2,298 research outputs found
Cosmological Perturbations in Ho\v{r}ava-Lifshitz Gravity
We study cosmological perturbations in Ho\v{r}ava-Lifshitz Gravity. We
consider scalar metric fluctuations about a homogeneous and isotropic
space-time. Starting from the most general metric, we work out the complete
second order action for the perturbations. We then make use of the residual
gauge invariance and of the constraint equations to reduce the number of
dynamical degrees of freedom. After introducing the Sasaki-Mukhanov variable,
the combination of spatial metric fluctuation and matter inhomogeneity for
which the action in General Relativity has canonical form, we find that this
variable has the standard time derivative term in the second order action, and
that the extra degree of freedom is non-dynamical. The limit is
well-behaved, unlike what is obtained when performing incomplete analyses of
cosmological fluctuations. Thus, there is no strong coupling problem for
Ho\v{r}ava-Lifshitz gravity when considering cosmological solutions. We also
compute the spectrum of cosmological perturbations. If the potential in the
action is taken to be of "detailed balance" form, we find a cancelation of the
highest derivative terms in the action for the curvature fluctuations. As a
consequence, the initial spectrum of perturbations will not be scale-invariant
in a general spacetime background. As an application, we consider fluctuations
in an inflationary background and draw connections with the "trans-Planckian
problem" for cosmological perturbations. In the special case in which the
potential term in the action is of detailed balance form and in which , the equation of motion for cosmological perturbations in the far UV takes
the same form as in GR. However, in general the equation of motion is
characterized by a modified dispersion relation.Comment: 17 pages, 2 eps figure
A universal average spectral energy distribution for quasars from optical to extreme ultraviolet
The well-known anti-correlation between the optical/ultraviolet (UV) emission
line equivalent widths of active galactic nuclei and the continuum luminosity
(the so-called Baldwin effect) is a long-standing puzzle. One common hypothesis
is that more luminous sources have softer spectral energy distribution (SED) in
the extreme UV (EUV), as revealed by some observational studies. In this work
we revisit this issue through cross-matching SDSS quasars with GALEX
far-UV/near-UV catalogs and correcting the effect of a severe observational
bias of significant UV detection incompleteness, i.e., the more luminous in
observed-frame optical, the more likely detected in observed-frame UV. We find
that, for GALEX detected quasars at 1.8 < z < 2.2, the rest-frame mean UV SED
(~ 500 -- 3000 Angstrom) bewilderingly shows no luminosity dependence at
log(\nu L_\nu(2200 Angstrom)) > 45 (up to 47.3), contrary to the standard thin
disc model predictions and the observed Baldwin effect in this luminosity
range. Probably, the universal mean UV SED is the result of a local
atomic-originated process, and in fainter quasars stronger disk turbulence
launching more clouds is the main origin of the Baldwin effect. After
correcting for the absorption of the intergalactic medium, a rest-frame
intrinsic mean EUV SED is derived from a sub-sample of bright quasars and is
found to be much redder in the EUV than all previous quasar composite spectra,
highlighting the significance of properly accounting for the sample
incompleteness. Interestingly, the global consistence between our extremely red
mean EUV SED and the line-driven wind model again supports an origin of a local
physical process.Comment: 27 pages, 15 figures, author's initial version submitted to Nature
Astronom
Spittlebug genus Kanozata Matsumura (Hemiptera, Cercopoidea: Cercopidae) with a new distribution record of K. contermina (Distant) to China
Male and female genitalia of species in the spittlebug genus Kanozata Matsumura, 1940 are described and illustrated. Kanozata contermina (Distant, 1916) is reported from China for the first time. Keys to species and a distribution map are provided
Black holes in 4D Einstein-Maxwell-Gauss-Bonnet gravity coupled with scalar fields
Einstein-Maxwell-Gauss-Bonnet-axion theory in -dimensional spacetime is
investigated in this paper through a "Kaluza-Klein-like" process. Dual to
systems at finite temperature with background magnetic field on three
dimensions, the four-dimensional dyonic black hole solution coupled with higher
derivative terms is obtained. After the tensor-type perturbation is added, the
shear viscosity to entropy density ratio is calculated at high temperature and
low temperature separately. The behaviour of shear viscosity to entropy density
ratio of uncharged black holes is found to be similar with that in
-dimensional spacetime, violating the Kovtun-Starinets-Son bound as well
when temperature becomes lower. In addition, the main feature of this ratio
remains almost unchanged in dimensions, which is characterised by
at low temperature , with proportional to the
coefficient from scalar fields. The difficulty in causal analysis is
also discussed, which is mainly caused by the vanishing momentum term in
equations of motion.Comment: 19 pages, 2 figures; more detail, minor corrections, references
added. Accepted in EPJ
The Metabolism of Baicalin in Rat and the Biological Activities of the Metabolites
Baicalin is one of the major bioactive constituents of Scutellariae Radix, but the biotransformation of it is poorly understood. In this paper, the metabolism of baicalin in rat was studied. Nine metabolites including one new compound were isolated and identified structurally. The plausible scheme for the biotransformation pathways of baicalin in the rats was deduced. And the main metabolites were evaluated for their antioxidation and anti-inflammation biological activities for the first time
Experiment and Simulation Investigation on the Tensile Behavior of Composite Laminate with Stitching Reinforcement
The experiments and finite element simulations of composite laminate with stitching are carried out. Firstly, the monotonous tensile experiments with and without stitching are conducted to investigate the influence of stitch reinforcement on the composite laminate. Secondly, the finite element method (FEM) is employed to simulate the tensile process of specimens, and the link element is introduced to simulate the stitching. The experiment results shows that the stitching has little influence on the damage load under monotonous tensile load, while there is a significant influence on the changing of strain. The FEM results are consistent with the experiment results, which means that the link element can be used to study the stitching of the composite laminate. The simulation results also show that the distributions of strain are changed obviously due to the existence of the stitching. Research results have a significant role on the design of the composite structures with and without stitching
Ballistic Thermal Rectification in Asymmetric Three-Terminal Mesoscopic Dielectric Systems
By coupling the asymmetric three-terminal mesoscopic dielectric system with a
temperature probe, at low temperature, the ballistic heat flux flow through the
other two asymmetric terminals in the nonlinear response regime is studied
based on the Landauer formulation of transport theory. The thermal
rectification is attained at the quantum regime. It is a purely quantum effect
and is determined by the dependence of the ratio
on , the phonon's frequency.
Where and are respectively the
transmission coefficients from two asymmetric terminals to the temperature
probe, which are determined by the inelastic scattering of ballistic phonons in
the temperature probe. Our results are confirmed by extensive numerical
simulations.Comment: 10 pages, 4 figure
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