* Astronomy

Blobrana · L

RE: Dark matter

Blobrana · L

Using one of the most powerful supercomputers in the world to simulate the halo of dark matter that envelopes our galaxy, researchers found dense clumps and streams of the mysterious stuff lurking in the inner regions of the halo, in the same neighbourhood as our solar system.

"In previous simulations, this region came out smooth, but now we have enough detail to see clumps of dark matter" - Piero Madau, professor of astronomy and astrophysics at the University of California, Santa Cruz.

The results, reported in the August 7 issue of the journal Nature, may help scientists figure out what the dark matter is. So far, it has been detected only through its gravitational effects on stars and galaxies. According to one theory, however, dark matter consists of weakly interacting massive particles (WIMPs), which can annihilate each other and emit gamma rays when they collide. Gamma rays from dark matter annihilation could be detected by the recently launched Gamma-ray Large Area Space Telescope (GLAST), which UCSC physicists helped build.

Source

-- Edited by Blobrana at 13:08, 2008-08-07

Blobrana · L

Title: Dark Matter in the Solar System
Authors: X. Xu, E. R. Siegel

We determine the density and mass distribution of dark matter within our Solar System. We explore the three-body interactions between dark matter particles, the Sun, and the planets to compute the amount of dark matter gravitationally captured over the lifetime of the Solar System. We provide an analytical framework for performing these calculations and detail our numerical simulations accordingly. We find that the local density of dark matter is enhanced by between three and five orders of magnitude over the background halo density, dependent on the radial distance from the Sun. This has profound implications for terrestrial direct dark matter detection searches. We also discuss our results in the context of gravitational signatures, including existing constraints, and find that dark matter captured in this fashion is not responsible for the Pioneer anomaly. We conclude that dark matter appears to, overall, play a much more important role in our Solar System than previously thought.

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Blobrana · L

Title: DM particles: how warm they can be?
Authors: Julio C. Fabris, Ilya L. Shapiro, Flavia Sobreira

One of important questions concerning particles which compose the Dark Matter (DM) is their average speed. We consider the model of relativistic weakly interacting massive particles and try to impose an upper bound on their actual and past warmness through the analysis of density perturbations and comparison with the LSS data. It is assumed that the DM can be described by the recently invented model of reduced relativistic gas (RRG). The equation of state of the RRG model is closely reproducing the one of the Maxwell distribution, while being much simpler. This advantage of the RRG model makes our analysis very efficient. As a result we arrive at the rigid and model-independent bound for the DM warmness without using the standard (much more sophisticated) approach based on the Einstein-Boltzmann system of equations.

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Blobrana · L

Title: Two components of dark matter in the DAMA data
Authors: Yukio Tomozawa

It is shown that the DAMA data indicate two dark matter components, one that circulates around the galactic centre (GC) and another that is emitted from the GC. From the location of the maximum yearly variation, one can compute the ratio of the two components.

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Blobrana · L

Title: Unparticle Dark Matter
Authors: Tatsuru Kikuchi, Nobuchika Okada
(Version v3)

Once a parity is introduced in unparticle physics, under which unparticle provided in a hidden conformal sector is odd while all Standard Model particles are even, unparticle can be a suitable candidate for the cold dark matter (CDM) in the present universe through its coupling to the Standard Model Higgs doublet. We find that for Higgs boson mass in the range, 114.4 GeV < m_h < 250 GeV, the relic abundance of unparticle with mass 50 GeV < m_U < 80 GeV can be consistent with the currently observed CDM density. In this scenario, Higgs boson with mass m_h < 160 GeV dominantly decays into a pair of unparticles and such an invisible Higgs boson may be discovered in future collider experiments.

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Blobrana · L

Over the past three years, the Sloan Digital Sky Survey has identified Ursa Major II, Willman I and Coma Berenices Dwarf as small satellite galaxies of the Milky Way. Louis Strigari of the University of California, Irvine, analysed the motion of their stars and found that they appear to be subject to a gravitational field equivalent to that of at least 1 million solar masses distributed around each galaxy. Yet each of these galaxies only shines as bright as 1000 suns, a discrepancy which leads Strigari to suggest that these galaxies are rich in unseen dark matter.

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Blobrana · L

It's the top contender for dark matter, the stuff thought to make up 90 per cent of the mass of the universe. Yet is the hypothetical neutralino particle really the best candidate? By reassessing the reasons for making the neutralino the front runner, physicists have opened up the field to many other dark matter candidates.
The neutralino has captivated cosmologists because it wasn't dreamed up to explain dark matter. Rather, it popped out of theories trying to address problems in the standard model of particle physics, which attempts to explain all the known particles that make up normal matter and the forces that affect them. For instance, the standard model cannot explain why the weak nuclear force is so much stronger than gravity.
The theory of supersymmetry was developed to explain such discrepancies. As yet unconfirmed, it says that every particle in the standard model has a heavier supersymmetric partner. ...

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Blobrana · L

Title: Can the flyby anomaly be attributed to earth-bound dark matter?
Authors: Stephen L. Adler

We make preliminary estimates to assess whether the recently reported flyby anomaly could be attributed to dark matter interactions. We consider both elastic and exothermic inelastic scattering from dark matter constituents; for isotropic dark matter velocity distributions, the former decrease, while the latter increase, the final flyby velocity. Since the observed flyby velocity anomaly shows examples with both positive and negative signs, a two-component model is indicated, involving both elastic and inelastic scatterers with differing spatial distributions. The magnitude of the observed anomalies requires dark matter densities many orders of magnitude greater than the galactic halo density. Such a large density could result from an accumulation cascade, in which the solar system-bound dark matter density is much higher than the galactic halo density, and the earth-bound density is much higher than the solar system-bound density. Constraints on this picture are discussed.

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Blobrana · L

Title: Mirror dark matter and the new DAMA/LIBRA results: A simple explanation for a beautiful experiment
Authors: R. Foot

Recently, the DAMA/LIBRA experiment has convincingly confirmed the DAMA/NaI annual modulation signal, experimentally demonstrating the existence of non-baryonic dark matter in the halo of our galaxy. Meanwhile, in another part of town, other (higher threshold) experiments such as CDMS and XENON10 have not detected any evidence for dark matter. One promising dark matter candidate which can reconcile the positive DAMA annual modulation signal with the null results from the other experiments, is mirror dark matter. We re-analyse the mirror matter interpretation of the DAMA annual modulation signal utilizing a) the new data from DAMA/LIBRA, including the measured energy dependence of the annual modulation signal b) an updated quenching factor which takes into account the channeling effect in NaI crystals and c) the latest constraints from CDMS/Ge, CDMS/Si and XENON10 experiments. We show that the simplest possibility of a He' (and/or H') dominated halo with a small O' component is sufficient to fully explain all of the dark matter experiments. We also point out that a certain class of hidden sector dark matter models, although theoretically less appealing and less constrained, can mimic the success of the mirror dark matter model and hence are also viable.

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