62 research outputs found
On Di\'osi-Penrose criterion of gravity-induced quantum collapse
It is shown that the Di\'osi-Penrose criterion of gravity-induced quantum
collapse may be inconsistent with the discreteness of space-time, which is
generally considered as an indispensable element in a complete theory of
quantum gravity. Moreover, the analysis also suggests that the discreteness of
space-time may result in rapider collapse of the superposition of energy
eigenstates than required by the Di\'osi-Penrose criterion.Comment: 5 pages, no figure
Weisskopf-Wigner Decay Theory for the Energy-Driven Stochastic Schr\"odinger Equation
We generalize the Weisskopf-Wigner theory for the line shape and transition
rates of decaying states to the case of the energy-driven stochastic
Schr\"odinger equation that has been used as a phenomenology for state vector
reduction. Within the standard approximations used in the Weisskopf-Wigner
analysis, and assuming that the perturbing potential inducing the decay has
vanishing matrix elements within the degenerate manifold containing the
decaying state, the stochastic Schr\"odinger equation linearizes. Solving the
linearized equations, we find no change from the standard analysis in the line
shape or the transition rate per unit time. The only effect of the stochastic
terms is to alter the early time transient behavior of the decay, in a way that
eliminates the quantum Zeno effect. We apply our results to estimate
experimental bounds on the parameter governing the stochastic effects.Comment: 29 pages in RevTeX, Added Note, references adde
Constraining slow-roll inflation with WMAP and 2dF
We constrain slow-roll inflationary models using the recent WMAP data
combined with data from the VSA, CBI, ACBAR and 2dF experiments. We find the
slow-roll parameters to be and . For inflation models
we find that at the 2 and levels,
indicating that the model is under very strong pressure from
observations. We define a convergence criterion to judge the necessity of
introducing further power spectrum parameters such as the spectral index and
running of the spectral index. This criterion is typically violated by models
with large negative running that fit the data, indicating that the running
cannot be reliably measured with present data.Comment: 8 pages RevTeX4 file with six figures incorporate
Quantum phase transition in the Frenkel-Kontorova chain: from pinned instanton glass to sliding phonon gas
We study analytically and numerically the one-dimensional quantum
Frenkel-Kontorova chain in the regime when the classical model is located in
the pinned phase characterized by the gaped phonon excitations and devil's
staircase. By extensive quantum Monte Carlo simulations we show that for the
effective Planck constant smaller than the critical value the
quantum chain is in the pinned instanton glass phase. In this phase the
elementary excitations have two branches: phonons, separated from zero energy
by a finite gap, and instantons which have an exponentially small excitation
energy. At the quantum phase transition takes place and for
the pinned instanton glass is transformed into the sliding
phonon gas with gapless phonon excitations. This transition is accompanied by
the divergence of the spatial correlation length and appearence of sliding
modes at .Comment: revtex 16 pages, 18 figure
Cosmological parameters from SDSS and WMAP
We measure cosmological parameters using the three-dimensional power spectrum
P(k) from over 200,000 galaxies in the Sloan Digital Sky Survey (SDSS) in
combination with WMAP and other data. Our results are consistent with a
``vanilla'' flat adiabatic Lambda-CDM model without tilt (n=1), running tilt,
tensor modes or massive neutrinos. Adding SDSS information more than halves the
WMAP-only error bars on some parameters, tightening 1 sigma constraints on the
Hubble parameter from h~0.74+0.18-0.07 to h~0.70+0.04-0.03, on the matter
density from Omega_m~0.25+/-0.10 to Omega_m~0.30+/-0.04 (1 sigma) and on
neutrino masses from <11 eV to <0.6 eV (95%). SDSS helps even more when
dropping prior assumptions about curvature, neutrinos, tensor modes and the
equation of state. Our results are in substantial agreement with the joint
analysis of WMAP and the 2dF Galaxy Redshift Survey, which is an impressive
consistency check with independent redshift survey data and analysis
techniques. In this paper, we place particular emphasis on clarifying the
physical origin of the constraints, i.e., what we do and do not know when using
different data sets and prior assumptions. For instance, dropping the
assumption that space is perfectly flat, the WMAP-only constraint on the
measured age of the Universe tightens from t0~16.3+2.3-1.8 Gyr to
t0~14.1+1.0-0.9 Gyr by adding SDSS and SN Ia data. Including tensors, running
tilt, neutrino mass and equation of state in the list of free parameters, many
