768 research outputs found
Cosmological solutions in generalized hybrid metric-Palatini gravity
We construct exact solutions representing a
Friedmann-Lema\^itre-Robsertson-Walker (FLRW) universe in a generalized hybrid
metric-Palatini theory. By writing the gravitational action in a scalar-tensor
representation, the new solutions are obtained by either making an ansatz on
the scale factor or on the effective potential. Among other relevant results,
we show that it is possible to obtain exponentially expanding solutions for
flat universes even when the cosmology is not purely vacuum. We then derive the
classes of actions for the original theory which generate these solutions.Comment: 14 pages, 17 figure
The evolution of cosmological gravitational waves in f(R) gravity
We give a rigorous and mathematically clear presentation of the Covariant and
Gauge Invariant theory of gravitational waves in a perturbed
Friedmann-Lemaitre-Robertson-Walker universe for Fourth Order Gravity, where
the matter is described by a perfect fluid with a barotropic equation of state.
As an example of a consistent analysis of tensor perturbations in Fourth Order
Gravity, we apply the formalism to a simple background solution of R^n gravity.
We obtain the exact solutions of the perturbation equations for scales much
bigger than and smaller than the Hubble radius. It is shown that the evolution
of tensor modes is highly sensitive to the choice of n and an interesting new
feature arises. During the radiation dominated era, their exist a growing
tensor perturbation for nearly all choices of n. This occurs even when the
background model is undergoing accelerated expansion as opposed to the case of
General Relativity. Consequently, cosmological gravitational wave modes can in
principle provide a strong constraint on the theory of gravity independent of
other cosmological data sets.Comment: 19 pages, 4 figures; v2: corrected to match version accepted for
publication in PR
DarkMem: Fine-grained power management of local memories for accelerators in embedded systems
SRAM consumes a growing fraction of the static power in heterogeneous SoCs, as embedded memories take 70% to 90% of the area of specialized accelerators. We present Dark-Mem as a comprehensive solution for fine-grained power management of accelerator local memories. The DarkMem methodology optimizes at design time the bank configuration for each given accelerator to maximize power-gating opportunities. The DarkMem microarchitecture dynamically varies the operating mode of each memory bank according to the accelerator workload. In our experiments, DarkMem reduces the SRAM static power by more than 40% on average, which translates into a reduction of the total power by almost 18% on average with less than 1% overhead
Efficient Method for the Computation of Lightning Current Distributions in Wind Turbine Blades using the Fourier Transform and the Finite Element Method
Rotor blades of large, modern wind turbines are susceptible to lightning strikes. In order to produce a design that resists lightning strikes, it is crucial to simulate lightning current propagation in the blade components. Since the current in the blade is generated by the superposition of potential and induced electric fields, a coupling exists between electric and magnetic fields which need to be calculated by an imposed integral constraint at each time step. Commercial software packages can deal with such constraints, but it results in time-consuming computations. Therefore, this work aims to develop a numerical methodology able to compute the voltage which drives the lightning current through the structure. In this way, the problem is reformulated as a voltage-driven one which in turn allows a simple subsequent coupling of electrical and magnetic problems. The computation of the voltage waveform was accomplished using the fast Fourier transform and the finite element method (FEM) to compute the structure impedance in the frequency domain. The developed procedure showed high efficiency for a blade subjected to different lightning impulses. It allows a description of the time-dependent lightning current to be given, as well as the distribution of current within the blade conductors
Cosmological dynamics of exponential gravity
We present a detailed investigation of the cosmological dynamics based on
gravity. We apply the dynamical system approach to both
the vacuum and matter cases and obtain exact solutions and their stability in
the finite and asymptotic regimes. The results show that cosmic histories exist
which admit a double de-Sitter phase which could be useful for describing the
early and the late-time accelerating universe.Comment: 17 pages LaTeX, 3 figure
The evolution of tensor perturbations in scalar-tensor theories of gravity
The evolution equations for tensor perturbations in a generic scalar tensor
theory of gravity are presented. Exact solution are given for a specific class
of theories and Friedmann-Lema\^{i}tre-Robertson-Walker backgrounds. In these
cases it is shown that, although the evolution of tensor models depends on the
choice of parameters of the theory, no amplification is possible if the
gravitational interaction is attractive.Comment: 11 pages, 2 figures, submitted to Physical Review
Agile SoC Development with Open ESP
ESP is an open-source research platform for heterogeneous SoC design. The
platform combines a modular tile-based architecture with a variety of
application-oriented flows for the design and optimization of accelerators. The
ESP architecture is highly scalable and strikes a balance between regularity
and specialization. The companion methodology raises the level of abstraction
to system-level design and enables an automated flow from software and hardware
development to full-system prototyping on FPGA. For application developers, ESP
offers domain-specific automated solutions to synthesize new accelerators for
their software and to map complex workloads onto the SoC architecture. For
hardware engineers, ESP offers automated solutions to integrate their
accelerator designs into the complete SoC. Conceived as a heterogeneous
integration platform and tested through years of teaching at Columbia
University, ESP supports the open-source hardware community by providing a
flexible platform for agile SoC development.Comment: Invited Paper at the 2020 International Conference On Computer Aided
Design (ICCAD) - Special Session on Opensource Tools and Platforms for Agile
Development of Specialized Architecture
Bounce Conditions in f(R) Cosmologies
We investigate the conditions for a bounce to occur in
Friedmann-Robertson-Walker cosmologies for the class of fourth order gravity
theories. The general bounce criterion is determined and constraints on the
parameters of three specific models are given in order to obtain bounces
solutions. It is found that unlike the case of General Relativity, a bounce
appears to be possible in open and flat cosmologies.Comment: 11 pages LaTe
One-loop corrections to the Drell--Yan process in SANC (II). The neutral current case
Radiative corrections to the neutral current Drell--Yan-like processes are
considered. Complete one-loop electroweak corrections are calculated within the
SANC system. Theoretical uncertainties are discussed. Numerical results are
presented for typical conditions of LHC experiments.Comment: 17 pages, 9 figures, 3 table
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