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TOPIC: Dark matter


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Dark matter
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Astronomers now have a new "eye" for determining the distance to certain mysterious bodies in and around our Milky Way galaxy. By taking advantage of the unique position of NASA's Spitzer's Space Telescope millions of miles from Earth, and a depth-perceiving trick called parallax, they were able to pin down the most probable location of one such object. The findings will ultimately help astronomers better understand the different components of our galaxy.

"Forty years ago a visionary astronomer named Dr. Sjur Refsdal theorised that dark bodies could be located using parallax and a space telescope. It is truly remarkable that we have been able to prove him right with this Spitzer observation" - Andrew Gould of Ohio State University, Columbus, Ohio, who led the project.

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An international team of astronomers led by Ohio State University has examined dark matter in the outer reaches of our galaxy in a new way.
For the first time, they were able to employ triangulation -- a method rooted in ancient Greek geometry -- to estimate the location of dark matter and calculate its mass.
The results, reported May 30 at the meeting of the American Astronomical Society in Honolulu, suggest that this technique could help astronomers detect dark matter of a particular mass range for which there were previously no reliable tests.
It could also settle longstanding questions about the composition of dark matter in the outer reaches of the Milky Way -- the so-called galactic "halo."
Dark matter is, by its nature, invisible. But astronomers can watch the sky for those rare moments when dark matter affects visible objects. One such opportunity is a gravitational lensing event -- when one dark object in space acts like a lens to magnify the light from a star shining behind it.

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Title: Can dark matter be a Bose-Einstein condensate?
Authors: C. G. Boehmer, T. Harko

We consider the possibility that the dark matter, which is required to explain the dynamics of the neutral hydrogen clouds at large distances from the galactic centre, could be in the form of a Bose-Einstein condensate. To study the condensate we use the non-relativistic Gross-Pitaevskii equation. By introducing the Madelung representation of the wave function, we formulate the dynamics of the system in terms of the continuity equation and of the hydrodynamic Euler equations. Hence dark matter can be described as a non-relativistic, Newtonian Bose-Einstein gravitational condensate gas, whose density and pressure are related by a barotropic equation of state. In the case of a condensate with quartic non-linearity, the equation of state is polytropic with index n=1. To test the validity of the model we fit the Newtonian tangential velocity equation of the model with a sample of rotation curves of low surface brightness and dwarf galaxies, respectively. We find a very good agreement between the theoretical rotation curves and the observational data for the low surface brightness galaxies. The deflection of photons passing through the dark matter halos is also analysed, and the bending angle of light is computed. The bending angle obtained for the Bose-Einstein condensate is larger than that predicted by standard general relativistic and dark matter models. Therefore the study of the light deflection by galaxies and the gravitational lensing could discriminate between the Bose-Einstein condensate dark matter model and other dark matter models.

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Most of the Universe is a complete and total mystery. And one of these mysteries is dark matter. Its out there, and astronomers are slowly teasing out its characteristics, but its not giving up its secrets easily.
The problem is, dark matter only interacts with regular matter through gravity (and maybe through the weak nuclear force). It doesnt shine, it doesnt give off heat or radio waves, and it passes through regular matter like it isnt there. But when dark matter is destroyed, it might give astronomers the clues theyre looking for.
Researchers have theorised that one productive way to search for dark matter might not be to search for it directly, but to look for the resulting particles and energy which are emitted when its destroyed. In the environment around the centre of our galaxy, dark matter might be dense enough that particles regularly collide, releasing a cascade of energy and additional particles; which could be detected.

