582 research outputs found
Plane-Symmetric Inhomogeneous Bulk Viscous Cosmological Models with Variable
A plane-symmetric non-static cosmological model representing a bulk viscous
fluid distribution has been obtained which is inhomogeneous and anisotropic and
a particular case of which is gravitationally radiative. Without assuming any
{\it adhoc} law, we obtain a cosmological constant as a decreasing function of
time. The physical and geometric features of the models are also discussed.Comment: 11 page
On the T-dependence of the magnetic penetration depth in unconventional superconductors at low temperatures: can it be linear?
We present a thermodynamics argument against a strictly linear temperature
dependence of the magnetic penetration depth, which applies to superconductors
with arbitrary pairing symmetry at low temperatures.Comment: 5 pages, expanded version of cond-mat/971102
Bianchi Type I Magnetofluid Cosmological Models with Variable Cosmological Constant Revisited
The behaviour of magnetic field in anisotropic Bianchi type I cosmological
model for bulk viscous distribution is investigated. The distribution consists
of an electrically neutral viscous fluid with an infinite electrical
conductivity. It is assumed that the component of shear tensor
is proportional to expansion () and the coefficient of
bulk viscosity is assumed to be a power function of mass density. Some physical
and geometrical aspects of the models are also discussed in presence and also
in absence of the magnetic field.Comment: 13 page
Ellipsoidal Universe Can Solve The CMB Quadrupole Problem
The recent three-year WMAP data have confirmed the anomaly concerning the low
quadrupole amplitude compared to the best-fit \Lambda CDM prediction. We show
that, allowing the large-scale spatial geometry of our universe to be
plane-symmetric with eccentricity at decoupling or order 10^{-2}, the
quadrupole amplitude can be drastically reduced without affecting higher
multipoles of the angular power spectrum of the temperature anisotropy.Comment: 4 pages, 2 figures, minor changes, reference added, to appear in
Phys. Rev. Let
Big Bang Nucleosynthesis with Gaussian Inhomogeneous Neutrino Degeneracy
We consider the effect of inhomogeneous neutrino degeneracy on Big Bang
nucleosynthesis for the case where the distribution of neutrino chemical
potentials is given by a Gaussian. The chemical potential fluctuations are
taken to be isocurvature, so that only inhomogeneities in the electron chemical
potential are relevant. Then the final element abundances are a function only
of the baryon-photon ratio , the effective number of additional neutrinos
, the mean electron neutrino degeneracy parameter , and
the rms fluctuation of the degeneracy parameter, . We find that for
fixed , , and , the abundances of helium-4,
deuterium, and lithium-7 are, in general, increasing functions of .
Hence, the effect of adding a Gaussian distribution for the electron neutrino
degeneracy parameter is to decrease the allowed range for . We show that
this result can be generalized to a wide variety of distributions for .Comment: 9 pages, 3 figures, added discussion of neutrino oscillations,
altered presentation of figure
The spherical collapse model in time varying vacuum cosmologies
We investigate the virialization of cosmic structures in the framework of
flat FLRW cosmological models, in which the vacuum energy density evolves with
time. In particular, our analysis focuses on the study of spherical matter
perturbations, as they decouple from the background expansion, "turn around"
and finally collapse. We generalize the spherical collapse model in the case
when the vacuum energy is a running function of the Hubble rate,
. A particularly well motivated model of this type is the
so-called quantum field vacuum, in which is a quadratic function,
, with . This model was previously studied
by our team using the latest high quality cosmological data to constrain its
free parameters, as well as the predicted cluster formation rate. It turns out
that the corresponding Hubble expansion history resembles that of the
traditional CDM cosmology. We use this CDM framework to
illustrate the fact that the properties of the spherical collapse model (virial
density, collapse factor, etc.) depend on the choice of the considered vacuum
energy (homogeneous or clustered). In particular, if the distribution of the
vacuum energy is clustered, then, under specific conditions, we can produce
more concentrated structures with respect to the homogeneous vacuum energy
case.Comment: 14 pages, 4 figures, minor changes, accepted for publication in Phys.
