3,615 research outputs found
Power spectra from an inflaton coupled to the Gauss-Bonnet term
We consider power-law inflation with a Gauss-Bonnet correction inspired by
string theory. We analyze the stability of cosmological perturbations and
obtain the allowed parameter space. We find that for GB-dominated inflation
ultra-violet instabilities of either scalar or tensor perturbations show up on
small scales. The Gauss-Bonnet correction with a positive (or negative)
coupling may lead to a reduction (or enhancement) of the tensor-to-scalar ratio
in the potential-dominated case. We place tight constraints on the model
parameters by making use of the WMAP 5-year data.Comment: 5 pages, 4 figures, RevTeX, references added, published versio
Realizing Scale-invariant Density Perturbations in Low-energy Effective String Theory
We discuss the realization of inflation and resulting cosmological
perturbations in the low-energy effective string theory. In order to obtain
nearly scale-invariant spectra of density perturbations and a suppressed
tensor-to-scalar ratio, it is generally necessary that the dilaton field
is effectively decoupled from gravity together with the existence of a slowly
varying dilaton potential. We also study the effect of second-order corrections
to the tree-level action which are the sum of a Gauss-Bonnet term coupled to
and a kinetic term . We find that it is possible to
realize observationally supported spectra of scalar and tensor perturbations
provided that the correction is dominated by the term even in
the absence of the dilaton potential. When the Gauss-Bonnet term is dominant,
tensor perturbations exhibit violent negative instabilities on small-scales
about a de Sitter background in spite of the fact that scale-invariant scalar
perturbations can be achieved.Comment: 13 pages; v2: minor corrections, refs. added, version to appear in
PR
Hybrid Chaplygin gas and phantom divide crossing
Hybrid Chaplygin gas model is put forward, in which the gases play the role
of dark energy. For this model the coincidence problem is greatly alleviated.
The effective equation of state of the dark energy may cross the phantom divide
. Furthermore, the crossing behaviour is decoupled from any gravity
theories. In the present model, is only a transient behaviour. There is
a de Sitter attractor in the future infinity. Hence, the big rip singularity,
which often afflicts the models with matter whose effective equation of state
less than -1, is naturally disappear. There exist stable scaling solutions,
both at the early universe and the late universe. We discuss the perturbation
growth of this model. We find that the index is consistent with observations.Comment: 11 pages, 4 figures, V3: discussions on the perturbation growth
added, V4: minor corrections, to match the published versio
[Report of] Specialist Committee V.4: ocean, wind and wave energy utilization
The committee's mandate was :Concern for structural design of ocean energy utilization devices, such as offshore wind turbines, support structures and fixed or floating wave and tidal energy converters. Attention shall be given to the interaction between the load and the structural response and shall include due consideration of the stochastic nature of the waves, current and wind
Quantifying the relative contributions of substorm injections and chorus waves to the rapid outward extension of electron radiation belt
Abstract We study the rapid outward extension of the electron radiation belt on a timescale of several hours during three events observed by Radiation Belt Storm Probes and Time History of Events and Macroscale Interactions during Substorms satellites and particularly quantify the contributions of substorm injections and chorus waves to the electron flux enhancement near the outer boundary of radiation belt. A comprehensive analysis including both observations and simulations is performed for the first event on 26 May 2013. The outer boundary of electron radiation belt moved from L = 5.5 to L \u3e 6.07 over about 6 h, with up to 4 orders of magnitude enhancement in the 30 keV to 5 MeV electron fluxes at L = 6. The observations show that the substorm injection can cause 100% and 20% of the total subrelativistic (∼0.1 MeV) and relativistic (2-5 MeV) electron flux enhancements within a few minutes. The data-driven simulation supports that the strong chorus waves can yield 60%-80% of the total energetic (0.2-5.0 MeV) electron flux enhancement within about 6 h. Some simple analyses are further given for the other two events on 2 and 29 June 2013, in which the contributions of substorm injections and chorus waves are shown to be qualitatively comparable to those for the first event. These results clearly illustrate the respective importance of substorm injections and chorus waves for the evolution of radiation belt electrons at different energies on a relatively short timescale. Key Points Rapid outward extension of electron radiation belt observed by RBSP and THEMIS A two-step scenario to explain the rapid flux enchantment Differentiating between contributions of substorm injections and chorus waves
