4,994 research outputs found
Dark energy cosmology from higher-order, string-inspired gravity and its reconstruction
In this paper we investigate the cosmological effects of modified gravity
with string curvature corrections added to Einstein-Hilbert action in the
presence of a dynamically evolving scalar field coupled to Riemann invariants.
The scenario exhibits several features of cosmological interest for late
universe. It is shown that higher order stringy corrections can lead to a class
of dark energy models consistent with recent observations. The model can give
rise to quintessence, deSitter or phantom dark energy, in last case without
recourse to negative kinetic energy field. The detailed treatment of
reconstruction program for general scalar-Gauss-Bonnet gravity is presented for
any given cosmology. The explicit examples of reconstructed scalar potentials
are given for accelerated (quintessence, cosmological constant or phantom)
universe. Finally, the relation with modified gravity is established on
classical level and is extended to include third order terms on curvature.Comment: 15 pages, 3 figures, minor changes, published versio
Formation of caustics in Dirac-Born-Infeld type scalar field systems
We investigate the formation of caustics in Dirac-Born-Infeld type scalar
field systems for generic classes of potentials, viz., massive rolling scalar
with potential, and inverse
power-law potentials with . We find that in the case
of\texttt{} exponentially decreasing rolling massive scalar field potential,
there are multi-valued regions and regions of likely to be caustics in the
field configuration. However there are no caustics in the case of exponentially
increasing potential. We show that the formation of caustics is inevitable for
the inverse power-law potentials under consideration in Minkowski space time
whereas caustics do not form in this case in the FRW universe.Comment: 16 pages, 14 figures, major revision, conclusions strengthen, to
appear in PR
Renin-angiotensin system and its relationship to arterial pressure regulation
The renal system is known to participate in the natural regulation of blood pressure. A renal system model that simulates interaction responses of renal output to arterial blood pressure was developed. The major components of this model are glomerular filtration rate, tubular sodium reabsorption, the renin-angiotensin system, Aldosterone secretion, and antidiuretic hormone secretion. The model\u27s differential equations were coded using Vissim 1.2 student version. It was found that, the renal output increased as glomerular filtration rate, Aldosterone concentration, and blood pressure increased. Conversely, increasing antidiuretic hormone concentration lead to decreased renal output.
Additionally, Angiotensin II and Aldosterone concentrations increased as the arterial pressure decreased. The model suggests that increasing renal output leads to decreased blood volume, which aids in the long term regulation of blood pressure
Direct perturbations of aggregate excess demand
We establish that an exchange economy, i.e., preferences and endowments, that generates a giiven aggregate excess demand (AED) function is close to the economy generating the AED obtained by an arbitrary perturbation of the original one.Arrow-Debreu, transversality, exchange economy.
Scalar Field Oscillations Contributing to Dark Energy
We use action-angle variables to describe the basic physics of coherent
scalar field oscillations in the expanding universe. These analytical mechanics
methods have some advantages, like the identification of adiabatic invariants.
As an application, we show some instances of potentials leading to equations of
state with , thus contributing to the dark energy that causes the
observed acceleration of the universe.Comment: 17 pages, 6 figures, Latex file. Sec.II reduced, discussion on sound
speed added in Sec.IV, new references added. Accepted for publication in
Physical Review
Millimeter radiation from a 3D model of the solar atmosphere II. Chromospheric magnetic field
We use state-of-the-art, three-dimensional non-local thermodynamic
equilibrium (non-LTE) radiative magnetohydrodynamic simulations of the quiet
solar atmosphere to carry out detailed tests of chromospheric magnetic field
diagnostics from free-free radiation at millimeter and submillimeter
wavelengths (mm/submm). The vertical component of the magnetic field was
deduced from the mm/submm brightness spectra and the degree of circular
polarization synthesized at millimeter frequencies. We used the frequency bands
observed by the Atacama Large Millimeter/Submillimeter Array (ALMA) as a
convenient reference. The magnetic field maps obtained describe the
longitudinal magnetic field at the effective formation heights of the relevant
wavelengths in the solar chromosphere. The comparison of the deduced and model
chromospheric magnetic fields at the spatial resolution of both the model and
current observations demonstrates a good correlation, but has a tendency to
underestimate the model field. The systematic discrepancy of about 10 percent
is probably due to averaging of the restored field over the heights
contributing to the radiation, weighted by the strength of the contribution. On
the whole, the method of probing the longitudinal component of the magnetic
field with free-free emission at mm/submm wavelengths is found to be applicable
to measurements of the weak quiet-Sun magnetic fields. However, successful
exploitation of this technique requires very accurate measurements of the
polarization properties (primary beam and receiver polarization response) of
the antennas, which will be the principal factor that determines the level to
which chromospheric magnetic fields can be measured. Consequently,
high-resolution and high-precision observations of circularly polarized
radiation at millimeter wavelengths can be a powerful tool for producing
chromospheric longitudinal magnetograms.Comment: 12 pages, 13 figures, accepted for publication in A&
A new SATIRE-S spectral solar irradiance reconstruction for solar cycles 21--23 and its implications for stratospheric ozone
We present a revised and extended total and spectral solar irradiance (SSI)
reconstruction, which includes a wavelength-dependent uncertainty estimate,
spanning the last three solar cycles using the SATIRE-S model. The SSI
reconstruction covers wavelengths between 115 and 160,000 nm and all dates
between August 1974 and October 2009. This represents the first full-wavelength
SATIRE-S reconstruction to cover the last three solar cycles without data gaps
and with an uncertainty estimate. SATIRE-S is compared with the NRLSSI model
and SORCE/SOLSTICE ultraviolet (UV) observations. SATIRE-S displays similar
cycle behaviour to NRLSSI for wavelengths below 242 nm and almost twice the
variability between 242 and 310 nm. During the decline of last solar cycle,
between 2003 and 2008, SSI from SORCE/SOLSTICE version 12 and 10 typically
displays more than three times the variability of SATIRE-S between 200 and 300
nm. All three datasets are used to model changes in stratospheric ozone within
a 2D atmospheric model for a decline from high solar activity to solar minimum.
The different flux changes result in different modelled ozone trends. Using
NRLSSI leads to a decline in mesospheric ozone, while SATIRE-S and
SORCE/SOLSTICE result in an increase. Recent publications have highlighted
increases in mesospheric ozone when considering version 10 SORCE/SOLSTICE
irradiances. The recalibrated SORCE/SOLSTICE version 12 irradiances result in a
much smaller mesospheric ozone response than when using version 10 and now
similar in magnitude to SATIRE-S. This shows that current knowledge of
variations in spectral irradiance is not sufficient to warrant robust
conclusions concerning the impact of solar variability on the atmosphere and
climate.Comment: 25 pages (18 pages in main article with 6 figures; 7 pages in
supplementary materials with 6 figures) in draft mode using the American
Meteorological Society package. Submitted to Journal of Atmospheric Sciences
for publicatio
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