697 research outputs found
Method of determination of the mass composition of ring current ions
A method for individual registration of protons, and helium and oxygen ions, with energies E for a charge on the order of 100 kev/q in the ring currents of the Earth's magnetosphere was examined. The method is based on the various specific losses in energy by these ions in matter. The ion current, selected according to E/q, is passed through a solid target, after which identification of the masses is carried out, based on the energy losses in the possibly to reliably divide the flows of protons, and helium and oxygen ions
Crossing Statistic: Bayesian interpretation, model selection and resolving dark energy parametrization problem
By introducing Crossing functions and hyper-parameters I show that the
Bayesian interpretation of the Crossing Statistics [1] can be used trivially
for the purpose of model selection among cosmological models. In this approach
to falsify a cosmological model there is no need to compare it with other
models or assume any particular form of parametrization for the cosmological
quantities like luminosity distance, Hubble parameter or equation of state of
dark energy. Instead, hyper-parameters of Crossing functions perform as
discriminators between correct and wrong models. Using this approach one can
falsify any assumed cosmological model without putting priors on the underlying
actual model of the universe and its parameters, hence the issue of dark energy
parametrization is resolved. It will be also shown that the sensitivity of the
method to the intrinsic dispersion of the data is small that is another
important characteristic of the method in testing cosmological models dealing
with data with high uncertainties.Comment: 14 pages, 4 figures, discussions extended, 1 figure and two
references added, main results unchanged, matches the final version to be
published in JCA
Transitions to palliative care in acute hospitals in England: qualitative study
Objective To explore how transitions to a palliative care approach are perceived to be managed in acute hospital settings in England
The Importance of Lens Galaxy Environments
While many strong gravitational lens galaxies are suspected to lie in groups
or clusters of galaxies, environmental effects in lens models are often
unconstrained and sometimes ignored. We show that this creates significant
biases in a variety of lensing applications, by creating mock lenses associated
with each of 13 galaxies in a realistic model group, and then analyzing them
with standard techniques. We find that standard models of double lenses, which
neglect environment, grossly overestimate the ellipticity of the lens galaxy
(de/e~0.5) and the Hubble constant (dh/h~0.22). Standard models of quad lenses,
which approximate the environment as a tidal shear, recover the ellipticity
reasonably well (|de/e|<~0.24) but overestimate the Hubble constant
(dh/h~0.15), and have significant (~30%) errors in the millilensing analyses
used to constrain the amount of substructure in dark matter halos. For both
doubles and quads, standard models slightly overestimate the velocity
dispersion of the lens galaxy (d(sigma)/sigma~0.06), and underestimate the
magnifications of the images (d(mu)/mu ~ -0.25). Standard analyses of lens
statistics overestimate Omega_Lambda (by 0.05-0.14), and underestimate the
ratio of quads to doubles (by a factor of 2). These biases help explain some
long-standing puzzles (such as the high observed quad/double ratio), but
aggravate others (such as the low value of H_0 inferred from lensing). Most of
the biases are caused by neglect of the convergence from the mass associated
with the environment, but additional uncertainty is introduced by neglect of
higher-order terms. Fortunately, we show that directly observing and modeling
lens environments should make it possible to remove the biases and reduce the
uncertainties associated with environments to the few percent level. (Abridged)Comment: 14 emulateapj pages; accepted in Ap
Median Statistics, H_0, and the Accelerating Universe
(Abridged) We develop median statistics that provide powerful alternatives to
chi-squared likelihood methods and require fewer assumptions about the data.
Applying median statistics to Huchra's compilation of nearly all estimates of
the Hubble constant, we find a median value H_0=67 km/s/Mpc. Median statistics
assume only that the measurements are independent and free of systematic
errors. This estimate is arguably the best summary of current knowledge because
it uses all available data and, unlike other estimates, makes no assumption
about the distribution of measurement errors. The 95% range of purely
statistical errors is +/- 2 km/s/Mpc. The statistical precision of this result
leads us to analyze the range of possible systematic errors in the median,
which we estimate to be roughly +/- 5 km/s/Mpc (95% limits), dominating over
the statistical errors. A Bayesian median statistics treatment of high-redshift
Type Ia supernovae (SNe Ia) apparent magnitude versus redshift data from Riess
et al. yields a posterior probability that the cosmological constant Lambda > 0
of 70 or 89%, depending on the prior information used. The posterior
probability of an open universe is about 47%. Analysis of the Perlmutter et al.