constraints are still quite weak, but future cosmological measurements from
SDSS and other sources should allow these to be substantially tightened.Comment: Minor revisions to match accepted PRD version. SDSS data and ppt
figures available at http://www.hep.upenn.edu/~max/sdsspars.htm
A Person-Centered Approach to Poststroke Care: The COMprehensive Post-Acute Stroke Services Model
Many individuals who have had a stroke leave the hospital without postacute care services in place. Despite high risks of complications and readmission, there is no standard in the United States for postacute stroke care after discharge home. We describe the rationale and methods for the development of the COMprehensive Post-Acute Stroke Services (COMPASS) care model and the structure and quality metrics used for implementation. COMPASS, an innovative, comprehensive extension of the TRAnsition Coaching for Stroke (TRACS) program, is a clinician-led quality improvement model providing early supported discharge and transitional care for individuals who have had a stroke and have been discharged home. The effectiveness of the COMPASS model is being assessed in a cluster-randomized pragmatic trial in 41 sites across North Carolina, with a recruitment goal of 6,000 participants. The COMPASS model is evidence based, person centered, and stakeholder driven. It involves identification and education of eligible individuals in the hospital; telephone follow-up 2, 30, and 60 days after discharge; and a clinic visit within 14 days conducted by a nurse and advanced practice provider. Patient and caregiver self-reported assessments of functional and social determinants of health are captured during the clinic visit using a web-based application. Embedded algorithms immediately construct an individualized care plan. The COMPASS model's pragmatic design and quality metrics may support measurable best practices for postacute stroke care
Density perturbations in generalized Einstein scenarios and constraints on nonminimal couplings from the Cosmic Microwave Background
We study cosmological perturbations in generalized Einstein scenarios and
show the equivalence of inflationary observables both in the Jordan frame and
the Einstein frame. In particular the consistency relation relating the
tensor-to-scalar ratio with the spectral index of tensor perturbations
coincides with the one in Einstein gravity, which leads to the same likelihood
results in terms of inflationary observables. We apply this formalism to
nonminimally coupled chaotic inflationary scenarios with potential
and place constraints on the strength of nonminimal couplings using a
compilation of latest observational data. In the case of the quadratic
potential (), the nonminimal coupling is constrained to be for negative from the observational contour
bound. Although the quartic potential () is under a strong observational
pressure for , this property is relaxed by taking into account negative
nonminimal couplings. We find that inflationary observables are within the
contour bound as long as . We also show that
the cases are disfavoured even in the presence of nonminimal
couplings.Comment: 16 pages, 4 eps figure
The last stand before MAP: cosmological parameters from lensing, CMB and galaxy clustering
Cosmic shear measurements have now improved to the point where they deserve
to be treated on par with CMB and galaxy clustering data for cosmological
parameter analysis, using the full measured aperture mass variance curve rather
than a mere phenomenological parametrization thereof. We perform a detailed
9-parameter analysis of recent lensing (RCS), CMB (up to Archeops) and galaxy
clustering (2dF) data, both separately and jointly. CMB and 2dF data are
consistent with a simple flat adiabatic scale-invariant model with
Omega_Lambda=0.72+/-0.09, omega_cdm=0.115+/- 0.013, omega_b=0.024+/-0.003, and
a hint of reionization around z~8. Lensing helps further tighten these
constraints, but reveals tension regarding the power spectrum normalization:
including the RCS survey results raises sigma8 significantly and forces other
parameters to uncomfortable values. Indeed, sigma8 is emerging as the currently
most controversial cosmological parameter, and we discuss possible resolutions
of this sigma8 problem. We also comment on the disturbing fact that many recent
analyses (including this one) obtain error bars smaller than the Fisher matrix
bound. We produce a CMB power spectrum combining all existing experiments, and
using it for a "MAP versus world" comparison next month will provide a powerful
test of how realistic the error estimates have been in the cosmology community.Comment: Added references and Fisher error discussion. Combined CMB data,
window and covariance matrix for January "MAP vs World" contest at
http://www.hep.upenn.edu/~max/cmblsslens.html or from [email protected]
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