1,857 research outputs found
Generation and measurement of nonstationary random processes technical note no. 3
Generation and measurement of nonstationary stochastic processes related to Monte Carlo studies with analog compute
Breaking Cosmological Degeneracies in Galaxy Cluster Surveys with a Physical Model of Cluster Structure
Forthcoming large galaxy cluster surveys will yield tight constraints on
cosmological models. It has been shown that in an idealized survey, containing
> 10,000 clusters, statistical errors on dark energy and other cosmological
parameters will be at the percent level. It has also been shown that through
"self-calibration", parameters describing the mass-observable relation and
cosmology can be simultaneously determined, though at a loss in accuracy by
about an order of magnitude. Here we examine the utility of an alternative
approach of self-calibration, in which a parametrized ab-initio physical model
is used to compute cluster structure and the resulting mass-observable
relations. As an example, we use a modified-entropy ("pre-heating") model of
the intracluster medium, with the history and magnitude of entropy injection as
unknown input parameters. Using a Fisher matrix approach, we evaluate the
expected simultaneous statistical errors on cosmological and cluster model
parameters. We study two types of surveys, in which a comparable number of
clusters are identified either through their X-ray emission or through their
integrated Sunyaev-Zel'dovich (SZ) effect. We find that compared to a
phenomenological parametrization of the mass-observable relation, using our
physical model yields significantly tighter constraints in both surveys, and
offers substantially improved synergy when the two surveys are combined. These
results suggest that parametrized physical models of cluster structure will be
useful when extracting cosmological constraints from SZ and X-ray cluster
surveys. (abridged)Comment: 22 pages, 8 figures, accepted to Ap
Hydrodynamical Simulations of the Lyman Alpha Forest: Model Comparisons
We investigate the properties of the Lyman alpha forest as predicted by
numerical simulations for a range of currently viable cosmological models. This
is done in order to understand the dependencies of the forest on cosmological
parameters. Focusing on the redshift range from two to four, we show that: (1)
most of the evolution in the distributions of optical depth, flux and column
density can be understood by simple scaling relations, (2) the shape of optical
depth distribution is a sensitive probe of the amplitude of density
fluctuations on scales of a few hundred kpc, (3) the mean of the b distribution
(a measure of the width of the absorption lines) is also very sensitive to
fluctuations on these scales, and decreases as they increase. We perform a
preliminary comparison to observations, where available. A number of other
properties are also examined, including the evolution in the number of lines,
the two-point flux distribution and the HeII opacity.Comment: 37 pages, 21 figures, submitted to Ap
Cross-correlation between the soft X-ray background and SZ Sky
While both X-ray emission and Sunyaev-Zel'dovich (SZ) temperature
fluctuations are generated by the warm-hot gas in dark matter halos, the two
observables have different dependence on the underlying physical properties,
including the gas distribution. A cross-correlation between the soft X-ray
background (SXRB) and the SZ sky may allow an additional probe on the
distribution of warm-hot gas at intermediate angular scales and redshifts
complementing studies involving clustering within SXRB and SZ separately. Using
a halo approach, we investigate this cross-correlation analytically. The two
contributions are correlated mildly with a correlation coefficient of
, and this relatively low correlation presents a significant challenge
for its detection. The correlation, at small angular scales, is affected by the
presence of radiative cooling or preheating and provides a probe on the thermal
history of the hot gas in dark halos. While the correlation remains
undetectable with CMB data from the WMAP satellite and X-ray background data
from existing catalogs, upcoming observations with CMB missions such as Planck,
for the SZ side, and an improved X-ray map of the large scale structure, such
as the one planned with DUET mission, may provide a first opportunity for a
reliable detection of this cross-correlation.Comment: 8 pages, 6 figures, accepted for publication in A&
Transgenerational latent early-life associated regulation unites environment and genetics across generations