high-redshift SNe Ia data show the best-fit flat-Lambda model favored over the
best-fit Lambda = 0 open model by odds of 366:1; corresponding Riess et al.
odds are 3:1 (assuming prior odds of 1:1).Median statistics analyses of the SNe
Ia data do not rule out a time-variable Lambda model, and may even favor it
over a time-independent Lambda and a Lambda = 0 open model.Comment: Significant revisions include discussion of systematic errors in the
median of H_0. Accepted for publication in The Astrophysical Journal, v548,
February 20, 2001 issue. 47 pages incl. figures and table
Resonance Structure for Preheating with Massless Fields
We extend recent work on the resonance structure for post-inflation
reheating, providing an analytic treatment for models in which both the
inflaton and the fields into which it decays are massless. Solutions are
derived which are valid for either a spatially-flat or spatially-open metric.
Closed-form solutions are given for the characteristic exponent, which measures
the rate of particle production during preheating. It is demonstrated that in
certain regions of parameter space, the maximum values of the characteristic
exponent in an open universe are several times greater than the maximum values
in a spatially-flat universe. It is further demonstrated that the solutions
found here by means of a simple algebraic construction match the two
previously-known exact solutions, which were derived in terms of special
functions.Comment: 13pp, revtex. Minor revisions. Forthcoming in Phys Rev
Preheating in an Expanding Universe: Analytic Results for the Massless Case
Analytic results are presented for preheating in both flat and open models of
chaotic inflation, for the case of massless inflaton decay into further
inflaton quanta. It is demonstrated that preheating in both these cases closely
resembles that in Minkowski spacetime. Furthermore, quantitative differences
between preheating in spatially-flat and open models of inflation remain of
order for the chaotic inflation initial conditions considered here.Comment: 15pp, revtex. No figures. Very minor revisions; forthcoming in Phys
Rev
Dark matter and non-Newtonian gravity from General Relativity coupled to a fluid of strings
An exact solution of Einstein's field equations for a point mass surrounded
by a static, spherically symmetric fluid of strings is presented. The solution
is singular at the origin. Near the string cloud limit there is a
correction to Newton's force law. It is noted that at large distances and small
accelerations, this law coincides with the phenomenological force law invented
by Milgrom in order to explain the flat rotation curves of galaxies without
introducing dark matter. When interpreted in the context of a cosmological
model with a string fluid, the new solution naturally explains why the critical
acceleration of Milgrom is of the same order of magnitude as the Hubble
parameter.Comment: 12 pages, REVTeX, no figure
Topologically massive gravito-electrodynamics: exact solutions
We construct two classes of exact solutions to the field equations of
topologically massive electrodynamics coupled to topologically massive gravity
in 2 + 1 dimensions. The self-dual stationary solutions of the first class are
horizonless, asymptotic to the extreme BTZ black-hole metric, and regular for a
suitable parameter domain. The diagonal solutions of the second class, which
exist if the two Chern-Simons coupling constants exactly balance, include
anisotropic cosmologies and static solutions with a pointlike horizon.Comment: 15 pages, LaTeX, no figure
Energy Production in the Formation of a Finite Thickness Cosmic String
The classical electromagnetic modes outside a long, straight, superconducting
cosmic string are calculated, assuming the string to be surrounded by a
superconducting cylindric surface of radius R. Thereafter, by use of a
Bogoliubov-type argument, the electromagnetic energy W produced per unit length
in the lowest two modes is calculated when the string is formed "suddenly". The
essential new element in the present analysis as compared with prior work of
Parker [Phys. Rev. Lett. {\bf 59}, 1369 (1987)] and Brevik and Toverud [Phys.
Rev. D {\bf 51}, 691 (1995)], is that the radius {\it a} of the string is
assumed finite, thus necessitating Neumann functions to be included in the
fundamental modes. We find that the theory is changed significantly: W is now
strongly concentrated in the lowest mode , whereas the
proportionality that is characteristic for zero-width
strings is found in the next mode (1,1). Here G is the gravitational constant,
the string mass per unit length, and t the GUT time.Comment: 20 pages, LaTeX, no figure
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