3,385 research outputs found
The fields of uniformly accelerated charges in de Sitter spacetime
The scalar and electromagnetic fields of charges uniformly accelerated in de
Sitter spacetime are constructed. They represent the generalization of the Born
solutions describing fields of two particles with hyperbolic motion in flat
spacetime. In the limit Lambda -> 0, the Born solutions are retrieved. Since in
the de Sitter universe the infinities I^+- are spacelike, the radiative
properties of the fields depend on the way in which a given point of I^+- is
approached. The fields must involve both retarded and advanced effects: Purely
retarded fields do not satisfy the constraints at the past infinity I^-.Comment: 5 pages, 3 figures, RevTeX; Slightly expanded version of the paper
published in Physical Review Letters. (The published version can be generated
from the same TeX source.); problem with the postscript fixe
Increasing evidence for hemispherical power asymmetry in the five-year WMAP data
(Abridged)Motivated by the recent results of Hansen et al. (2008) concerning
a noticeable hemispherical power asymmetry in the WMAP data on small angular
scales, we revisit the dipole modulated signal model introduced by Gordon et
al. (2005). This model assumes that the true CMB signal consists of a Gaussian
isotropic random field modulated by a dipole, and is characterized by an
overall modulation amplitude, A, and a preferred direction, p. Previous
analyses of this model has been restricted to very low resolution due to
computational cost. In this paper, we double the angular resolution, and
compute the full corresponding posterior distribution for the 5-year WMAP data.
The results from our analysis are the following: The best-fit modulation
amplitude for l <= 64 and the ILC data with the WMAP KQ85 sky cut is A=0.072
+/- 0.022, non-zero at 3.3sigma, and the preferred direction points toward
Galactic coordinates (l,b) = (224 degree, -22 degree) +/- 24 degree. The
corresponding results for l <~ 40 from earlier analyses was A = 0.11 +/- 0.04
and (l,b) = (225 degree,-27 degree). The statistical significance of a non-zero
amplitude thus increases from 2.8sigma to 3.3sigma when increasing l_max from
40 to 64, and all results are consistent to within 1sigma. Similarly, the
Bayesian log-evidence difference with respect to the isotropic model increases
from Delta ln E = 1.8 to Delta ln E = 2.6, ranking as "strong evidence" on the
Jeffreys' scale. The raw best-fit log-likelihood difference increases from
Delta ln L = 6.1 to Delta ln L = 7.3. Similar, and often slightly stronger,
results are found for other data combinations. Thus, we find that the evidence
for a dipole power distribution in the WMAP data increases with l in the 5-year
WMAP data set, in agreement with the reports of Hansen et al. (2008).Comment: 6 pages, 2 figures; added references and minor comments. Accepted for
publication in Ap
Asymmetries in the CMB anisotropy field
We report on the results from two independent but complementary statistical
analyses of the WMAP first-year data, based on the power spectrum and N-point
correlation functions. We focus on large and intermediate scales (larger than
about 3 degrees) and compare the observed data against Monte Carlo ensembles
with WMAP-like properties. In both analyses, we measure the amplitudes of the
large-scale fluctuations on opposing hemispheres and study the ratio of the two
amplitudes. The power-spectrum analysis shows that this ratio for WMAP, as
measured along the axis of maximum asymmetry, is high at the 95%-99% level
(depending on the particular multipole range included). The axis of maximum
asymmetry of the WMAP data is weakly dependent on the multipole range under
consideration but tends to lie close to the ecliptic axis. In the N-point
correlation function analysis we focus on the northern and southern hemispheres
defined in ecliptic coordinates, and we find that the ratio of the large-scale
fluctuation amplitudes is high at the 98%-99% level. Furthermore, the results
are stable with respect to choice of Galactic cut and also with respect to
frequency band. A similar asymmetry is found in the COBE-DMR map, and the axis
of maximum asymmetry is close to the one found in the WMAP data.Comment: 6 pages, 5 figures; version to appear in ApJ, textual improvements,
added reference
Soil nutrient levels define herbage yield but not root biomass in a multispecies grass-legume ley
The response of above- and below-ground biomass to soil nutrient availability is crucial for estimating belowground carbon input and predicting changes in soil carbon storage. However, the response is far from clear at plant community level, especially for grassland systems. Using a long-term field experiment initiated 123 years ago with varying soil nutrient levels (deficient, sub-optimal, optimal and over-optimal) established by use of two nutrient sources (animal manure or mineral fertiliser), we examined the effects of soil nutrient level and source on herbage yield and composition, root biomass and root-to-shoot (R/S) ratio of an unfertilised multispecies grass-legume ley. Increased nutrient levels enhanced herbage yield, but did not affect root biomass. The R/S ratio decreased from deficient to sub-optimal level, but remained constant from optimal to over-optimal level. Nutrient source did not influence herbage yield, root biomass or R/S ratio, but the legume proportion increased in soils previously receiving mineral fertiliser. The R/S ratio decreased with herbage yield, but did not vary with herbage composition. We conclude that soil nutrient level and herbage yield rather than nutrient source and herbage composition determine biomass allocation between aboveground and belowground in temperate grassland leys
Evidence of vorticity and shear at large angular scales in the WMAP data: a violation of cosmological isotropy?
