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TOPIC: Dark matter
Blobrana


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Cosmic positron excess
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Title: Cosmic positron excess: is the dark matter solution a good bet?
Authors: Julien Lavalle

The recent observation by the PAMELA satellite of a rising positron fraction up to ~ 100 GeV has triggered a considerable amount of putative interpretations in terms of dark matter (DM) annihilation or decay. Here, we make a critical reassessment of such a possibility, recalling the elementary conditions with respect to the standard astrophysical background that would make it likely, showing that they are not fulfilled. Likewise, we argue that, as now well accepted, DM would need somewhat contrived properties to contribute significantly to the observed positron signal, even when including e.g. clumpiness effects. This means that most of natural DM candidates arising in particle physics beyond the standard model are not expected to be observed in the cosmic antimatter spectrum, unfortunately. However, this does not prevent them from remaining excellent DM candidates, this only points towards the crucial need of developing much more complex detection strategies (multimessenger, multiwavelength, multiscale searches).

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Blobrana


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Dark Matter
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Title: Physics at Underground Laboratories: Direct Detection of Dark Matter
Authors: Igor G. Irastorza

Underground laboratories host two kind of experiments at the frontier of our knowledge in Particle Physics, Astrophysics and Cosmology: the direct detection of the Dark Matter of the Universe and the search for the Neutrinoless Double Beta Decay of the nuclei. Both experimental quests pose great technical challenges which are being addressed in different ways by an important number of groups. Here a updated review of the efforts being done to detect Dark Matter particles is presented, emphasizing latest achievements.

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Blobrana


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Title: How Dark Matter Reionised The Universe
Authors: Alexander V. Belikov, Dan Hooper
(10 Apr 2009, version, v2)

Although empirical evidence indicates that that the universe's gas had become ionised by redshift z ~ 6, the mechanism by which this transition occurred remains unclear. In this article, we explore the possibility that dark matter annihilations may have played the dominant role in this process. Energetic electrons produced in these annihilations can scatter with the cosmic microwave background to generate relatively low energy gamma rays, which ionise and heat gas far more efficiently than higher energy prompt photons. In contrast to previous studies, we find that viable dark matter candidates with electroweak scale masses can naturally provide the dominant contribution to the reionisation of the universe. Intriguingly, we find that dark matter candidates capable of producing the recent cosmic ray excesses observed by PAMELA and/or ATIC are also predicted to lead to the full reionisation of the universe by z ~ 6.

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Blobrana


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A group of scientists is hoping to replicate a controversial Italian experiment that claims to have detected dark matter. But they might have to do so without the help, or the equipment, of the original group.
Dark matter is thought to make up around 85% of the matter in the Universe, but it rarely interacts with regular matter except through the force of gravity. Researchers working on the DAMA experiment at the Gran Sasso National Laboratory near L'Aquila, Italy, claim they have spotted direct signs of it.
The detector used by the DAMA team consists of 250 kilograms of ultrapure sodium iodide crystals placed 1,400 metres beneath Gran Sasso mountain. Over the past decade, the researchers have collected data showing that nuclei in the crystals periodically release flashes of light, which could be caused by interactions with dark matter. Crucially, the number of flashes varies with the seasons, which would be consistent with Earth's motion through a galactic dark-matter stream
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Blobrana


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Precise picture of early universe supports 'dark matter' theory
A detailed picture of the seeds of structures in the universe has been unveiled by an international team co-led by a Cardiff University scientist.
The team has obtained extremely precise data about the early universe, using a telescope near the South Pole in the Antarctic.
Their measurements of the cosmic microwave background - a faintly glowing relic of the hot, dense, young universe - provide further support for the standard cosmological model of the universe. The findings confirm the model's prediction that dark matter and dark energy make up 95% of everything in existence, while ordinary matter makes up just 5%.
In a paper published in the November 1 issue of The Astrophysical Journal, researchers on the QUaD telescope project have released detailed maps of the cosmic microwave background (CMB). The researchers focused their measurements on variations in the CMB's temperature and polarization to learn about the distribution of matter in the early universe. Polarization is the direction in which vibrations travel from all light rays, which is at right angles to the ray's direction of travel.

