3,082 research outputs found
Generalized gravity and the late-time cosmic acceleration
High-precision observational data have confirmed with startling evidence that
the Universe is currently undergoing a phase of accelerated expansion. This
phase, one of the most important and challenging current problems in cosmology,
represents a new imbalance in the governing gravitational equations.
Historically, physics has addressed such imbalances by either identifying
sources that were previously unaccounted for, or by altering the gravitational
theory. Several candidates, responsible for this expansion, have been proposed
in the literature, in particular, dark energy models and modified gravity
models, amongst others. Outstanding questions are related to the nature of this
so-called "dark energy" that is driving this acceleration, and whether it is
due to the vacuum energy or a dynamical field. On the other hand, the late-time
cosmic acceleration may be due to modifications of General Relativity. In this
work we explore a generalised modified gravity theory, namely
gravity, where is the Ricci scalar, is a scalar field, and is a
kinetic term. This theory contains a wide range of dark energy and modified
gravity models. We considered specific models and applications to the late-time
cosmic acceleration.Comment: 13 pages, 1 figure; slightly revised version, displayed name
corrected. arXiv admin note: text overlap with arXiv:1412.086
Internships for Higher Education Students to Promote the Local Sustainability of Rural Places
Sustainability and population have a complex relationship. Popular versions of the first are usually based on the premise of reducing the population. However, in some remote rural areas with a low density, this condition needs to be reconsidered, as it would put the viability of the local community at risk and lead to environmental, economic and, above all, social losses. Although repopulation is impossible, since remote rural areas have an ageing demographic structure and an extremely low fertility rate, we can propose the regeneration of their human, social and relational capital by attracting talent and commitment. To achieve this, the Desafio Programme (Rural Erasmus) enables university students to do internships in companies and institutions in sparsely populated areas, living alongside their residents. Despite being an experimental policy with a narrow scope in statistical terms, the Desafio Programme is an inspiring experience supported by those who have participated in it and contributes to the social and economic regeneration of areas in decline. This is a practical case of applying a sustainable strategy to a rural area in crisis, which can be transferred to many other countries facing a similar problem
RF Power Amplifier Linearization in Professional Mobile Radio Communications Using Artificial Neural Networks
This paper is focused on the linearization of the radio frequency power amplifier of a professional digital handheld by means of an artificial neural network. The simplicity of the neural network that is used, together with the fact that a feedback path is unnecessary, makes this solution ideal to reduce both the cost of a handheld and its hardware complexity, while fully maintaining its performance. A compensation system is also needed to keep the linearization characteristics of the neural network stable against frequency, temperature, and voltage variations. The whole solution that comprises both the neural network and the compensation system has been implemented in the digital signal processor of a real handheld and afterward fully tested. It has proved to be satisfactory to meet the telecommunication standard requirements in all frequency, temperature, and voltage ranges under consideration while efficient to lower the computational cost of the handheld and to make its internal hardware simpler in comparison with other traditional linearization techniques. The results obtained demonstrate that a neural network can be used to linearize the power amplifiers that are used in transmitters of telecommunication equipment, leading to a significant reduction of both their hardware cost and complexity
Confirmation of and Variable Energy Injection by a Near-Relativistic Outflow in APM 08279+5255
We present results from multi-epoch spectral analysis of XMM-Newton and
Chandra observations of the broad absorption line (BAL) quasar APM 08279+5255.
