51,911 research outputs found
Massive sterile neutrinos as warm Dark Matter
We show that massive sterile neutrinos mixed with the ordinary ones may be
produced in the early universe in the right amount to be natural warm dark
matter particles. Their mass should be below 40 keV and the corresponding
mixing angles sin^2 2\theta > 10^{-11} for mixing with \nu_\mu or \nu_\tau,
while mixing with \nu_e is slightly stronger bounded with mass less than 30
keV.Comment: 13 pages, 1 figure, references and acknowledgement added; discussion
on SN bound updated, matches version in Astropart.phy
Spectral distortion of cosmic background radiation by scattering on hot electrons. Exact calculations
The spectral distortion of the cosmic background radiation produced by the
inverse Compton scattering on hot electrons in clusters of galaxies (thermal
Sunyaev-Zeldovich effect) is calculated for arbitrary optical depth and
electron temperature. The distortion is found by a numerical solution of the
exact Boltzmann equation for the photon distribution function. In the limit of
small optical depth and low electron temperature our results confirm the
previous analyses. In the opposite limits, our method is the only one that
permits to make accurate calculations.Comment: 18 pages, 7 figures, to be published in Ap
Why does the Jeans Swindle work?
When measuring the mass profile of any given cosmological structure through
internal kinematics, the distant background density is always ignored. This
trick is often refereed to as the "Jeans Swindle". Without this trick a
divergent term from the background density renders the mass profile undefined,
however, this trick has no formal justification. We show that when one includes
the expansion of the Universe in the Jeans equation, a term appears which
exactly cancels the divergent term from the background. We thereby establish a
formal justification for using the Jeans Swindle.Comment: 5 pages, 2 figures, Accepted for publication in MNRAS Letter
Metric adjusted skew information: Convexity and restricted forms of superadditivity
We give a truly elementary proof of the convexity of metric adjusted skew
information following an idea of Effros. We extend earlier results of weak
forms of superadditivity to general metric adjusted skew informations.
Recently, Luo and Zhang introduced the notion of semi-quantum states on a
bipartite system and proved superadditivity of the Wigner-Yanase-Dyson skew
informations for such states. We extend this result to general metric adjusted
skew informations. We finally show that a recently introduced extension to
parameter values of the WYD-information is a special case of
(unbounded) metric adjusted skew information.Comment: An error in the literature is pointed ou
Shear Viscosity of Quark Matter
We consider the shear viscosity of a system of quarks and its ratio to the
entropy density above the critical temperature for deconfinement. Both
quantities are derived and computed for different modeling of the quark
self-energy, also allowing for a temperature dependence of the effective mass
and width. The behaviour of the viscosity and the entropy density is argued in
terms of the strength of the coupling and of the main characteristics of the
quark self-energy. A comparison with existing results is also discussed.Comment: 15 pages, 4 figure
Constraining neutrino physics with BBN and CMBR
We perform a likelihood analysis of the recent results on the anisotropy of
Cosmic Microwave Background Radiation from the BOOMERanG and DASI experiments
to show that they single out an effective number of neutrinos in good agreement
with standard Big Bang Nucleosynthesis. We also consider degenerate Big Bang
Nucleosynthesis to provide new bounds on effective relativistic degrees of
freedom and, in particular, on neutrino chemical potential
. When including Supernova Ia data we find, at , and , .Comment: 6 pages, 3 figures, some reference adde
Dynamical density functional theory with hydrodynamic interactions and colloids in unstable traps
A density functional theory for colloidal dynamics is presented which
includes hydrodynamic interactions between the colloidal particles. The theory
is applied to the dynamics of colloidal particles in an optical trap which
switches periodically in time from a stable to unstable confining potential. In
the absence of hydrodynamic interactions, the resulting density breathing mode,
exhibits huge oscillations in the trap center which are almost completely
damped by hydrodynamic interactions. The predicted dynamical density fields are
in good agreement with Brownian dynamics computer simulations
Dynamical arrest and replica symmetry breaking in attractive colloids
Within the Replica Symmetry Breaking (RSB) framework developed by M.Mezard
and G.Parisi we investigate the occurrence of structural glass transitions in a
model of fluid characterized by hard sphere repulsion together with short range
attraction. This model is appropriate for the description of a class of
colloidal suspensions. The transition line in the density-temperature plane
displays a reentrant behavior, in agreement with Mode Coupling Theory (MCT), a
dynamical approach based on the Mori-Zwanzig formalism. Quantitative
differences are however found, together with the absence of the predicted
glass-glass transition at high density. We also perform a systematic study of
the pure hard sphere fluid in order to ascertain the accuracy of the adopted
method and the convergence of the numerical procedure.Comment: 7 pages, 6 figure
Quantum scissors: teleportation of single-mode optical states by means of a nonlocal single photon
We employ the quantum state of a single photon entangled with the vacuum
(|1,0>-|0,1>), generated by a photon incident upon a symmetric beam splitter,
to teleport single-mode quantum states of light by means of the Bennett
protocol. Teleportation of coherent states results in truncation of their Fock
expansion to the first two terms. We analyze the teleported ensembles by means
of homodyne tomography and obtain fidelities of up to 99 per cent for low
source state amplitudes. This work is an experimental realization of the
quantum scissors device proposed by Pegg, Phillips and Barnett (Phys. Rev.
Lett. 81, 1604 (1998)
Kinetics of Surfactant Adsorption at Fluid/Fluid Interfaces: Non-ionic Surfactants
We present a model treating the kinetics of adsorption of soluble
surface-active molecules at the interface between an aqueous solution and
another fluid phase. The model accounts for both the diffusive transport inside
the solution and the kinetics taking place at the interface using a free-energy
formulation. In addition, it offers a general method of calculating dynamic
surface tensions. Non-ionic surfactants are shown, in general, to undergo a
diffusion-limited adsorption, in accord with experimental findings.Comment: 6 pages, 3 figures, see also cond-mat/960814
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