339 research outputs found
Long Range Forces from Two Neutrino Exchange Revisited
The exchange of two massless neutrinos gives rise to a long range force which
couples to weakly charged matter. As has been noted previously in the
literature, the potential for this force is \VN \propto G_{F}^2 / r^5 with
monopole-monople, spin-spin and more complicated interactions. Unfortunately,
this is far too small to be observed in present day experiments. We calculate
\VN explicitly in the electroweak theory, and show that under very general
assumptions forces arising from the exchange of two massless fermions can at
best yield potentials.Comment: 5 pages + 1 figure (not included), UFIFT-HEP-92-28/HUTP-92-A04
Symmetry, singularities and integrability in complex dynamics III: approximate symmetries and invariants
The different natures of approximate symmetries and their corresponding first
integrals/invariants are delineated in the contexts of both Lie symmetries of
ordinary differential equations and Noether symmetries of the Action Integral.
Particular note is taken of the effect of taking higher orders of the
perturbation parameter. Approximate symmetries of approximate first
integrals/invariants and the problems of calculating them using the Lie method
are considered
Modulator noise suppression in the LISA Time-Delay Interferometric combinations
We previously showed how the measurements of some eighteen time series of
relative frequency or phase shifts could be combined (1) to cancel the phase
noise of the lasers, (2) to cancel the Doppler fluctuations due to non-inertial
motions of the six optical benches, and (3) to remove the phase noise of the
onboard reference oscillators required to track the photodetector fringes, all
the while preserving signals from passinggravitational waves. Here we analyze
the effect of the additional noise due to the optical modulators used for
removing the phase fluctuations of the onboard reference oscillators. We use a
recently measured noise spectrum of an individual modulator to quantify the
contribution of modulator noise to the first and second-generation Time-Delay
Interferometric (TDI) combinations as a function of the modulation frequency.
We show that modulator noise can be made smaller than the expected proof-mass
acceleration and optical-path noises if the modulation frequencies are larger
than MHz in the case of the unequal-arm Michelson TDI combination
, GHz for the Sagnac TDI combination , and
MHz for the symmetrical Sagnac TDI combination . These
modulation frequencies are substantially smaller than previously estimated and
may lead to less stringent requirements on the LISA's oscillator noise
calibration subsystem.Comment: 17 pages, 5 figures. Submitted to: Phys. Rev. D 1
Progress in Interferometry for LISA at JPL
Recent advances at JPL in experimentation and design for LISA interferometry
include the demonstration of Time Delay Interferometry using electronically
separated end stations, a new arm-locking design with improved gain and
stability, and progress in flight readiness of digital and analog electronics
for phase measurements.Comment: 11 pages, 9 figures, LISA 8 Symposium, Stanford University, 201
Benefits of Artificially Generated Gravity Gradients for Interferometric Gravitational-Wave Detectors
We present an approach to experimentally evaluate gravity gradient noise, a
potentially limiting noise source in advanced interferometric gravitational
wave (GW) detectors. In addition, the method can be used to provide sub-percent
calibration in phase and amplitude of modern interferometric GW detectors.
Knowledge of calibration to such certainties shall enhance the scientific
output of the instruments in case of an eventual detection of GWs. The method
relies on a rotating symmetrical two-body mass, a Dynamic gravity Field
Generator (DFG). The placement of the DFG in the proximity of one of the
interferometer's suspended test masses generates a change in the local
gravitational field detectable with current interferometric GW detectors.Comment: 16 pages, 4 figure
Evolution of the density contrast in inhomogeneous dust models
With the help of families of density contrast indicators, we study the
tendency of gravitational systems to become increasingly lumpy with time.
Depending upon their domain of definition, these indicators could be local or
global. We make a comparative study of these indicators in the context of
inhomogeneous cosmological models of Lemaitre--Tolman and Szekeres. In
particular, we look at the temporal asymptotic behaviour of these indicators
and ask under what conditions, and for which class of models, they evolve
monotonically in time. We find that for the case of ever-expanding models,
there is a larger class of indicators that grow monotonically with time,
whereas the corresponding class for the recollapsing models is more restricted.
