3,529 research outputs found
Constraining the Dark Universe
We combine complementary datasets to constrain dark energy. Using standard
Big Bang Nucleosynthesis and the observed abundances of primordial nuclides to
put constraints on at temperatures near , we find the
strong constraint at c.l.. Under the
assumption of flatness, using results from Cosmic Microwave Background (CMB)
anisotropy measurements, high redshift supernovae (SN-Ia) observations and data
from local cluster abundances we put a new constraint on the equation of state
parameter at 68% c.l..Comment: 5 pages, 3 figures, new reference added. To appear in the proceedings
of TAUP 2001 (7th international workshop on Topics in Astroparticle and
Underground Physics), Sep 2001, Laboratori Nazionali del Gran Sasso, Assergi,
Ital
Darboux class of cosmological fluids with time-dependent adiabatic indices
A one-parameter family of time dependent adiabatic indices is introduced for
any given type of cosmological fluid of constant adiabatic index by a
mathematical method belonging to the class of Darboux transformations. The
procedure works for zero cosmological constant at the price of introducing a
new constant parameter related to the time dependence of the adiabatic index.
These fluids can be the real cosmological fluids that are encountered at
cosmological scales and they could be used as a simple and efficient
explanation for the recent experimental findings regarding the present day
accelerating universe. In addition, new types of cosmological scale factors,
corresponding to these fluids, are presentedComment: document with the following three latex files: 1) quhm.tex: 17 pages,
10 figs, 16 numbered refs, Honorable Mention GRF 2000, 2) errad.tex: Errata
and Addenda (EaA) of 5 pages with 2 figs enclosed, 3) analogy.tex: Negative
friction of Darboux cosmological fluids of 4 page
Phantom Energy Accretion by Stringy Charged Black Hole
We investigate the dynamical behavior of phantom energy near stringy
magnetically charged black hole. For this purpose, we derive equations of
motion for steady-state spherically symmetric flow of phantom energy onto the
stringy magnetically charged black hole. It is found that phantom energy
accreting onto black hole decreases its mass. Further, the location of critical
points of accretion is explored, which yields mass to charge ratio. This ratio
implies that accretion process cannot transform a black hole into an extremal
black hole or a naked singularity, hence cosmic censorship hypothesis remains
valid here.Comment: 7 pages, no figur
Cosmological constant influence on cosmic string spacetime
We investigate the line element of spacetime around a linear cosmic string in
the presence of a cosmological constant. We obtain the metric and argue that it
should be discarded because of asymptotic considerations. Then a time dependent
and consistent form of the metric is obtained and its properties are discussed.Comment: 3 page
Phantom Accretion onto the Schwarzschild de-Sitter Black Hole
We deal with phantom energy accretion onto the Schwarzschild de-Sitter black
hole. The energy flux conservation, relativistic Bernoulli equation and mass
flux conservation equation are formulated to discuss the phantom accretion. We
discuss the conditions for critical accretion. It is found that mass of the
black hole decreases due to phantom accretion. There exist two critical points
which lie in the exterior of horizons (black hole and cosmological horizons).
The results for the phantom energy accretion onto the Schwarzschild black hole
can be recovered by taking .Comment: 9 pages, no figur
Stochastic simulation of vertically nonhomogeneous gusts
The small-scale horizontal gust structure of detailed wind profiles along the vertical in the first 20 km of the atmosphere is a vertically nonhomogeneous process. A linear stochastic model was developed based on the process covariance function. This model was formulated through the use of a scaling hypothesis which transforms the nonhomogeneous gust process into a nondimensional gust process which is homogeneous in a nondimensional gust height coordinate. The velocity scaling parameter for the gust process is the gust standard deviation, and the length scale used to nondimensionalize the altitude is the vertical space lag associated with the first zero of the gust covariance function. State space theory was used to derive a digital filter from the model, which can be readily used to simulate gusts for space vehicle design applications
Natural extension of the Generalised Uncertainty Principle
We discuss a gedanken experiment for the simultaneous measurement of the
position and momentum of a particle in de Sitter spacetime. We propose an
extension of the so-called generalized uncertainty principle (GUP) which
implies the existence of a minimum observable momentum. The new GUP is directly
connected to the nonzero cosmological constant, which becomes a necessary
ingredient for a more complete picture of the quantum spacetime.Comment: 4 pages, 1 figure, v2 with added references, revised and extended as
published in CQ
Quantitative assessment of cell fate decision between autophagy and apoptosis
Abstract Autophagy and apoptosis are cellular processes that regulate cell survival and death, the former by eliminating dysfunctional components in the cell, the latter by programmed cell death. Stress signals can induce either process, and it is unclear how cells ‘assess’ cellular damage and make a ‘life’ or ‘death’ decision upon activating autophagy or apoptosis. A computational model of coupled apoptosis and autophagy is built here to analyze the underlying signaling and regulatory network dynamics. The model explains the experimentally observed differential deployment of autophagy and apoptosis in response to various stress signals. Autophagic response dominates at low-to-moderate stress; whereas the response shifts from autophagy (graded activation) to apoptosis (switch-like activation) with increasing stress intensity. The model reveals that cytoplasmic Ca2+ acts as a rheostat that fine-tunes autophagic and apoptotic responses. A G-protein signaling-mediated feedback loop maintains cytoplasmic Ca2+ level, which in turn governs autophagic response through an AMP-activated protein kinase (AMPK)-mediated feedforward loop. Ca2+/calmodulin-dependent kinase kinase β (CaMKKβ) emerges as a determinant of the competing roles of cytoplasmic Ca2+ in autophagy regulation. The study demonstrates that the proposed model can be advantageously used for interrogating cell regulation events and developing pharmacological strategies for modulating cell decisions
Is the Pre-WMAP CMB Data Self-consistent?
Although individual observational groups vigorously test their data sets for
systematic errors, the pre-WMAP CMB observational data set has not yet been
collectively tested. Under the assumption that the concordance model is the
correct model, we have explored residuals of the observational data with
respect to this model to see if any patterns emerge that can be identified with
systematic errors. We found no significant trends associated with frequency,
frequency channels, calibration source, pointing uncertainty, instrument type,
platform and altitude. We did find some evidence at the ~ 1 to ~ 2 sigma level
for trends associated with angular scale (l range) and absolute galactic
latitude. The slope of the trend in galactic latitude is consistent with low
level galactic contamination. The residuals with respect to l may indicate that
the concordance model used here needs slight modification. See Griffiths &
Lineweaver (2003) for more detail.Comment: 8 pages, 4 figures, to be published in the proceedings of "The Cosmic
Microwave Background and its Polarization", New Astronomy Reviews, (eds. S.
Hanany and K.A. Olive
Is the Lambda CDM Model Consistent with Observations of Large-Scale Structure?
The claim that large-scale structure data independently prefers the Lambda
Cold Dark Matter model is a myth. However, an updated compilation of
large-scale structure observations cannot rule out Lambda CDM at 95%
confidence. We explore the possibility of improving the model by adding Hot
Dark Matter but the fit becomes worse; this allows us to set limits on the
neutrino mass.Comment: To appear in Proceedings of "Sources and Detection of Dark
Matter/Energy in the Universe", ed. D. B. Cline. 6 pages, including 2 color
figure
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