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Blobrana


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In the latest episode of their continuing efforts to embrace and understand the dark side of creation, astronomers sifting data from a new satellite say they have discerned the existence of a mysterious haze of high-energy particles surrounding the center of the Milky Way galaxy.
Nobody knows where the particles came from, and the five astronomers who posted their results online on Monday did not offer a formal opinion. But one tantalizing prospect, they admit, is that the particles are the decayed remains of the long-sought dark matter that constitutes 25 percent of the universe.

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Blobrana


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Dark Matter Annihilation
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Title: Possible Evidence For Dark Matter Annihilation In The Inner Milky Way From The Fermi Gamma Ray Space Telescope
Authors: Lisa Goodenough, Dan Hooper

We study the gamma rays observed by the Fermi Gamma Ray Space Telescope from the direction of the Galactic Centre and find that their angular distribution and energy spectrum are well described by a dark matter annihilation scenario. In particular, we find a good fit to the data for dark matter particles with a 25-30 GeV mass, an annihilation cross section of ~9x10^-26 cm^3/s, and that are distributed with a cusped halo profile within the inner kiloparsec of the Galaxy.

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Blobrana


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RE: Dark matter
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Dark Matter Dark Energy & the Unknown Universe



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Blobrana


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Title: Universality of galactic surface densities within one dark halo scale-length
Authors: Gianfranco Gentile, Benoit Famaey, HongSheng Zhao  &  Paolo Salucci

It was recently discovered that the mean dark matter surface density within one dark halo scale-length (the radius within which the volume density profile of dark matter remains approximately flat) is constant across a wide range of galaxies. This scaling relation holds for galaxies spanning a luminosity range of 14 magnitudes and the whole Hubble sequence. Here we report that the luminous matter surface density is also constant within one scale-length of the dark halo. This means that the gravitational acceleration generated by the luminous component in galaxies is always the same at this radius. Although the total luminous-to-dark matter ratio is not constant, within one halo scale-length it is constant. Our finding can be interpreted as a close correlation between the enclosed surface densities of luminous and dark matter in galaxies.

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An international team of astronomers has reported an unexpected link between mysterious dark matter and the visible stars and gas in galaxies. This could revolutionise our current understanding of gravity.
Hongsheng Zhao of the SUPA Centre of Gravity, University of St. Andrews, suggests that an unknown force is acting on dark matter. Only four percent of the universe is thought to made of known material. Stars and gas in galaxies move in such a way that astronomers have inferred that the gravity from a hypothetical invisible halo of dark matter is needed to keep galaxies together.
However, a solid understanding of dark matter, as well as direct evidence of its existence, has remained elusive. Now, the astronomers believe that the interactions between dark and ordinary matter could be more important and more complex than previously thought, and even speculate that dark matter might not exist, and that the anomalous motions of stars in galaxies are due to a modification of gravity on extragalactic scales.

"The dark matter seems to know how the visible matter is distributed. They seem to conspire with each other such that the gravity of the visible matter at the characteristic radius of the dark halo is always the same" - Benoit Famaey, Universities of Bonn and Strasbourg.

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Blobrana


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IceCube telescope
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Title: The Indirect Search for Dark Matter with IceCube
Authors: Francis Halzen, Dan Hooper
(Version v2)

We revisit the prospects for IceCube and similar kilometre-scale telescopes to detect neutrinos produced by the annihilation of weakly interacting massive dark matter particles (WIMPs) in the Sun. We emphasize that the astrophysics of the problem is understood; models can be observed or, alternatively, ruled out. In searching for a WIMP with spin-independent interactions with ordinary matter, IceCube is only competitive with direct detection experiments if the WIMP mass is sufficiently large. For spin-dependent interactions IceCube already has improved the best limits on spin-dependent WIMP cross sections by two orders of magnitude. This is largely due to the fact that models with significant spin-dependent couplings to protons are the least constrained and, at the same time, the most promising because of the efficient capture of WIMPs in the Sun. We identify models where dark matter particles are beyond the reach of any planned direct detection experiments while being within reach of neutrino telescopes. In summary, we find that, even when contemplating recent direct detection results, neutrino telescopes have the opportunity to play an important as well as complementary role in the search for particle dark matter.

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