Nevertheless, in the absence of decaying modes, indicators exist which grow
monotonically with time for both ever-expanding and recollapsing models
simultaneously. On the other hand, no such indicators may found which grow
monotonically if the decaying modes are allowed to exist. We also find the
conditions for these indicators to be non-divergent at the initial singularity
in both models. Our results can be of potential relevance for understanding
structure formation in inhomogeneous settings and in debates regarding
gravitational entropy and arrow of time. In particular, the spatial dependence
of turning points in inhomogeneous cosmologies may result in multiple density
contrast arrows in recollapsing models over certain epochs. We also find that
different notions of asymptotic homogenisation may be deduced, depending upon
the density contrast indicators used.Comment: 22 pages, 1 figure. To be published in Classical and Quantum Gravit
Constraints on Light Pseudoscalars Implied by Tests of the Gravitational Inverse-Square Law
The exchange of light pseudoscalars between fermions leads to a
spin-independent potential in order g^4, where g is the Yukawa
pseudoscalar-fermion coupling constant. This potential gives rise to detectable
violations of both the weak equivalence principle (WEP) and the gravitational
inverse-square law (ISL), even if g is quite small. We show that when
previously derived WEP constraints are combined with those arisingfrom ISL
tests, a direct experimental limit on the Yukawa coupling of light
pseudoscalars to neutrons can be inferred for the first time (g_n^2/4pi < 1.6
\times 10^-7), along with a new (and significantly improved) limit on the
coupling of light pseudoscalars to protons.Comment: 12 pages, Revtex, with 1 Postscript figure (submitted to Physical
Review Letters
The potential effects of climate change on air quality across the conterminous US at 2030 under three Representative Concentration Pathways
The potential impacts of climate change on regional ozone (O3) and
fine particulate (PM2.5) air quality in the United States (US) are
investigated by linking global climate simulations with regional-scale
meteorological and chemical transport models. Regional climate at 2000 and
at 2030 under three Representative Concentration Pathways (RCPs) is simulated by
using the Weather Research and Forecasting (WRF) model to downscale 11-year
time slices from the Community Earth System Model (CESM). The downscaled
meteorology is then used with the Community Multiscale Air Quality (CMAQ)
model to simulate air quality during each of these 11-year periods. The
analysis isolates the future air quality differences arising from
climate-driven changes in meteorological parameters and specific natural
emissions sources that are strongly influenced by meteorology. Other factors
that will affect future air quality, such as anthropogenic air pollutant
emissions and chemical boundary conditions, are unchanged across the
simulations. The regional climate fields represent historical daily maximum
and daily minimum temperatures well, with mean biases of less than 2 K for most
regions of the US and most seasons of the year and good representation of
variability. Precipitation in the central and eastern US is well simulated
for the historical period, with seasonal and annual biases generally less
than 25 %, with positive biases exceeding 25 % in the western US throughout
the year and in part of the eastern US during summer. Maximum daily 8 h
ozone (MDA8 O3) is projected to increase during summer and autumn in
the central and eastern US. The increase in summer mean MDA8 O3 is
largest under RCP8.5, exceeding 4 ppb in some locations, with smaller
seasonal mean increases of up to 2 ppb simulated during autumn and changes
during spring generally less than 1 ppb. Increases are magnified at the upper
end of the O3 distribution, particularly where projected increases in
temperature are greater. Annual average PM2.5 concentration changes
range from −1.0 to 1.0 µg m−3. Organic PM2.5
concentrations increase during summer and autumn due to increased biogenic
emissions. Aerosol nitrate decreases during winter, accompanied by lesser
decreases in ammonium and sulfate, due to warmer temperatures causing
increased partitioning to the gas phase. Among meteorological factors
examined to account for modeled changes in pollution, temperature and
isoprene emissions are found to have the largest changes and the greatest
impact on O3 concentrations.</p
Treatment preference and quality of life impact: Ravulizumab vs eculizumab for atypical hemolytic uremic syndrome
Limits on Gravitational-Wave Emission from Selected Pulsars Using LIGO Data
We place direct upper limits on the amplitude of gravitational waves from 28 isolated radio pulsars by a coherent multidetector analysis of the data collected during the second science run of the LIGO interferometric detectors. These are the first direct upper limits for 26 of the 28 pulsars. We use coordinated radio observations for the first time to build radio-guided phase templates for the expected gravitational-wave signals. The unprecedented sensitivity of the detectors allows us to set strain upper limits as low as a few times 10^(-24). These strain limits translate into limits on the equatorial ellipticities of the pulsars, which are smaller than 10^(-5) for the four closest pulsars
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