Motivated by the large-scale asymmetry observed in the cosmic microwave
background sky, we consider a specific class of anisotropic cosmological models
-- Bianchi type VII_h -- and compare them to the WMAP first-year data on large
angular scales. Remarkably, we find evidence of a correlation which is ruled
out as a chance alignment at the 3sigma level. The best fit Bianchi model
corresponds to x=0.55, Omega_0=0.5, a rotation axis in the direction
(l,b)=(222degr,-62degr), shear (sigma/H)_0=2.4e-10 and a right--handed
vorticity (omega/H)_0=6.1e-10. Correcting for this component greatly reduces
the significance of the large-scale power asymmetry, resolves several anomalies
detected on large angular scales (ie. the low quadrupole amplitude and
quadrupole/octopole planarity and alignment), and can account for a
non--Gaussian "cold spot" on the sky. Despite the apparent inconsistency with
the best-fit parameters required in inflationary models to account for the
acoustic peaks, we consider the results sufficiently provocative to merit
further consideration.Comment: 4 pages, 3 figures; emulateapj.cls; ApJL accepted version plus fixed
error in vorticity calculation (sqrt(2) off in Table 1, abstract, and
conclusions); basic conclusions unchange
Diversity promotes production of ryegrass-clover leys through inclusion of competitive forb species
Highly productive temporary grasslands in Europe are usually composed of only a few plant species, and are typically dominated by perennial ryegrass-clover mixtures. Including additional competitive forb species holds potential for enhancing productivity in temporary grasslands, but requires further demonstration. In a grassland biodiversity experiment, one or all of the three forb species: chicory (Cichorium intybus L.), caraway (Carum carvi L.) and plantain (Plantago lanceolata L.), were grown in different proportions with the perennial ryegrass (Lolium perenne L.)-red clover (Trifolium pratense L.)mixture under two slurry application levels (0 and 250 kg total N ha-1 year-1). Dry matter (DM) yield and botanical composition were determined in 2014 and 2015. Results showed that plantain-containing mixtures significantly increased DM yield by on average 9.5% (20% plantain in seed mixture) to 13.6% (60% plantain) compared to the ryegrass-clover mixture, while other mixtures with forb species produced yields similar to that of the ryegrass-clover mixture. These effects were independent of slurry application and consistent over two years. Moreover, plantain-containing mixtures produced higher yield than
chicory- and caraway-containing mixtures, through greater biomass of plantain and/or complementary effects on red clover. These findings firstly demonstrate that increasing species diversity through including certain competitive forbs promotes production of ryegrass-clover mixtures
Constraints on mode couplings and modulation of the CMB with WMAP data
We investigate a possible asymmetry in the statistical properties of the
cosmic microwave background temperature field and to do so we construct an
estimator aiming at detecting a dipolar modulation. Such a modulation is found
to induce correlations between multipoles with . Applying this
estimator, to the V and W bands of the WMAP data, we found a significant
detection in the V band. We argue however that foregrounds and in particular
point sources are the origin of this signal.Comment: 14 pages, 14 figure
Forbs enhance productivity of unfertilised grass-clover leys and support low-carbon bioenergy
