15,705 research outputs found
Proceedings of the distribution automation and control working group. Volume 1: Executive summary
The meeting was sponsored by the Department of Energy, Division of Electric Energy Systems. Its purpose was to bring together some members of the electric utility community so that they might reach a common understanding on: (1) key issues and uncertainties to be resolved, (2) the existing state of the art, and (3) specific requirements for further RD&D in the area of DAC. The statements and recommendations formulated by the group on various topics are presented
Lower Limit to the Scale of an Effective Quantum Theory of Gravitation
An effective quantum theory of gravitation in which gravity weakens at energies higher than ~10^-3 eV is one way to accommodate the apparent smallness of the cosmological constant. Such a theory predicts departures from the Newtonian inverse-square force law on distances below ~0.05 mm. However, it is shown that this modification also leads to changes in the long-range behavior of gravity and is inconsistent with observed gravitational lenses
The Limits of Quintessence
We present evidence that the simplest particle-physics scalar-field models of
dynamical dark energy can be separated into distinct behaviors based on the
acceleration or deceleration of the field as it evolves down its potential
towards a zero minimum. We show that these models occupy narrow regions in the
phase-plane of w and w', the dark energy equation-of-state and its
time-derivative in units of the Hubble time. Restricting an energy scale of the
dark energy microphysics limits how closely a scalar field can resemble a
cosmological constant. These results, indicating a desired measurement
resolution of order \sigma(w')\approx (1+w), define firm targets for
observational tests of the physics of dark energy.Comment: 4 pages, 2 figure
Spectral Distortion in a Radially Inhomogeneous Cosmology
The spectral distortion of the cosmic microwave background blackbody spectrum
in a radially inhomogeneous spacetime, designed to exactly reproduce a
LambdaCDM expansion history along the past light cone, is shown to exceed the
upper bound established by COBE-FIRAS by a factor of approximately 3700. This
simple observational test helps uncover a slew of pathological features that
lie hidden inside the past light cone, including a radially contracting phase
at decoupling and, if followed to its logical extreme, a naked singularity at
the radially inhomogeneous Big Bang.Comment: 16 pages, 8 figures (added references and clarified discussion; some
numbers revised
LISA For Cosmologists: Calculating The Signal-To-Noise Ratio For Stochastic And Deterministic Sources
We present the steps to forecast the sensitivity of the Laser Interferometer Space Antenna (LISA) to both a stochastic gravitational wave background and deterministic wave sources. We show how to use these expressions to estimate the precision with which LISA can determine parameters associated with these sources. Tools are included to enable easy calculation of the signal-to-noise ratio and draw sensitivity curves. Benchmark values are given for easy comparison and checking of methods in the case of three worked examples. The first benchmark is the threshold stochastic gravitational wave background ΩGWh2 that LISA can observe. The second is the signal-to-noise ratio that LISA would observe for a binary black hole system identical to GW150914, radiating four years before merger. The third is the case of a monotone source, such as a binary that is far from merger
LISA For Cosmologists: Calculating The Signal-To-Noise Ratio For Stochastic And Deterministic Sources
We present the steps to forecast the sensitivity of the Laser Interferometer Space Antenna (LISA) to both a stochastic gravitational wave background and deterministic wave sources. We show how to use these expressions to estimate the precision with which LISA can determine parameters associated with these sources. Tools are included to enable easy calculation of the signal-to-noise ratio and draw sensitivity curves. Benchmark values are given for easy comparison and checking of methods in the case of three worked examples. The first benchmark is the threshold stochastic gravitational wave background ΩGWh2 that LISA can observe. The second is the signal-to-noise ratio that LISA would observe for a binary black hole system identical to GW150914, radiating four years before merger. The third is the case of a monotone source, such as a binary that is far from merger
Towards a future singularity?
We discuss whether the future extrapolation of the present cosmological state
may lead to a singularity even in case of "conventional" (negative) pressure of
the dark energy field, namely . The discussion is based on an
often neglected aspect of scalar-tensor models of gravity: the fact that
different test particles may follow the geodesics of different metric frames,
and the need for a frame-independent regularization of curvature singularities.Comment: 8 pages. Essay written for the "2004 Awards for Essays on
Gravitation" (Gravity Research Foundation, Wellesley Hills, MA, USA), and
selected for "Honorable Mention
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