3,723 research outputs found
Accelerated expansion in modified gravity with a Yukawa-like term
We discuss the Palatini formulation of modified gravity including a
Yukawa-like term. It is shown that in this formulation, the Yukawa term offers
an explanation for the current exponential accelerated expansion of the
universe and reduces to the standard Friedmann cosmology in the appropriate
limit. We then discuss the scalar-tensor formulation of the model as a metric
theory and show that the Yukawa term predicts a power-law acceleration at
late-times. The Newtonian limit of the theory is also discussed in context of
the Palatini formalism.Comment: 9 pages, 2 figures, to appear in IJMP
Model-Independent Distance Measurements from Gamma-Ray Bursts and Constraints on Dark Energy
Gamma-Ray Bursts (GRB) are the most energetic events in the Universe, and
provide a complementary probe of dark energy by allowing the measurement of
cosmic expansion history that extends to redshifts greater than 6. Unlike Type
Ia supernovae (SNe Ia), GRBs must be calibrated for each cosmological model
considered, because of the lack of a nearby sample of GRBs for
model-independent calibration. For a flat Universe with a cosmological
constant, we find Omega_m=0.25^{+0.12}_{-0.11} from 69 GRBs alone. We show that
the current GRB data can be summarized by a set of model-independent distance
measurements, with negligible loss of information. We constrain a dark energy
equation of state linear in the cosmic scale factor using these distance
measurements from GRBs, together with the "Union" compilation of SNe Ia, WMAP
five year observations, and the SDSS baryon acoustic oscillation scale
measurement. We find that a cosmological constant is consistent with current
data at 68% confidence level for a flat Universe. Our results provide a simple
and robust method to incorporate GRB data in a joint analysis of cosmological
data to constrain dark energy.Comment: 8 pages, 5 color figures. Version expanded and revised for
clarification, and typo in Eqs.(3)(4)(12) corrected. PRD, in pres
Possible role of the phagocytic proteinases, cathepsin B and elastas, in orthotopic liver transplantation
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Time-dependent radiative transfer with PHOENIX
Aims. We present first results and tests of a time-dependent extension to the
general purpose model atmosphere code PHOENIX. We aim to produce light curves
and spectra of hydro models for all types of supernovae. Methods. We extend our
model atmosphere code PHOENIX to solve time-dependent non-grey, NLTE, radiative
transfer in a special relativistic framework. A simple hydrodynamics solver was
implemented to keep track of the energy conservation of the atmosphere during
free expansion. Results. The correct operation of the new additions to PHOENIX
were verified in test calculations. Conclusions. We have shown the correct
operation of our extension to time-dependent radiative transfer and will be
able to calculate supernova light curves and spectra in future work.Comment: 7 pages, 12 figure
Possible role of extracellularly released phagocytic proteinases in the coagulation disorder during liver transplantation
Orthotopic liver transplantation is frequently associated with a complex coagulation disorder, influencing the outcome of the procedure. In this respect, disseminated intravascular coagulation (DIC) had been suggested to be of causative importance for bleeding complications after reperfusion of the liver graft. In 10 consecutive patients undergoing orthotopic liver transplantations, we studied the occurrence of two phagocyte proteinases of different origin in the graft liver perfus-ate and in systemic blood during the operation, as well as their effects on hemostasis. As compared with plasma samples taken at the end of the anhepatic phase, highly significant increases of cathepsin B and thrombin-anti-thrombin III complexes (TAT), as well as highly significant decreases in antithrombin III, protein C, and C1-inhibitor were observed in graft liver perfusate. Von Willebrand factor and fibrinogen were slightly decreased, whereas the elastase-alpha1 proteinase inhibitor complexes (EPI) were elevated. In plasma the activity of cathepsin B remained unchanged during the prereperfusion phases, but immediately after revascularization of the graft this cysteine proteinase increased. The EPI showed a gradual increase in plasma during the preanhepatic and anhepatic phases but a more pronounced increase in the reperfusion phase. In parallel with the rise in these two proteinases TAT increased and the activities of antithrombin III and C1-inhibitor in plasma decreased after reperfusion. At 12 hr after revascularization plasma levels of TAT, antithrombin III, and C1-inhibitor had returned to the prereperfusion ranges, whereas cathepsin B and EPI were significantly above the baseline levels. These observations are consistent with the hypothesis that extracellularly released lysosomal proteinases may play a role in the development of a DIC-like constellation, including thrombin formation after revascularization of the liver graft. For the first time we could prove the occurrence of phagocyte proteinases in graft liver perfusate and evaluate the importance of these proteinases for the understanding of the pathophysiology leading to bleeding complications in patients undergoing orthotopic liver transplantation
The Deepest Supernova Search is Realized in the Hubble Ultra Deep Field Survey
The Hubble Ultra Deep Field Survey has not only provided the deepest optical
and near infrared views of universe, but has enabled a search for the most
distant supernovae to z~2.2. We have found four supernovae by searching spans
of integrations of the Ultra Deep Field and the Ultra Deep Field Parallels
taken with the Hubble Space Telescope paired with the Advanced Camera for
Surveys and the Near Infrared Multi Object Spectrometer. Interestingly, none of
these supernovae were at z>1.4, despite the substantially increased sensitivity
per unit area to such objects over the Great Observatories Origins Deep Survey.
We present the optical photometric data for the four supernovae. We also show
that the low frequency of Type Ia supernovae observed at z>1.4 is statistically
consistent with current estimates of the global star formation history combined
with the non-trivial assembly time of SN Ia progenitors.Comment: 24 pages (6 figures), submitted to the Astronomical Journa
Cosmological implications of massive gravitons
The van Dam-Veltman-Zakharov (vDVZ) discontinuity requires that the mass
of the graviton is exactly zero, otherwise measurements of the deflection of
starlight by the Sun and the precession of Mercury's perihelion would conflict
with their theoretical values. This theoretical discontinuity is open to
question for numerous reasons. In this paper we show from a phenomenological
viewpoint that the hypothesis is in accord with Supernova Ia and CMB
observations, and that the large scale structure of the universe suggests that
eV.Comment: 13 pages, 1 figur
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