Intensively managed grasslands are dominated by highly productive grass-clover mixtures. Increasing crop diversity by inclusion of competitive forbs may enhance biomass production and sustainable biofuel production. Here we examined if one or all of three forbs (chicory, Cichorium intybus L.; caraway, Carum carvi L.; plantain, Plantago lanceolata L.) included in ryegrass-red clover mixtures enhanced above- and below-ground productivity, and assessed their biofuel potentials, based on a three-year experiment with and without fertilisation as cattle slurry. We determined herbage yield, standing root biomass, and estimated methane energy output and greenhouse gas (GHG) emissions per energy unit using life cycle assessment. Results showed that plantain-containing grass-clover mixtures significantly increased herbage yield, while chicory- or caraway-containing mixtures maintained similar yields to the grass-clover mixture. Standing root biomass of the grass-clover mixture was enhanced by inclusion of caraway and plantain, with that of plantain further enhanced by fertilisation. The highest methane energy output was achieved in plantain-containing grass-clover mixtures. All unfertilised mixtures achieved the 60% reduction in GHG emissions compared to fossil fuel, whereas all fertilised mixtures did not meet the 60% reduction target. These findings suggest that including competitive forbs such as plantain in grass-clover mixtures enhances productivity, supporting low-carbon footprint bioenergy production
Interpretation of F106B and CV580 in-flight lightning data and form factor determination
Two topics of in-flight aircraft/lightning interaction are addressed. The first is the analysis of measured data from the NASA F106B Thunderstorm Research Aircraft and the CV580 research program run by the FAA and Wright-Patterson Air Force Base. The CV580 data was investigated in a mostly qualitative sense, while the F106B data was subjected to both statistical and quantitative analysis using linear triggered lightning finite difference models. The second main topic is the analysis of field mill data and the calibration of the field mill systems. The calibration of the F106B field mill system was investigated using an improved finite difference model of the aircraft having a spatial resolution of one-quarter meter. The calibration was applied to measured field mill data acquired during the 1985 thunderstorm season. The experimental determination of form factors useful for field mill calibration was also investigated both experimentally and analytically. The experimental effort involved the use of conducting scale models and an electrolytic tank. An analytic technique was developed to aid in the understanding of the experimental results
The scalar perturbation spectral index n_s: WMAP sensitivity to unresolved point sources
Precision measurement of the scalar perturbation spectral index, n_s, from
the Wilkinson Microwave Anisotropy Probe temperature angular power spectrum
requires the subtraction of unresolved point source power. Here we reconsider
this issue. First, we note a peculiarity in the WMAP temperature likelihood's
response to the source correction: Cosmological parameters do not respond to
increased source errors. An alternative and more direct method for treating
this error term acts more sensibly, and also shifts n_s by ~0.3 sigma closer to
unity. Second, we re-examine the source fit used to correct the power spectrum.
This fit depends strongly on the galactic cut and the weighting of the map,
indicating that either the source population or masking procedure is not
isotropic. Jackknife tests appear inconsistent, causing us to assign large
uncertainties to account for possible systematics. Third, we note that the WMAP
team's spectrum was computed with two different weighting schemes: uniform
weights transition to inverse noise variance weights at l = 500. The fit
depends on such weighting schemes, so different corrections apply to each
multipole range. For the Kp2 mask used in cosmological analysis, we prefer
source corrections A = 0.012 +/- 0.005 muK^2 for uniform weighting and A =
0.015 +/- 0.005 muK^2 for N_obs weighting. Correcting WMAP's spectrum
correspondingly, we compute cosmological parameters with our alternative
likelihood, finding n_s = 0.970 +/- 0.017 and sigma_8 = 0.778 +/- 0.045 . This
n_s is only 1.8 sigma from unity, compared to the ~2.6 sigma WMAP 3-year
result. Finally, an anomalous feature in the source spectrum at l<200 remains,
most strongly associated with W-band.Comment: 9 pages, 10 figures, 3 tables. Submitted to Ap
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