31,621 research outputs found
The Small-Scale Power Spectrum of Cold Dark Matter
One of the best motivated hypotheses in cosmology states that most of the
matter in the universe is in the form of weakly-interacting massive particles
that decoupled early in the history of the universe and cooled adiabatically to
an extremely low temperature. Nevertheless, the finite temperature and horizon
scales at which these particles decoupled imprint generic signatures on their
small scales density fluctuations. We show that the previously recognized
cut-off in the fluctuation power-spectrum due to free-streaming of particles at
the thermal speed of decoupling, is supplemented by acoustic oscillations owing
to the initial coupling between the cold dark matter (CDM) and the radiation
field. The power-spectrum oscillations appear on the scale of the horizon at
thermal decoupling which corresponds to a mass scale of
\~10^{-4}*(T_d/10MeV)^{-3} solar masses for a CDM decoupling temperature T_d.
The suppression of the power-spectrum on smaller scales by the acoustic
oscillations is physically independent from the free-streaming effect, although
the two cut-off scales are coincidentally comparable for T_d~10MeV and a
particle mass of M~100GeV. The initial conditions for recent numerical
simulations of the earliest and smallest objects to have formed in the
universe, need to be modified accordingly. The smallest dark matter clumps may
be detectable through gamma-ray production from particle annihilation, through
fluctuations in the event rate of direct detection experiments, or through
their tidal gravitational effect on wide orbits of objects near the outer edge
of the solar system.Comment: Physical Review D, in pres
A Thermal Gradient Approach for the Quasi-Harmonic Approximation and its Application to Improved Treatment of Anisotropic Expansion
We present a novel approach to efficiently implement thermal expansion in the
quasi-harmonic approximation (QHA) for both isotropic and more importantly,
anisotropic expansion. In this approach, we rapidly determine a crystal's
equilibrium volume and shape at a given temperature by integrating along the
gradient of expansion from zero Kelvin up to the desired temperature. We
compare our approach to previous isotropic methods that rely on a brute-force
grid search to determine the free energy minimum, which is infeasible to carry
out for anisotropic expansion, as well as quasi-anisotropic approaches that
take into account the contributions to anisotropic expansion from the lattice
energy. We compare these methods for experimentally known polymorphs of
piracetam and resorcinol and show that both isotropic methods agree to within
error up to 300 K. Using the Gr\"{u}neisen parameter causes up to 0.04 kcal/mol
deviation in the Gibbs free energy, but for polymorph free energy differences
there is a cancellation in error with all isotropic methods within 0.025
kcal/mol at 300 K.
Anisotropic expansion allows the crystals to relax into lattice geometries
0.01-0.23 kcal/mol lower in energy at 300 K relative to isotropic expansion.
For polymorph free energy differences all QHA methods produced results within
0.02 kcal/mol of each other for resorcinol and 0.12 kcal/mol for piracetam, the
two molecules tested here, demonstrating a cancellation of error for isotropic
methods.
We also find that when expanding in more than a single volume variable, there
is a non-negligible rate of failure of the basic approximations of QHA.
Specifically, while expanding into new harmonic modes as the box vectors are
increased, the system often falls into alternate, structurally distinct
harmonic modes unrelated by continuous deformation from the original harmonic
mode.Comment: 38 pages, including 9 pages supporting informatio
Global financial crisis and return of South Asian Gulf migrants: patterns and determinants of their integration to local labour markets
Studies record that a large number of South Asian migrant workers in the Middle–East had to return to their home countries owing to the global financial crisis and loss of jobs. However, their distress of loss of job in the gulf is compounded by the fact that in their own home countries the rehabilitation and reintegration of these workers is tedious and often the returnees are thrust with forced choices. This paper, based on a primary survey conducted in five south Asian countries, namely; Nepal, Sri Lanka, Bangladesh, Pakistan and India, concludes that on return, the employment status of REMs were in general worse off than in their host country with high share of casualisation, self employment and unemployment in the crisis year and a decline in their average monthly earnings. The analysis suggests that those who found employment on return was in fact driven by economic compulsions to reduce their job search period and cost.Global Financial Crisis; Return Migrants; South Asia; Employment; Wages
Solvent Induced Disulfide Bond Formation in 2,5-dimercapto-1,3,4-thiadiazole
Disulfide bond formation is the decisive event in the protein folding to determine the conformation and stability of protein. To achieve this disulfide bond formation in vitro, we took 2,5-dimercapto-1,3,4-thiadiazole (DMcT) as a model compound. We found that disulfide bond formation takes place between two sulfhydryl groups of DMcT molecules in methanol. UV-Vis, FT-IR and mass spectroscopic as well as cyclic voltammetry were used to monitor the course of reaction. We proposed a mechanism for the solvent induced disulfide bond formation on the basis of the results we obtained
Dietary factors affecting exogenous and endogenous sources of fat and carbohydrate for energy production and synthesis Annual progress report, 1 Oct. 1967 - 30 Jun. 1968
Dietary effects on total fatty acid content in rats, and changes in liver, adipose tissue, and carbohydrate metabolis
QueRIE: Collaborative Database Exploration
Interactive database exploration is a key task in information mining. However, users who lack SQL expertise or familiarity with the database schema face great difficulties in performing this task. To aid these users, we developed the QueRIE system for personalized query recommendations. QueRIE continuously monitors the user’s querying behavior and finds matching patterns in the system’s query log, in an attempt to identify previous users with similar information needs. Subsequently, QueRIE uses these “similar” users and their queries to recommend queries that the current user may find interesting. In this work we describe an instantiation of the QueRIE framework, where the active user’s session is represented by a set of query fragments. The recorded fragments are used to identify similar query fragments in the previously recorded sessions, which are in turn assembled in potentially interesting queries for the active user. We show through experimentation that the proposed method generates meaningful recommendations on real-life traces from the SkyServer database and propose a scalable design that enables the incremental update of similarities, making real-time computations on large amounts of data feasible. Finally, we compare this fragment-based instantiation with our previously proposed tuple-based instantiation discussing the advantages and disadvantages of each approach
The Pierre Auger Observatory: Results on Ultra-High Energy Cosmic Rays
The focus of this article is on recent results on ultra-high energy cosmic
rays obtained with the Pierre Auger Observatory. The world's largest instrument
of this type and its performance are described. The observations presented here
include the energy spectrum, the primary particle composition, limits on the
fluxes of photons and neutrinos and a discussion of the anisotropic
distribution of the arrival directions of the most energetic particles.
Finally, plans for the construction of a Northern Auger Observatory in
Colorado, USA, are discussed.Comment: Proceedings of the International Workshop on Advances in Cosmic Ray
Science, Waseda University, Shinjuku, Tokyo, Japan, March 2008; to be
published in the Journal of the Physical Society of Japan (JPSJ) supplemen
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