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Title: Evidence Of Dark Matter Annihilations In The WMAP Haze
Authors: Dan Hooper, Douglas P. Finkbeiner and Gregory Dobler

The WMAP experiment has revealed an excess of microwave emission from the region around the centre of our Galaxy. It has been suggested that this signal, known as the WMAP Haze, could be synchrotron emission from relativistic electrons and positrons generated in dark matter annihilations. In this letter, we revisit this possibility. We find that the angular distribution of the WMAP Haze matches the prediction for dark matter annihilations with a cusped density profile, (r) / r^1.2 in the inner kiloparsecs. Comparing the intensity in different WMAP frequency bands, we find that a wide range of possible WIMP annihilation modes are consistent with the spectrum of the haze for a WIMP with a mass in the 100 GeV to multi-TeV range. Most interestingly, we find that to generate the observed intensity of the haze, the dark matter annihilation cross section is required to be approximately equal to the value needed for a thermal relic, v 310^26 cm^3/s. No boost factors are required. If dark matter annihilations are in fact responsible for the WMAP Haze, and the slope of the halo profile continues into the inner Galaxy, GLAST is expected to detect gamma rays from the dark matter annihilations in the Galactic Centre if the WIMP mass is less than several hundred GeV.

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Mass-to-Light Ratios
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Title: The Baryon Fractions and Mass-to-Light Ratios of Early-Type Galaxies
Authors: Guangfei Jiang, C.S. Kochanek

We jointly model 22 early-type gravitational lens galaxies with stellar dynamical measurements using standard CDM halo models. The sample is inhomogeneous in both its mass distributions and the evolution of its stellar populations unless the true uncertainties are significantly larger than the reported measurement errors. In general, the individual systems cannot constrain halo models, in the sense that the data poorly constrains the stellar mass fraction of the halo. The ensemble of systems, however, strongly constrains the average stellar mass represented by the visible galaxies to 0.0260.006 of the halo mass if we neglect adiabatic compression, rising to 0.0560.011 of the halo mass if we include adiabatic compression. Both estimates are significantly smaller than the global baryon fraction, corresponding to a star formation efficiency for early-type galaxies of 10%-30%. In the adiabatically compressed models, we find an average local B-band stellar mass-to-light ratio of (M/L)_0 = (7.20.5)(M_{\sun}/L_{\sun}) that evolves by d\log(M/L)/dz = -0.720.08 per unit redshift. Adjusting the isotropy of the stellar orbits has little effect on the results. The adiabatically compressed models are strongly favoured if we impose either local estimates of the mass-to-light ratios of early-type galaxies or the weak lensing measurements for the lens galaxies on 100 kpc scales as model constraints.

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RE: Dark matter
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Title: Direct Dark Matter Searches
Authors: N. J. Spooner

For many working in particle physics and cosmology successful discovery and characterisation of the new particles that most likely explain the non-baryonic cold dark matter, known to comprise the majority of matter in the Universe, would be the most significant advance in physics for a century. Reviewed here is the current status of direct searches for such particles, in particular the so-called Weakly Interacting Massive Particles (WIMPs), together with a brief overview of the possible future direction of the field extrapolated from recent advances. Current best limits are at or below 10-7 pb for spin-independent neutralino coupling, sufficient that experiments are already probing SUSY models. However, new detectors with tonne-scale mass and/or capability to correlate signal events to our motion through the Galaxy will likely be needed to determine finally whether WIMPs exist.

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DAMA observatory
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Title: Results on Dark Matter and beta beta decay modes by DAMA at Gran Sasso
Authors: R. Bernabei (Univ. and INFN Roma Tor Vergata)
(Version v2)

DAMA is an observatory for rare processes and it is operative deep underground at the Gran Sasso National Laboratory of the I.N.F.N. (LNGS). Here some arguments will be presented on the investigation on dark matter particles by annual modulation signature and on some of the realised double beta decay searches.

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RE: Dark matter
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Title: Satellite survival in cold dark matter cosmology
Authors: C.M. Boily, N. Nakastao, R. Spurzem, T. Tsuchiya

We study the survival of substructures (clumps) within larger self-gravitating dark matter halos. Building on scaling relations obtained from N-body calculations of violent relaxation, we argue that the tidal field of galaxies and halos can only destroy substructures if spherical symmetry is imposed at formation. We explore other mechanisms that may tailor the number of halo substructures during the course of virialisation. Unless the larger halo is built up from a few large clumps, we find that clump-clump encounters are unlikely to homogenize the halo on a dynamical timescale. Phase mixing would proceed faster in the inner parts and allow for the secular evolution of a stellar disk.

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