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TOPIC: Dark matter


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RE: Dark matter
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Title: Is dark matter made of mirror matter? Evidence from cosmological data
Authors: Paolo Ciarcelluti, Quentin Wallemacq

We present new fast numerical simulations of cosmic microwave background and large scale structure in the case in which the cosmological dark matter is made entirely or partly of mirror matter. We consider scalar adiabatic primordial perturbations at linear scales in a flat Universe. The speed of the simulations allows us for the first time to use Markov Chain Monte Carlo analyses to constrain the mirror parameters. A Universe with pure mirror matter can fit very well the observations, equivalently to the case of an admixture with cold dark matter. In both cases, the analyses show a clear indication of the presence of a consistent amount of mirror dark matter, 0.05 < Omega_mirror h^2 < 0.12.

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Title: Constraining the substructure of dark matter haloes with galaxy-galaxy lensing
Authors: Ran Li, Houjun Mo, Zuhui Fan, Xiaohu Yang, Frank C. van den Bosch

With galaxy groups constructed from the Sloan Digital Sky Survey (SDSS), we analyse the expected galaxy-galaxy lensing signals around satellite galaxies residing in different host haloes and located at different halo-centric distances. We use Markov Chain Monte Carlo (MCMC) method to explore the potential constraints on the mass and density profile of subhaloes associated with satellite galaxies from SDSS-like surveys and surveys similar to the Large Synoptic Survey Telescope (LSST). Our results show that for SDSS-like surveys, we can only set a loose constraint on the mean mass of subhaloes. With LSST-like surveys, however, both the mean mass and the density profile of subhaloes can be well constrained.

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Title: Testing The Light Dark Matter Hypothesis With AMS
Authors: Dan Hooper, Wei Xue

The spectrum and morphology of gamma-rays from the Galactic Center and the spectrum of synchrotron emission observed from the Milky Way's radio filaments have each been interpreted as possible signals of \sim 7-10 GeV dark matter particles annihilating in the Inner Galaxy. In dark matter models capable of producing these signals, the annihilations should also generate significant fluxes of \sim 7-10 GeV positrons which can lead to a distinctive bump-like feature in local cosmic ray positron spectrum. In this letter, we show that while such a feature would be difficult to detect with PAMELA, it would likely be identifiable by the currently operating AMS experiment. As no known astrophysical sources or mechanisms are likely to produce such a sharp feature, the observation of a positron bump at around 7-10 GeV would significantly strengthen the case for a dark matter interpretation of the reported gamma-ray and radio anomalies.

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Cold Dark matter
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Title: Finding new signature effects on galactic dynamics to constrain Bose-Einstein-condensed cold dark matter
Authors: Tanja Rindler-Daller, Paul R. Shapiro

If cosmological cold dark matter (CDM) consists of light enough bosonic particles that their phase-space density exceeds unity, they will comprise a Bose-Einstein condensate (BEC). The nature of this BEC-CDM as a quantum fluid may then distinguish it dynamically from the standard form of CDM involving a collisionless gas of non-relativistic particles that interact purely gravitationally. We summarise some of the dynamical properties of BEC-CDM that may lead to observable signatures in galactic halos and present some of the bounds on particle mass and self-interaction coupling strength that result from a comparison with observed galaxies.

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Title: Can Effects of Dark Matter be Explained by the Turbulent Flow of Spacetime?
Authors: F. Elliott Koch, Angus H. Wright

For the past forty years the search for dark matter has been one of the primary foci of astrophysics, although there has yet to be any direct evidence for its existence (Porter et al. 2011). Indirect evidence for the existence of dark matter is largely rooted in the rotational speeds of stars within their host galaxies, where, instead of having a ~ r^½ radial dependence, stars appear to have orbital speeds independent of their distance from the galactic center, which led to proposed existence of dark matter (Porter et al. 2011; Peebles 1993). We propose an alternate explanation for the observed stellar motions within galaxies, combining the standard treatment of a fluid-like spacetime with the possibility of a "bulk flow" of mass through the Universe. The differential "flow" of spacetime could generate vorticies capable of providing the "perceived" rotational speeds in excess of those predicted by Newtonian mechanics. Although a more detailed analysis of our theory is forthcoming, we find a crude "order of magnitude" calculation can explain this phenomena. We also find that this can be used to explain the graviational lensing observed around globular clusters like "Bullet Cluster".

