circumstances the INTEGRAL/SPI 511 keV signal can originate from decays of MeV-scale composite states produced by: (A) thermonuclear (type Ia) or (B) core collapse supernovae (SNe). The requisite dynamical properties that would account for the observed data are quite distinct, for cases (A) and (B). We show that these requirements can be met in simple hidden valley models. An uncommon and novel feature of scenario (A) is that the dynamics of type Ia SNe, standard candles for cosmological measurements, might be affected by our mechanism. In case (A), the mass of the state mediating between the hidden and the SM e^+e^- could be a few hundred GeV and within the reach of a 500 GeV e^+e^- linear collider. In case (B), the mass of the mediator is typically hundreds of TeV and beyond the reach of near-future collider experiments. However, simple models for scenario (B) include a long-lived particle that could, under certain circumstances, be a viable dark matter candidate. The appearance of long-lived particles in typical models leads to cosmological constraints and we address how a consistent cosmic history may be achieved.

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Blobrana · L

RE: Dark matter

Blobrana · L

Rumours that first dark matter particle found

Update: in an email to the blog Resonaances, Nature's senior physical science editor Leslie Sage has squashed the rumours that a paper is about to appear in the journal
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Blobrana · L

Title: Dark Matter Search Using Chandra Observations of Willman 1, and a Spectral Feature Consistent with a Decay Line of a 5 keV Sterile Neutrino
Authors: Michael Loewenstein (UMD/CRESST/NASA-GSFC), Alexander Kusenko (UCLA/Univ. of Tokyo)

We report the results of a search for an emission line from radiatively decaying dark matter in the Chandra X-ray Observatory spectrum of the ultra-faint dwarf spheroidal galaxy Willman 1. 99% confidence line flux upper limits over the 0.4-7 keV Chandra bandpass are derived and mapped to an allowed region in the sterile neutrino mass-mixing angle plane that is consistent with recent constraints from Suzaku X-ray Observatory and Chandra observations of the Ursa Minor and Draco dwarf spheroidals. A significant excess to the continuum, detected by fitting the particle-background-subtracted source spectrum, indicates the presence of a narrow emission feature with energy 2.51 ±0.07 (0.11) keV and flux [3.53 ±1.95 (2.77)] X 10^(-6) photons/cm²/s at 68% (90%) confidence. Interpreting this as an emission line from sterile neutrino radiative decay, we derive the corresponding allowed range of sterile neutrino mass and mixing angle using two approaches. The first assumes that dark matter is solely composed of sterile neutrinos, and the second relaxes that requirement. The detection is consistent with the sterile neutrino mass of 5.0 ±0.2 keV and a mixing angle in a narrow range for which neutrino oscillations can produce all of the dark matter and for which sterile neutrino emission from the cooling neutron stars can explain pulsar kicks, thus bolstering both the statistical and physical significance of our measurement.

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Blobrana · L

Title: An algorithm for the direct reconstruction of the dark matter correlation function from weak lensing and galaxy clustering
Authors: Tobias Baldauf, Robert E. Smith, Uros Seljak, Rachel Mandelbaum

The clustering of matter on cosmological scales is an essential probe for studying the physical origin and composition of our Universe. To date, most of the direct studies have focused on shear-shear weak lensing correlations, but it is also possible to extract the dark matter clustering by combining galaxy-clustering and galaxy-galaxy-lensing measurements. In this study we develop a method that can constrain the dark matter correlation function from galaxy clustering and galaxy-galaxy-lensing measurements, by focusing on the correlation coefficient between the galaxy and matter overdensity fields. To generate a mock galaxy catalogue for testing purposes, we use the Halo Occupation Distribution approach applied to a large ensemble of N-body simulations to model pre-existing SDSS Luminous Red Galaxy sample observations. Using this mock catalogue, we show that a direct comparison between the excess surface mass density measured by lensing and its corresponding galaxy clustering quantity is not optimal. We develop a new statistic that suppresses the small-scale contributions to these observations and show that this new statistic leads to a cross-correlation coefficient that is within a few percent of unity down to 5 Mpc/h. Furthermore, the residual incoherence between the galaxy and matter fields can be explained using a theoretical model for scale-dependent bias, giving us a final estimator that is unbiased to within 1%. We also perform a comprehensive study of other physical effects that can affect the analysis, such as redshift space distortions and differences in radial windows between galaxy clustering and weak lensing observations. We apply the method to a range of cosmological models and show the viability of our new statistic to distinguish between cosmological models.

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