490 research outputs found
The gravitating perfect fluid-scalar field equations: quintessence and tachyonic
The system consisting of a self gravitating perfect fluid and scalar field is
considered in detail. The scalar fields considered are the quintessence and
``tachyonic'' forms which have important application in cosmology. Mathematical
properties of the general system of equations are studied including the
algebraic and differential identities as well as the eigenvalue structure. The
Cauchy problem for both quintessence and the tachyon is presented. We discuss
the initial constraint equations which must be satisfied by the initial data. A
Cauchy evolution scheme is presented in the form of a Taylor series about the
Cauchy surface. Finally, a simple numerical example is provided to illustrate
this scheme.Comment: 15 pages, 2 figures. Revised version contains more references.
Accepted for publication in General Relativity and Gravitatio
The Evolution of Black Holes in the Mini-Superspace Approximation of Loop Quantum Gravity
Using the improved quantization technique to the mini-superspace
approximation of loop quantum gravity, we study the evolution of black holes
supported by a cosmological constant. The addition of a cosmological constant
allows for classical solutions with planar, cylindrical, toroidal and higher
genus black holes. Here we study the quantum analog of these space-times. In
all scenarios studied, the singularity present in the classical counter-part is
avoided in the quantized version and is replaced by a bounce, and in the late
evolution, a series of less severe bounces. Interestingly, although there are
differences during the evolution between the various symmetries and topologies,
the evolution on the other side of the bounce asymptotes to space-times of
Nariai-type, with the exception of the planar black hole analyzed here, whose
-=constant subspaces seem to continue expanding in the long term
evolution. For the other cases, Nariai-type universes are attractors in the
quantum evolution, albeit with different parameters. We study here the quantum
evolution of each symmetry in detail.Comment: 26 pages, 7 figures.V2 has typos corrected, references added, and a
more careful analysis of the planar case. Accepted for publication in
Physical Review
The sensitivity of global wildfires to simulated past, present, and future lightning frequency
In this study, components of the Max Planck Institute Earth System Model were used to explore how changes in lightning induced by climate change alter wildfire activity. To investigate how climate change alters global flash frequency, simulations with the atmospheric general circulation model ECHAM6 were performed for the time periods preindustrial, present-day, and three future scenarios. The effect of changes in lightning activity on fire occurrence was derived from simulations with the land surface vegetation model JSBACH. Global cloud-to-ground lightning activity decreased by 3.3% under preindustrial climate and increased by up to 21.3% for the RCP85 projection at the end of the century when compared to present-day, respectively. Relative changes were most pronounced in North America and northeastern Asia. Global burned area was little affected by these changes and only increased by up to 3.3% for RCP85. However, on the regional scale, significant changes occurred. For instance, burned area increases of over 100% were found in high-latitude regions, while also several regions were identified where burned area declined, such as parts of South America and Africa. ©2014. American Geophysical Union. All Rights Reserved
Fire emission heights in the climate system - Part 1: Global plume height patterns simulated by ECHAM6-HAM2
We use the global circulation model ECHAM6 extended by the aerosol module HAM2 to simulate global patterns in wildfire emission heights. Prescribed plume heights in ECHAM6 are replaced by an implementation of a simple, semi-empirical plume height parametrization. In a first step, the global performance of the plume height parametrization is evaluated for plumes reported in the Multiangle Imaging Spectroradiometer (MISR) Plume Height Project (MPHP) data set. Our results show that the parametrization simulates a largely reasonable global distribution of plume heights. While the modeled global mean plume height (1411 ± 646 m) is in good agreement with the observed mean (1382 ± 702 m), the upper and lower tails of the plume height distribution tend to be slightly underrepresented. Furthermore, we compare plume heights simulated by the simple parametrization to a more complex, analytical plume model. Major differences in global plume height distributions are found for the lowest 1.5 km, but reasonable agreement is observed for higher plumes. In a second step, fire radiative power (FRP) as reported in the global fire assimilation system (GFAS) is used to simulate plume heights for observed fires globally for the period 2005–2011. The global fraction of simulated daytime plumes injecting emissions into the free troposphere (FT) ranges from 3.7 ± 0.7 to 5.2 ± 1.0 %. This range is comparable to results from observational studies, but it is much lower than results for prescribed plume heights in the ECHAM6-HAM2 standard setup. Nevertheless, occasionally deep emission injections exceeding 5–7 km in height are simulated for intense fires and favorable meteorological conditions. The application of a prescribed diurnal cycle in FRP turns out to be of minor importance. For a hypothetical doubling in FRP, moderate changes in plume heights of 100–400 m are simulated. These small changes indicate that a potential future increase in fire intensity will only slightly impact the emission heights on a global scale
Black hole entropy for the general area spectrum
We consider the possibility that the horizon area is expressed by the general
area spectrum in loop quantum gravity and calculate the black hole entropy by
counting the degrees of freedom in spin-network states related to its area.
Although the general area spectrum has a complex expression, we succeeded in
obtaining the result that the black hole entropy is proportional to its area as
in previous works where the simplified area formula has been used. This gives
new values for the Barbero-Immirzi parameter () which are larger than that of previous works.Comment: 5 page
How present aerosol pollution from North America impacts North Atlantic climate
This paper describes the potential effects of present-day aerosol pollution from North America (USA, Canada) on the climate of the North Atlantic region. The study has been performed by applying the comprehensive atmospheric general circulation model ECHAM5-HAM, which is coupled to a mixed-layer ocean with an embedded thermodynamic sea ice module. The model includes a microphysical aerosol model (HAM), which allows for the assessment of aerosol impacts on climate. Sulphate, black and organic carbon, sea salt and mineral dust are considered as aerosol species. Two equilibrium simulations with two different aerosol pollutant scenarios are compared for each season. We investigate the effect on radiation, temperature, hydrological quantities and dynamics, when human-induced aerosol emissions from North America were omitted. The decrease of both direct and indirect aerosol effects induces a positive change in top of the atmosphere (TOA) radiative fluxes resulting in an overall warming in the whole region. Our results demonstrate the vulnerability especially of the Arctic to the reduction in aerosol load. For fall we find an increase in precipitation over the North Atlantic, associated with a tendency to a larger number of cyclones with high-pressure gradients and a higher frequency in storm days
Simulation and analysis of in vitro DNA evolution
We study theoretically the in vitro evolution of a DNA sequence by binding to
a transcription factor. Using a simple model of protein-DNA binding and
available binding constants for the Mnt protein, we perform large-scale,
realistic simulations of evolution starting from a single DNA sequence. We
identify different parameter regimes characterized by distinct evolutionary
behaviors. For each regime we find analytical estimates which agree well with
simulation results. For small population sizes, the DNA evolutional path is a
random walk on a smooth landscape. While for large population sizes, the
evolution dynamics can be well described by a mean-field theory. We also study
how the details of the DNA-protein interaction affect the evolution.Comment: 11 pages, 11 figures. Submitted to PNA
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