6,524 research outputs found
Revisiting Agegraphic Dark Energy in Brans-Dicke Cosmology
We explore a spatially homogeneous and isotropic Friedmann-Robertson-Walker
(FRW) universe which is filled with agegraphic dark energy (ADE) with mutual
interaction with pressureless dark matter in the background of Brans-Dicke (BD)
theory. We consider both original and new type of agegraphic dark energy (NADE)
and further assume the sign of the interaction term can change during the
history of the Universe. We obtain the equation of state parameter, the
deceleration parameter and the evolutionary equation for the sign-changeable
interacting ADE and NADE in BD theory. We find that, in both models, the
equation of state parameter, , cannot cross the phantom line, although
they can predict the Universe evolution from the early deceleration phase to
the late time acceleration, compatible with observations. We also investigate
the sound stability of these models and find out that both models cannot show a
signal of stability for different model parameters.Comment: 10 pages, 14 figure
Stability of HDE model with sign-changeable interaction in Brans-Dicke theory
We consider the Brans-Dicke (BD) theory of gravity and explore the
cosmological implications of the sign-changeable interacting holographic dark
energy (HDE) model in the background of Friedmann-Robertson-Walker (FRW)
universe. As the system's infrared (IR) cutoff, we choose the future event
horizon, the Granda-Oliveros (GO) and the Ricci cutoffs. For each cutoff, we
obtain the density parameter, the equation of state (EoS) and the deceleration
parameter of the system. In case of future event horizon, we find out that the
EoS parameter, , can cross the phantom line, as a result the transition
from deceleration to acceleration expansion of the universe can be achieved
provided the model parameters are chosen suitably. Then, we investigate the
instability of the sign-changeable interacting HDE model against perturbations
in BD theory. For this purpose, we study the squared sound speed whose
sign determines the stability of the model. When the model is
unstable against perturbation. For future event horizon cutoff, our universe
can be stable () depending on the model parameters. Then, we focus
on GO and Ricci cutoffs and find out that although other features of these two
cutoffs seem to be consistent with observations, they cannot leads to stable
dominated universe, except in special case with GO cutoff. Our studies confirm
that for the sign-changeable HDE model in the setup of BD cosmology, the event
horizon is the most suitable horizon which can passes all conditions and leads
to a stable DE dominated universe.Comment: 19pages, 22figure
The Dirac point electron in zero-gravity Kerr--Newman spacetime
Dirac's wave equation for a point electron in the topologically nontrivial
maximal analytically extended electromagnetic Kerr--Newman spacetime is studied
in a zero-gravity limit; here, "zero-gravity" means , where is
Newton's constant of universal gravitation. The following results are obtained:
the formal Dirac Hamiltonian on the static spacelike slices is essentially
self-adjoint; the spectrum of the self-adjoint extension is symmetric about
zero, featuring a continuum with a gap about zero that, under two smallness
conditions, contains a point spectrum. Some of our results extend to a
generalization of the zero- Kerr--Newman spacetime with different
electric-monopole-to-magnetic-dipole-moment ratio.Comment: 49 pages, 17 figures; referee's comments implemented; the endnotes in
the published version appear as footnotes in this preprin
A Note on Tsallis Holographic Dark Energy
We explore the effects of considering various infrared (IR) cutoffs,
including the particle horizon, Ricci horizon and Granda-Oliveros (GO) cutoffs,
on the properties of Tsallis holographic dark energy (THDE) model, proposed
inspired by Tsallis generalized entropy formalism \cite{THDE}. Interestingly
enough, we find that for the particle horizon as IR cutoff, the obtained THDE
model can describe the accelerated universe. This is in contrast to the usual
HDE model which cannot lead to an accelerated universe, if one consider the
particle horizon as IR cutoff. We also investigate the cosmological
consequences of THDE under the assumption of a mutual interaction between the
dark sectors of the Universe. It is shown that the evolution history of the
Universe can be described by these IR cutoffs and thus the current cosmic
acceleration can also been realized. The sound instability of THDE models for
each cutoff are also explored, separately.Comment: 12 pages, 31 figure
Protracted Screening in the Periodic Anderson Model
The asymmetric infinite-dimensional periodic Anderson model is examined with
a quantum Monte Carlo simulation. For small conduction band filling, we find a
severe reduction in the Kondo scale, compared to the impurity value, as well as
protracted spin screening consistent with some recent controversial
photoemission experiments. The Kondo screening drives a ferromagnetic
transition when the conduction band is quarter-filled and both the RKKY and
superexchange favor antiferromagnetism. We also find RKKY-driven ferromagnetic
and antiferromagnetic transitions.Comment: 5 pages, LaTeX and 4 PS figure
The Variability of Polarized Radiation from Sgr A*
Sgr A* is variable at radio and submillimeter wavelengths on hourly time
scales showing time delays between the peaks of flare emission as well as
linearly polarized emission at millimeter and sub-mm wavelengths. To determine
the polarization characteristics of this variable source at radio frequencies,
we present VLA observations of Sgr A* and report the detection of polarized
emission at a level of 0.77\pm0.01% and 0.2\pm0.01% at 43 and 22 GHz,
respectively. The change in the time averaged polarization angle between 22 and
43 GHz corresponds to a RM of -2.5\pm0.6 x10^3 rad m{-2} with no phase wrapping
(or \sim 5x10^4 rad m^2 with 2\pi phase wrap). We also note a rise and fall
time scale of 1.5 -- 2 hours in the total polarized intensity. The light curves
of the degree of linearly polarized emission suggests a a correlation with the
variability of the total intensity at 43 GHz. The available polarization data
at radio and sub-mm wavelengths suggest that the rotation measure decreases
with decreasing frequency. This frequency dependence, and observed changes in
polarization angle during flare events, may be caused by the reduction in
rotation measure associated with the expansion of synchrotron-emitting blobs.Comment: 11 pages, 3 figures, ApJL (in press
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