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Ed ~ Unfortunately, these 'Spacetime vorticies' would be detected by looking at the light from distant quasars and measuring the time-lag effect (which has already been done). 



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Dark Matter Simulations
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Title: A Novel Approach to Visualising Dark Matter Simulations
Authors: Ralf Kaehler, Oliver Hahn, Tom Abel

In the last decades cosmological N-body dark matter simulations have enabled ab initio studies of the formation of structure in the Universe. Gravity amplified small density fluctuations generated shortly after the Big Bang, leading to the formation of galaxies in the cosmic web. These calculations have led to a growing demand for methods to analyse time-dependent particle based simulations. Rendering methods for such N-body simulation data usually employ some kind of splatting approach via point based rendering primitives and approximate the spatial distributions of physical quantities using kernel interpolation techniques, common in SPH (Smoothed Particle Hydrodynamics)-codes. This paper proposes three GPU-assisted rendering approaches, based on a new, more accurate method to compute the physical densities of dark matter simulation data. It uses full phase-space information to generate a tetrahedral tessellation of the computational domain, with mesh vertices defined by the simulation's dark matter particle positions. Over time the mesh is deformed by gravitational forces, causing the tetrahedral cells to warp and overlap. The new methods are well suited to visualise the cosmic web. In particular they preserve caustics, regions of high density that emerge, when several streams of dark matter particles share the same location in space, indicating the formation of structures like sheets, filaments and halos. We demonstrate the superior image quality of the new approaches in a comparison with three standard rendering techniques for N-body simulation data.

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Dark matter
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Title: A new determination of the local dark matter density from the kinematics of K dwarfs
Authors: Silvia Garbari, Chao Liu, Justin I. Read, George Lake
(Version, v2)

We apply a new method to determine the local disc matter and dark halo matter density to kinematic and position data for \sim2000 K dwarf stars taken from the literature. Our method assumes only that the disc is locally in dynamical equilibrium, and that the 'tilt' term in the Jeans equations is small up to \sim1 kpc above the plane. We present a new calculation of the photometric distances to the K dwarf stars, and use a Monte Carlo Markov Chain to marginalise over uncertainties in both the baryonic mass distribution, and the velocity and distance errors for each individual star. We perform a series of tests to demonstrate that our results are insensitive to plausible systematic errors in our distance calibration, and we show that our method recovers the correct answer from a dynamically evolved N-body simulation of the Milky Way. We find a local dark matter density of {rho}dm = 0.025+0.014-0.013 solar masses pc^{-3} (0.95+0.53-0.49 GeV cm^{-3}) at 90% confidence assuming no correction for the non-flatness of the local rotation curve, and {rho}dm = 0.022+0.015-0.013 solar masses pc^-3 (0.85+0.57-0.50 GeV cm^{-3}) if the correction is included. Our 90% lower bound on {rho}dm is larger than the canonical value typically assumed in the literature, and is at mild tension with extrapolations from the rotation curve that assume a spherical halo. Our result can be explained by a larger normalisation for the local Milky Way rotation curve, an oblate dark matter halo, a local disc of dark matter, or some combination of these.

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Plenty of dark matter near the Sun

Astronomers at the University of Zürich and the ETH Zürich, together with other international researchers, have found large amounts of invisible "dark matter" near the Sun. Their results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive "halo" of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high quality simulations. The authors also find tantalising hints of a new dark matter component in our Galaxy.
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Title: Search for a dark matter particle family
Authors: Yukio Tomozawa

I suggest a simple signature for new particles which are unstable partners of a dark matter particle. The suggested mass range is from 8 TeV to 3 PeV, the former being the mass of the dark matter particle and the latter being the knee energy mass scale from the cosmic ray energy spectrum. It can be the energy spectrum of a specific particle such as a muon, a neutrino, jets or any other particles produced in cosmic ray showers, as long as the spectrum is measured. As for the detection of a 3 PeV particle by the neutrino energy spectrum, all dark matter targets throughout the galaxy that are bombarded by high energy cosmic rays and high energy dark matter particles contribute to the process. This is new in the study of dark matter physics.

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Dark matter underpinnings of cosmic web found

The skeleton of dark matter that undergirds the cosmic web of matter in the universe has been clearly detected for first time.
We know that matter in the cosmos forms a web, with galaxies and clusters linked by filaments across mostly empty space.
Jörg Dietrich at the University Observatory in Munich, Germany, and his team have detected the dark matter component in a filament in a supercluster about 2.7 billion light years from us, called Abell 222/223.

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