Rev.
Neutrino Oscillations and the Early Universe
The observational and theoretical status of neutrino oscillations in
connection with solar and atmospheric neutrino anomalies is presented in brief.
The effect of neutrino oscillations on the early Universe evolution is
discussed in detail. A short review is given of the standard Big Bang
Nucleosynthesis and the influence of resonant and nonresonant neutrino
oscillations on active neutrinos and on primordial nucleosynthesis of He-4. BBN
cosmological constraints on neutrino oscillation parameters are discussed.Comment: 21 p., 6 figures, a review based on raview talk at NCYA Conference
and a presentation at CAPP200
Breaking Of Conformal Invariance And Electromagnetic Field Generation In The Universe
It is shown that the breaking of the conformal invariance in quantum
electrodynamics due to the trace anomaly results in the generation of long wave
electromagnetic fields during inflationary stage of the universe evolution. If
the coefficient of the logarithmic charge renormalization is large (due to a
large number of charged particles species), these primordial electromagnetic
fields can be strong enough to create the observed galactic magnetic fields.Comment: 6 pages, UM - TH - 93 - 0
Production of a sterile species via active-sterile mixing: an exactly solvable model
The production of a sterile species via active-sterile mixing in a thermal
medium is studied in an exactly solvable model. The \emph{exact} time evolution
of the sterile distribution function is determined by the dispersion relations
and damping rates for the quasiparticle modes. These depend on
\wtg = \Gamma_{aa}/2\Delta E, with the interaction rate of the
active species in absence of mixing and the oscillation frequency in
the medium without damping. \wtg \ll1,\wtg \gg 1 describe the weak and strong
damping limits respectively. For \wtg\ll1, \Gamma_1 = \Gamma_{aa}\cos^2\tm ;
\Gamma_{2}=\Gamma_{aa}\sin^2\tm where \tm is the mixing angle in the medium
and the sterile distribution function \emph{does not} obey a simple rate
equation. For \wtg \gg 1, and \Gamma_2 = \Gamma_{aa}
\sin^22\tm/4\wtg^2, is the sterile production rate. In this regime sterile
production is suppressed and the oscillation frequency \emph{vanishes} at an
MSW resonance, with a breakdown of adiabaticity. These are consequences of
quantum Zeno suppression. For active neutrinos with standard model interactions
the strong damping limit is \emph{only} available near an MSW resonance
\emph{if} with the vacuum mixing angle.
The full set of quantum kinetic equations for sterile production for arbitrary
\wtg are obtained from the quantum master equation. Cosmological resonant
sterile neutrino production is quantum Zeno suppressed relieving potential
uncertainties associated with the QCD phase transition.Comment: To appear in Phys. Rev.
Production of a sterile species: quantum kinetics
Production of a sterile species is studied within an effective model of
active-sterile neutrino mixing in a medium in thermal equilibrium. The quantum
kinetic equations for the distribution functions and coherences are obtained
from two independent methods: the effective action and the quantum master
equation. The decoherence time scale for active-sterile oscillations is
, but the evolution of the distribution functions
is determined by the two different time scales associated with the damping
rates of the quasiparticle modes in the medium: \Gamma_1=\Gamma_{aa}\cos^2\tm
; \Gamma_2=\Gamma_{aa}\sin^2\tm where is the interaction rate of
the active species in absence of mixing and \tm the mixing angle in the
medium. These two time scales are widely different away from MSW resonances and
preclude the kinetic description of active-sterile production in terms of a
simple rate equation. We give the complete set of quantum kinetic equations for
the active and sterile populations and coherences and discuss in detail the
various approximations. A generalization of the active-sterile transition
probability \emph{in a medium} is provided via the quantum master equation. We
derive explicitly the usual quantum kinetic equations in terms of the
``polarization vector'' and show their equivalence to those obtained from the
quantum master equation and effective action.Comment: To appear in Phys. Rev.
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