The origin of idiopathic diseases is still poorly understood. The latent early-life associated regulation (LEARn) model unites environmental exposures and gene expression while providing a mechanistic underpinning for later-occurring disorders. We propose that this process can occur across generations via transgenerational LEARn (tLEARn). In tLEARn, each person is a 'unit' accumulating preclinical or subclinical 'hits' as in the original LEARn model. These changes can then be epigenomically passed along to offspring. Transgenerational accumulation of 'hits' determines a sporadic disease state. Few significant transgenerational hits would accompany conception or gestation of most people, but these may suffice to 'prime' someone to respond to later-life hits. Hits need not produce symptoms or microphenotypes to have a transgenerational effect. Testing tLEARn requires longitudinal approaches. A recently proposed longitudinal epigenome/envirome-wide association study would unite genetic sequence, epigenomic markers, environmental exposures, patient personal history taken at multiple time points and family history
Simulations of Pregalactic Structure Formation with Radiative Feedback
We present results from three-dimensional hydrodynamic simulations of the
high redshift collapse of pregalactic clouds including feedback effects from a
soft H2 photodissociating UV radiation field. The simulations use an Eulerian
adaptive mesh refinement technique to follow the nonequilibrium chemistry of
nine chemical species with cosmological initial conditions drawn from a popular
Lambda-dominated cold dark matter model. The results confirm that the soft UV
background can delay the cooling and collapse of small halos (~10^6 Msun). For
reasonable values of the photo-dissociating flux, the H2 fraction is in
equilibrium throughout most of the objects we simulate. We determine the mass
threshold for collapse for a range of soft-UV fluxes and also derive a simple
analytic expression. Continuing the simulations beyond the point of initial
collapse demonstrates that the fraction of gas which can cool depends mostly on
the virial mass of the halo and the amount of soft-UV flux, with remarkably
little scatter. We parameterize this relation, for use in semi-analytic models.Comment: 18 pages, 7 figures, submitted to Ap
Complex phylogeographic history of central African forest elephants and its implications for taxonomy
Background: Previous phylogenetic analyses of African elephants have included limited numbers of forest elephant samples. A large-scale assessment of mitochondrial DNA diversity in forest elephant populations here reveals a more complex evolutionary history in African elephants as a whole than two-taxon models assume. Results: We analysed hypervariable region 1 of the mitochondrial control region for 71 new central African forest elephants and the mitochondrial cytochrome b gene from 28 new samples and compare these sequences to other African elephant data. We find that central African forest elephant populations fall into at least two lineages and that west African elephants (both forest and savannah) share their mitochondrial history almost exclusively with central African forest elephants. We also find that central African forest populations show lower genetic diversity than those in savannahs, and infer a recent population expansion. Conclusion: Our data do not support the separation of African elephants into two evolutionary lineages. The demographic history of African elephants seems more complex, with a combination of multiple refugial mitochondrial lineages and recurrent hybridization among them rendering a simple forest/savannah elephant split inapplicable to modern African elephant populations
Effects of the halo concentration distribution on strong-lensing optical depth and X-ray emission
We use simulated merger trees of galaxy-cluster halos to study the effect of
the halo concentration distribution on strong lensing and X-ray emission. Its
log-normal shape typically found in simulations favors outliers with high
concentration. Since, at fixed mass, more concentrated halos tend to be more
efficient lenses, the scatter in the concentration increases the strong-lensing
optical depth by . Within cluster samples, mass and concentration
have counteracting effects on strong lensing and X-ray emission because the
concentration decreases for increasing mass. Selecting clusters by
concentration thus has no effect on the lensing cross section. The most
efficiently lensing and hottest clusters are typically the \textit{least}
concentrated in samples with a broad mass range. Among cluster samples with a
narrow mass range, however, the most strongly lensing and X-ray brightest
clusters are typically 10% to 25% more concentrated.Comment: 12 pages, 10 figures. Version accepted by A&
Ground-State Dynamical Correlation Functions: An Approach from Density Matrix Renormalization Group Method
A numerical approach to ground-state dynamical correlation functions from
Density Matrix Renormalization Group (DMRG) is developed. Using sum rules,
moments of a dynamic correlation function can be calculated with DMRG, and with
the moments the dynamic correlation function can be obtained by the maximum
entropy method. We apply this method to one-dimensional spinless fermion
system, which can be converted to the spin 1/2 Heisenberg model in a special
case. The dynamical density-density correlation function is obtained.Comment: 11 pages, latex, 4 figure
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