140 research outputs found
Gradient instability for ω < - 1
We show that in single scalar field models of the dark energy with equations of state satisfying w ≡ p/ρ < − 1, the effective Lagrangian for fluctuations about the homogeneous background has a wrong sign spatial kinetic term. In most cases, spatial gradients are ruled out by microwave background observations. The sign of w + 1 is not connected to the sign of the time derivative kinetic term in the effective Lagrangian
Gradient instability for w < -1
We show that in single scalar field models of the dark energy with equations
of state satisfying , the effective Lagrangian for
fluctuations about the homogeneous background has a wrong sign spatial kinetic
term. In most cases, spatial gradients are ruled out by microwave background
observations. The sign of is not connected to the sign of the time
derivative kinetic term in the effective Lagrangian.Comment: revtex4, 8 pages, 1 figure. v2: corrected typo in Eq. 16, added
references and a paragraph on quintessence models; v3: reordering of
references. To appear in Phys. Lett.
Anthropic Distribution for Cosmological Constant and Primordial Density Perturbations
The anthropic principle has been proposed as an explanation for the observed
value of the cosmological constant. Here we revisit this proposal by allowing
for variation between universes in the amplitude of the scale-invariant
primordial cosmological density perturbations. We derive a priori probability
distributions for this amplitude from toy inflationary models in which the
parameter of the inflaton potential is smoothly distributed over possible
universes. We find that for such probability distributions, the likelihood that
we live in a typical, anthropically-allowed universe is generally quite small.Comment: 12 pages, 2 tables. v3: Replaced to match published version (minor
corrections of form
State minimum wage laws and newly diagnosed cases of HIV among heterosexual black residents of US metropolitan areas
This ecologic cohort study explores the relationship between state minimum wage laws and rates of HIV diagnoses among heterosexual black residents of U.S metropolitan areas over an 8-year span. Specifically, we applied hierarchical linear modeling to investigate whether state-level variations in minimum wage laws, adjusted for cost-of-living and inflation, were associated with rates of new HIV diagnoses among heterosexual black residents of metropolitan statistical areas (MSAs; n=73), between 2008 and 2015. Findings suggest that an inverse relationship exists between baseline state minimum wages and initial rates of newly diagnosed HIV cases among heterosexual black individuals, after adjusting for potential confounders. MSAs with a minimum wage that was $1 higher at baseline had a 27.12% lower rate of newly diagnosed HIV cases. Exploratory analyses suggest that income inequality may mediate this relationship. If subsequent research establishes a causal relationship between minimum wage and this outcome, efforts to increase minimum wages should be incorporated into HIV prevention strategies for this vulnerable population
The -dependence of the generalised Gerasimov-Drell-Hearn integral for the deuteron, proton and neutron
The Gerasimov-Drell-Hearn (GDH) sum rule connects the anomalous contribution
to the magnetic moment of the target nucleus with an energy-weighted integral
of the difference of the helicity-dependent photoabsorption cross sections. The
data collected by HERMES with a deuterium target are presented together with a
re-analysis of previous measurements on the proton. This provides a measurement
of the generalised GDH integral covering simultaneously the nucleon-resonance
and the deep inelastic scattering regions. The contribution of the
nucleon-resonance region is seen to decrease rapidly with increasing . The
DIS contribution is sizeable over the full measured range, even down to the
lowest measured . As expected, at higher the data are found to be in
agreement with previous measurements of the first moment of . From data on
the deuteron and proton, the GDH integral for the neutron has been derived and
the proton--neutron difference evaluated. This difference is found to satisfy
the fundamental Bjorken sum rule at GeV.Comment: 12 pages, 10 figure
Measurement of the Proton Spin Structure Function g1p with a Pure Hydrogen Target
A measurement of the proton spin structure function g1p(x,Q^2) in
deep-inelastic scattering is presented. The data were taken with the 27.6 GeV
longitudinally polarised positron beam at HERA incident on a longitudinally
polarised pure hydrogen gas target internal to the storage ring. The kinematic
range is 0.021<x<0.85 and 0.8 GeV^2<Q^2<20 GeV^2. The integral
Int_{0.021}^{0.85} g1p(x)dx evaluated at Q0^2 of 2.5 GeV^2 is
0.122+/-0.003(stat.)+/-0.010(syst.).Comment: 7 pages, 3 figures, 1 table, RevTeX late
Measurement of single-spin azimuthal asymmetries in semi-inclusive electroproduction of pions and kaons on a longitudinally polarised deuterium target
Single-spin asymmetries have been measured for semi-inclusive
electroproduction of , , and mesons in
deep-inelastic scattering off a longitudinally polarised deuterium target. The
asymmetries appear in the distribution of the hadrons in the azimuthal angle
around the virtual photon direction, relative to the lepton scattering
plane. The corresponding analysing powers in the moment of the
cross section are for ,
for ,
for and for . The moments are
compatible with zero for all particles.Comment: Revised version shortened 9 pages, 3 tables, 7 figure
Double-Spin Asymmetry in the Cross Section for Exclusive rho^0 Production in Lepton-Proton Scattering
Evidence for a positive longitudinal double-spin asymmetry = 0.24
+-0.11 (stat) +-0.02 (syst) in the cross section for exclusive diffractive
rho^0(770) vector meson production in polarised lepton-proton scattering was
observed by the HERMES experiment. The longitudinally polarised 27.56 GeV HERA
positron beam was scattered off a longitudinally polarised pure hydrogen gas
target. The average invariant mass of the photon-proton system has a value of
= 4.9 GeV, while the average negative squared four-momentum of the virtual
photon is = 1.7 GeV^2. The ratio of the present result to the
corresponding spin asymmetry in inclusive deep-inelastic scattering is in
agreement with an early theoretical prediction based on the generalised vector
meson dominance model.Comment: 10 pages, 4 embedded figures, LaTe
Measurement of the Neutron Spin Structure Function with a Polarized ^3He Target
Results are reported from the HERMES experiment at HERA on a measurement of
the neutron spin structure function in deep inelastic scattering
using 27.5 GeV longitudinally polarized positrons incident on a polarized
He internal gas target. The data cover the kinematic range
and . The integral evaluated at a fixed of is . Assuming Regge behavior at low , the first
moment is .Comment: 4 pages TEX, text available at
http://www.krl.caltech.edu/preprints/OAP.htm
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