Our analysis shows significant X-ray BALs in all epochs with rest-frame
energies lying in the range of ~ 6.7-18 keV. The X-ray BALs and 0.2-10 keV
continuum show significant variability on timescales as short as 3.3 days
(proper time) implying a source size-scale of ~ 10 r_g, where r_g is the
gravitational radius. We find a large gradient in the outflow velocity of the
X-ray absorbers with projected outflow velocities of up to 0.76 c. The maximum
outflow velocity constrains the angle between the wind velocity and our line of
sight to be less than ~ 22 degrees. We identify the following components of the
outflow: (a) Highly ionized X-ray absorbing material (2.9 < logxi < 3.9) and a
column density of log N_H ~ 23 outflowing at velocities of up to 0.76 c. (b)
Low-ionization X-ray absorbing gas with log N_H ~ 22.8. We find that flatter
spectra appear to result in lower outflow velocities. Based on our spectral
analysis of observations of APM 08279+5255 over a period of 1.2 years (proper
time) we estimate the mass-outflow rate and efficiency of the outflow to have
varied between 16(-8,+12) M_solar yr^-1 and 64(-40,+66) M_solar yr^-1 and
0.18(-0.11,+0.15) to 1.7(-1.2+1.9), respectively. Assuming that the outflow
properties of APM 08279+5255 are a common property of most quasars at similar
redshifts, our results then imply that quasar winds are massive and energetic
enough to influence significantly the formation of the host galaxy, provide
significant metal enrichment to the interstellar medium and intergalactic
medium, and are a viable mechanism for feedback at redshifts near the peak in
the number density of galaxy mergers.Comment: 27 pages, includes 12 figures, accepted for publication in Ap
A domain-oriented approach to the reduction of combinatorial complexity in signal transduction networks
BACKGROUND: Receptors and scaffold proteins possess a number of distinct domains and bind multiple partners. A common problem in modeling signaling systems arises from a combinatorial explosion of different states generated by feasible molecular species. The number of possible species grows exponentially with the number of different docking sites and can easily reach several millions. Models accounting for this combinatorial variety become impractical for many applications. RESULTS: Our results show that under realistic assumptions on domain interactions, the dynamics of signaling pathways can be exactly described by reduced, hierarchically structured models. The method presented here provides a rigorous way to model a large class of signaling networks using macro-states (macroscopic quantities such as the levels of occupancy of the binding domains) instead of micro-states (concentrations of individual species). The method is described using generic multidomain proteins and is applied to the molecule LAT. CONCLUSION: The presented method is a systematic and powerful tool to derive reduced model structures describing the dynamics of multiprotein complex formation accurately
Unveiling the Dynamics of the Universe
We explore the dynamics and evolution of the Universe at early and late
times, focusing on both dark energy and extended gravity models and their
astrophysical and cosmological consequences. Modified theories of gravity not
only provide an alternative explanation for the recent expansion history of the
universe, but they also offer a paradigm fundamentally distinct from the
simplest dark energy models of cosmic acceleration. In this review, we perform
a detailed theoretical and phenomenological analysis of different modified
gravity models and investigate their consistency. We also consider the
cosmological implications of well motivated physical models of the early
universe with a particular emphasis on inflation and topological defects.
Astrophysical and cosmological tests over a wide range of scales, from the
solar system to the observable horizon, severely restrict the allowed models of
the Universe. Here, we review several observational probes -- including
gravitational lensing, galaxy clusters, cosmic microwave background temperature
and polarization, supernova and baryon acoustic oscillations measurements --
and their relevance in constraining our cosmological description of the
Universe.Comment: 94 pages, 14 figures. Review paper accepted for publication in a
Special Issue of Symmetry. "Symmetry: Feature Papers 2016". V2: Matches
published version, now 79 pages (new format
Fabrication of Robust Thermal Transition Modules and First Cryogenic Experiment with the Refurbished COLDDIAG
Two sets of thermal transition modules as a key component for the COLDDIAG (cold vacuum chamber for beam heat load diagnostics) refurbishment were manufactured, based on the previous design study. The modules are installed in the existing COLDDIAG cryostat and tested with an operating temperature of approximately 50 K at both a cold bore and a thermal shield. This cool-down experiment is a preliminary investigation aiming at beam heat-load studies at the FCC-hh where the beam screens will be operated at almost the same temperature. In this contribution, we report the fabrication processes of the mechanically robust transition modules and the first thermal measurement results with the refurbished COLDDIAG in a cryogenic environment. The static heat load in the refurbished cryostat remains unchanged, compared to that in the former one (4-K cold bore and 50-K shield with thin transitions), despite the increase in the transition thickness. It originates from the identical temperature at the cold bore and the shield, which can theoretically allow the heat intakes by thermal conduction and radiation between them to vanish
- …