An Implantable Stimulator with Safety Sensors in Standard CMOS Process for Active Books
This paper presents a second-generation integrated circuit for the Active Books neural stimulation microsystem. It provides multi-channel stimulation with versatile control of stimulation profiles and reduced crosstalk from other stimulation channels. The new design features enhanced safety by monitoring the temperature and humidity inside the micropackage, and the peak electrode voltage at any stimulating electrode. The humidity sensor uses an interdigitated capacitor covered by a passivation layer and a polyimide covering. To boost sensitivity in the operating range of interest, the temperature sensor uses a temperature-insensitive current that is subtracted from a proportional-to-absolute-temperature current. A 3-b analog-todigital converter is used to record the peak electrode voltage. All sensor data is sent to an implanted central hub using bidirectional connection with error checking. Both the stimulation electronics and sensors are integrated on a 6.2 mm × 4 mm silicon die using XFAB's 0.6-μm CMOS high-voltage process. No post-processing steps are involved. The stimulator uses a fivewire cable to provide the power supply and bidirectional data signals. The chip operates from a 7.5-18 V power supply and can generate stimulation currents of 1 mA, 4 mA or 8 mA with a pulse duration of 2 μs-1.07 ms. The humidity sensor output varies linearly with relative humidity (RH) with a normalized sensitivity of 0.04%/%RH over the range of 20-90%RH. The temperature sensor has a nonlinearity of 0.4% over the range of 20-90 °C and a resolution of 0.12 °C. The stimulator is the first of its kind to include integrated temperature and humidity sensors. Index Terms-Active Books, humidity sensor, implant safety, integrated stimulator, temperature sensor, voltage sensor
Calibration of GRB Luminosity Relations with Cosmography
For the use of Gamma-Ray Bursts (GRBs) to probe cosmology in a
cosmology-independent way, a new method has been proposed to obtain luminosity
distances of GRBs by interpolating directly from the Hubble diagram of SNe Ia,
and then calibrating GRB relations at high redshift. In this paper, following
the basic assumption in the interpolation method that objects at the same
redshift should have the same luminosity distance, we propose another approach
to calibrate GRB luminosity relations with cosmographic fitting directly from
SN Ia data. In cosmography, there is a well-known fitting formula which can
reflect the Hubble relation between luminosity distance and redshift with
cosmographic parameters which can be fitted from observation data. Using the
Cosmographic fitting results from the Union set of SNe Ia, we calibrate five
GRB relations using GRB sample at and deduce distance moduli of GRBs
at by generalizing above calibrated relations at high
redshift. Finally, we constrain the dark energy parameterization models of the
Chevallier-Polarski-Linder (CPL) model, the Jassal-Bagla-Padmanabhan (JBP)
model and the Alam model with GRB data at high redshift, as well as with the
Cosmic Microwave Background radiation (CMB) and the baryonic acoustic
oscillation (BAO) observations, and we find the CDM model is
consistent with the current data in 1- confidence region.Comment: 15 pages, 4 figures, 2 tables; accepted for publication in IJMP
Numerical framework for transcritical real-fluid reacting flow simulations using the flamelet progress variable approach
An extension to the classical FPV model is developed for transcritical
real-fluid combustion simulations in the context of finite volume, fully
compressible, explicit solvers. A double-flux model is developed for
transcritical flows to eliminate the spurious pressure oscillations. A hybrid
scheme with entropy-stable flux correction is formulated to robustly represent
large density ratios. The thermodynamics for ideal-gas values is modeled by a
linearized specific heat ratio model. Parameters needed for the cubic EoS are
pre-tabulated for the evaluation of departure functions and a quadratic
expression is used to recover the attraction parameter. The novelty of the
proposed approach lies in the ability to account for pressure and temperature
variations from the baseline table. Cryogenic LOX/GH2 mixing and reacting cases
are performed to demonstrate the capability of the proposed approach in
multidimensional simulations. The proposed combustion model and numerical
schemes are directly applicable for LES simulations of real applications under
transcritical conditions.Comment: 55th AIAA Aerospace Sciences Meeting, Dallas, T
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