7,341 research outputs found
Probing gravitational non-minimal coupling with dark energy surveys
We investigate observational constraints on a specific one-parameter
extension to the minimal quintessence model, where the quintessence field
acquires a quadratic coupling to the scalar curvature through a coupling
constant . The value of is highly suppressed in typical tracker
models if the late-time cosmic acceleration is driven at some field values near
the Planck scale. We test in a second class of models in which the field
value today becomes a free model parameter. We use the combined data from
type-Ia supernovae, cosmic microwave background, baryon acoustic oscillations
and matter power spectrum, to weak lensing measurements and find a best-fit
value where is excluded outside the 95 per cent
confidence region. The effective gravitational constant subject
to the hint of a non-zero is constrained to at the same confidence level on cosmological scales, and can be narrowed
down to when combining with Solar
System tests.Comment: Context extended, figures and references added, title changed to
match with accepted version for publicatio
"Teleparallel" Dark Energy
Using the "teleparallel" equivalent of General Relativity as the
gravitational sector, which is based on torsion instead of curvature, we add a
canonical scalar field, allowing for a nonminimal coupling with gravity.
Although the minimal case is completely equivalent to standard quintessence,
the nonminimal scenario has a richer structure, exhibiting quintessence-like or
phantom-like behavior, or experiencing the phantom-divide crossing. The richer
structure is manifested in the absence of a conformal transformation to an
equivalent minimally-coupled model.Comment: 5 pages, 1 figure, Version published in PLB704 (2011) 384-38
Selective resolution of phonon modes in STM-IETS on clean and oxygen-adsorbed Cu(100) surfaces
The observation of surface phonon dispersion using local probes can provide
important information related to local structural and thermal properties. In
this study, surface phonon modes on a Cu(100) surface were measured using the
inelastic tunneling spectroscopy of scanning tunneling microscopy (STM-IETS)
with atomically sharp tips. Different phonon modes were selectively measured
depending on the structures of the probing tips or the surfaces. Two different
surface phonon modes, at 19.0 meV on a clean Cu(100) surface and at 13.5 meV on
an oxygen-adsorbed Cu(100) surface, are explained by the selection rules.
Additionally, the spatial variation in STM-IETS showed surface stress
relaxation.Comment: 19 pages, 4 figures, supplementary material with 2 figure
Research on the Motivation and Attitude of College students' Physical Education in Taiwan
College students' physical education plays an important role in physical activity and cultivates the concept of independent health management. At present, what kind of learning attitude do Taiwan college students face in physical education? What motivation does the student influence the attitude of the physical education? What is the relevance? All of the above are the purpose of this study. The research method adopts the questionnaire survey method, and the survey data adopts descriptive statistical analysis, independent sample t test, single factor variance analysis, LSD post hoc comparison method, and typical correlation analysis. Research results: 1. The different background variables of Taiwanese college students are that the main motivation factor of physical education is to obtain good health fitness for "physical health". 2. Taiwanese college students have different background variables. They all think that the "cognitive learning" of physical education is the main factor of attitude, that is, the knowledge about health care and sports skills. 3. There is a positive correlation between learning motivation and learning attitude (ρ=.90). Learning motivation is one of the important factors affecting learning attitude. Research conclusions: 1. The factors of Taiwanese male and female college students' motivation for learning in physical education are mainly based on "physical health". 2. Freshmen have higher motivations and learning attitudes in physical education than second-grade to fourth-grade. 3. Taiwan female college students average 1 or 2 times per week, male college students have the most athletes 2 to 3 times per week, more than 90% of college students like sports. 4. There is a positive correlation between learning motivation and learning attitude, indicating that the stronger the attribute of learning motivation "physical health", the higher the student's learning attitude. 5. Satisfying students' motivation for learning helps students to learn positively. 6. Another important task of the college physical education class is to prepare students for future lifelong sports
ATHENA : a pagerank-based scheme to solve the thundering herd in authentication
Vehicles in intelligent transport systems (ITS) react to an emergency situation by broadcasting critical messages like Decentralized Environmental Notification Messages (DENMs). A digital signature is attached to each message to secure the integrity of communication, and this message is inoperative until the authentication completes. This creates a challenge for vehicles to verify massive messages in some scenarios where it could incur the thundering herd in authentication, if there is a critical situation happening in heavy road traffic. To address this problem, we propose ATHENA, a pagerank-based scheme to solve the thundering herd in ITS authentication that utilises the transmission of messages and pagerank algorithm to rank the broadcasting vehicles. Simulation results show the efficiency of ATHENA and the effectiveness of performance enhancements compared with others
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