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

Dark Energy Accretion onto a Black Hole in an Expanding Universe

By using the solution describing a black hole embedded in the FLRW universe, we obtain the evolving equation of the black hole mass expressed in terms of the cosmological parameters. The evolving equation indicates that in the phantom dark energy universe the black hole mass becomes zero before the Big Rip is reached.Comment: 7 pages, no figures, errors is correcte

Possible role of the phagocytic proteinases, cathepsin B and elastas, in orthotopic liver transplantation

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Naked Singularity in a Modified Gravity Theory

The cosmological constant induced by quantum fluctuation of the graviton on a given background is considered as a tool for building a spectrum of different geometries. In particular, we apply the method to the Schwarzschild background with positive and negative mass parameter. In this way, we put on the same level of comparison the related naked singularity (-M) and the positive mass wormhole. We discuss how to extract information in the context of a f(R) theory. We use the Wheeler-De Witt equation as a basic equation to perform such an analysis regarded as a Sturm-Liouville problem . The application of the same procedure used for the ordinary theory, namely f(R)=R, reveals that to this approximation level, it is not possible to classify the Schwarzschild and its naked partner into a geometry spectrum.Comment: 8 Pages. Contribution given to DICE 2008. To appear in the proceeding

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

Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

The property of dark energy and the physical reason for acceleration of the present universe are two of the most difficult problems in modern cosmology. The dark energy contributes about two-thirds of the critical density of the present universe from the observations of type-Ia supernova (SNe Ia) and anisotropy of cosmic microwave background (CMB).The SN Ia observations also suggest that the universe expanded from a deceleration to an acceleration phase at some redshift, implying the existence of a nearly uniform component of dark energy with negative pressure. We use the ``gold'' sample containing 157 SNe Ia and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the properties of dark energy and the transition redshift. For a flat universe with the cosmological constant, we measure $\Omega_{M}=0.28_{-0.05}^{+0.04}$, which is consistent with Riess et al. The transition redshift is $z_{T}=0.60_{-0.08}^{+0.06}$. We also discuss several dark energy models that define the $w(z)$ of the parameterized equation of state of dark energy including one parameter and two parameters ($w(z)$ being the ratio of the pressure to energy density). Our calculations show that the accurately calculated transition redshift varies from $z_{T}=0.29_{-0.06}^{+0.07}$ to $z_{T}=0.60_{-0.08}^{+0.06}$ across these models. We also calculate the minimum redshift $z_{c}$ at which the current observations need the universe to accelerate.Comment: 16 pages, 5 figures, 1 tabl

Is there Evidence for a Hubble bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies

We examine recent evidence from the luminosity-redshift relation of Type Ia Supernovae (SNe Ia) for the $\sim 3 \sigma$ detection of a ``Hubble bubble'' -- a departure of the local value of the Hubble constant from its globally averaged value \citep{Jha:07}. By comparing the MLCS2k2 fits used in that study to the results from other light-curve fitters applied to the same data, we demonstrate that this is related to the interpretation of SN color excesses (after correction for a light-curve shape-color relation) and the presence of a color gradient across the local sample. If the slope of the linear relation ($\beta$) between SN color excess and luminosity is fit empirically, then the bubble disappears. If, on the other hand, the color excess arises purely from Milky Way-like dust, then SN data clearly favors a Hubble bubble. We demonstrate that SN data give $\beta \simeq 2$, instead of the $\beta \simeq 4$ one would expect from purely Milky-Way-like dust. This suggests that either SN intrinsic colors are more complicated than can be described with a single light-curve shape parameter, or that dust around SN is unusual. Disentangling these possibilities is both a challenge and an opportunity for large-survey SN Ia cosmology.Comment: Further information and data at http://qold.astro.utoronto.ca/conley/bubble/ Accepted for publication in ApJ

Late-Time Photometry of Type Ia Supernova SN 2012cg Reveals the Radioactive Decay of 57^{57}Co

Seitenzahl et al. (2009) have predicted that roughly three years after its explosion, the light we receive from a Type Ia supernova (SN Ia) will come mostly from reprocessing of electrons and X-rays emitted by the radioactive decay chain $^{57}{\rm Co}~\to~^{57}{\rm Fe}$, instead of positrons from the decay chain $^{56}{\rm Co}~\to~^{56}{\rm Fe}$ that dominates the SN light at earlier times. Using the {\it Hubble Space Telescope}, we followed the light curve of the SN Ia SN 2012cg out to $1055$ days after maximum light. Our measurements are consistent with the light curves predicted by the contribution of energy from the reprocessing of electrons and X-rays emitted by the decay of $^{57}$Co, offering evidence that $^{57}$Co is produced in SN Ia explosions. However, the data are also consistent with a light echo $\sim14$ mag fainter than SN 2012cg at peak. Assuming no light-echo contamination, the mass ratio of $^{57}$Ni and $^{56}$Ni produced by the explosion, a strong constraint on any SN Ia explosion model, is $0.043^{+0.012}_{-0.011}$, roughly twice Solar. In the context of current explosion models, this value favors a progenitor white dwarf with a mass near the Chandrasekhar limit.Comment: Updated to reflect the final version published by ApJ. For a video about the paper, see https://youtu.be/t3pUbZe8wq

A Possible Late Time Λ\LambdaCDM-like Background Cosmology in Relativistic MOND Theory

In the framework of Relativistic MOND theory (TeVeS), we show that a late time background $\Lambda$CDM cosmology can be attained by choosing a specific $F(\mu)$ that also meets the requirement for the existence of Newtonian and MOND limits. We investigate the dynamics of the scalar field $\phi$ under our chosen $F(\mu)$ and show that the "slow roll" regime of $\phi$ corresponds to a dynamical attractor, where the whole system reduces to $\Lambda$CDM cosmology.Comment: Major revisions made; Matching the version to be published in IJMP

Constraints on holographic dark energy models using the differential ages of passively evolving galaxies

Using the absolute ages of passively evolving galaxies observed at different redshifts, one can obtain the differential ages, the derivative of redshift $z$ with respect to the cosmic time $t$ (i.e. ${\rm d} z/{\rm d}t$). Thus, the Hubble parameter $H(z)$ can be measured through the relation $H(z)=-({\rm d} z/{\rm d}t)/(1+z)$. By comparing the measured Hubble parameter at different redshifts with the theoretical one containing free cosmological parameters, one can constrain current cosmological models. In this paper, we use this method to present the constraint on a spatially flat Friedman-Robert-Walker Universe with a matter component and a holographic dark energy component, in which the parameter $c$ plays a significant role in this dark energy model. Firstly we consider three fixed values of $c$=0.6, 1.0 and 1.4 in the fitting of data. If we set $c$ free, the best fitting values are $c=0.26$, $\Omega_{\rm m0}=0.16$, $h=0.9998$. It is shown that the holographic dark energy behaves like a quintom-type at the $1\sigma$ level. This result is consistent with some other independent cosmological constrains, which imply that $c<1.0$ is favored. We also test the results derived from the differential ages using another independent method based on the lookback time to galaxy clusters and the age of the universe. It shows that our results are reliable.Comment: 18 pages including 7 figures and 1 tables. Final version for publication in Modern Physics Letters A (MPLA)[minor revision to match the appear version