1,144 research outputs found
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
with respect to the cosmic time (i.e. ). Thus, the
Hubble parameter can be measured through the relation . 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 plays a significant role in this dark energy model. Firstly we
consider three fixed values of =0.6, 1.0 and 1.4 in the fitting of data. If
we set free, the best fitting values are , ,
. It is shown that the holographic dark energy behaves like a
quintom-type at the level. This result is consistent with some other
independent cosmological constrains, which imply that 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
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 , which
is consistent with Riess et al. The transition redshift is
. We also discuss several dark energy models that
define the of the parameterized equation of state of dark energy
including one parameter and two parameters ( being the ratio of the
pressure to energy density). Our calculations show that the accurately
calculated transition redshift varies from to
across these models. We also calculate the minimum
redshift at which the current observations need the universe to
accelerate.Comment: 16 pages, 5 figures, 1 tabl
The Case for an Accelerating Universe from Supernovae
The unexpected faintness of high-redshift Type Ia supernovae (SNe Ia), as
measured by two teams, has been interpreted as evidence that the expansion of
the Universe is accelerating. We review the current challenges to this
interpretation and seek to answer whether the cosmological implications are
compelling. We discuss future observations of SNe Ia which could offer
extraordinary evidence to test acceleration.Comment: To appear as an Invited Review for PASP 20 pages, 13 figure
Yang-Mills condensate dark energy coupled with matter and radiation
The coincidence problem is studied for the dark energy model of effective
Yang-Mills condensate in a flat expanding universe during the matter-dominated
stage. The YMC energy is taken to represent the dark energy, which
is coupled either with the matter, or with both the matter and the radiation
components. The effective YM Lagrangian is completely determined by quantum
field theory up to 1-loop order. It is found that under very generic initial
conditions and for a variety of forms of coupling, the existence of the scaling
solution during the early stages and the subsequent exit from the scaling
regime are inevitable. The transition to the accelerating stage always occurs
around a redshift . Moreover, when the Yang-Mills
condensate transfers energy into matter or into both matter and radiation, the
equation of state of the Yang-Mills condensate can cross over -1 around
, and takes on a current value . This is consistent with
the recent preliminary observations on supernovae Ia. Therefore, the
coincidence problem can be naturally solved in the effective YMC dark energy
models.Comment: 24 pages, 18 figure
Detection of a redshift 3.04 filament
The filamentary structure of the early universe has until now only been seen
in numerical simulations. Despite this lack of direct observational evidence,
the prediction of early filamentary structure formation in a Cold Dark Matter
dominated universe has become a paradigm for our understanding of galaxy
assembly at high redshifts. Clearly observational confirmation is required.
Lyman Break galaxies are too rare to be used as tracers of filaments and we
argue that to map out filaments in the high z universe, one will need to
identify classes of objects fainter than those currently accessible via the
Lyman Break technique. Objects selected via their Ly-alpha emission, and/or as
DLA absorbers, populate the faintest accessible part of the high redshift
galaxy luminosity function, and as such make up good candidates for objects
which will map out high redshift filaments. Here we present the first direct
detection of a filament (at z=3.04) mapped by those classes of objects. The
observations are the deepest yet to have been done in Ly-alpha imaging at high
redshift, and they reveal a single string of proto-galaxies spanning about 5
Mpc (20 Mpc comoving). Expanding the cosmological test proposed by Alcock &
Paczynski (1979), we outline how observations of this type can be used to
determine Omega_Lambda at z=3.Comment: 5 pages, LaTeX, 3 PostScript figures; Accepted for publication in
A&A-Letter
No realistic wormholes from ghost-free scalar-tensor phantom dark energy
It is proved that no wormholes can be formed in viable scalar-tensor models
of dark energy admitting its phantom-like () behaviour in cosmology,
even in the presence of electric or magnetic fields, if the non-minimal
coupling function is everywhere positive and the scalar field
itself is not a ghost. Some special static, spherically symmetric wormhole
solutions may exist if is allowed to reach zero or to become
negative, so that the effective gravitational constant becomes negative in some
region making the graviton a ghost. If remains non-negative, such solutions
require severe fine tuning and a very peculiar kind of model. If is
allowed, it is argued (and confirmed by previous investigations) that such
solutions are generically unstable under non-static perturbations, the
instability appearing right near transition surfaces to negative .Comment: 8 pages, late
Parametrization of dark energy equation of state Revisited
A comparative study of various parametrizations of the dark energy equation
of state is made. Astrophysical constraints from LSS, CMB and BBN are laid down
to test the physical viability and cosmological compatibility of these
parametrizations. A critical evaluation of the 4-index parametrizations reveals
that Hannestad-M\"{o}rtsell as well as Lee parametrizations are simple and
transparent in probing the evolution of the dark energy during the expansion
history of the universe and they satisfy the LSS, CMB and BBN constraints on
the dark energy density parameter for the best fit values.Comment: 11 page
Blood group determinates incidence for pancreatic cancer in Germany
Background: Genetic risk factors for sporadic pancreatic cancer are largely unknown but actually under high exposure. Findings of correlations between the AB0 blood group system (Chromosome 9q34,1-q34,2) and the risk of pancreatic cancer (PC) in patients from Asia, America and south Europe have already been published. So far it is unclear, whether this correlation between blood group an PC incidence can be found in German patients as well. Methods: One hundred and sixty-six patients who underwent a resection of PC were evaluated in a period between 2000 and 2010. Blood group reference distribution for the German population is given as: 0: 41%; A: 43%; B: 11%; AB: 5%; Rhesus positive: 85%; Rhesus negative: 15%. Analyses were done using the non-parametric Chi(2)-test (p-value two sided; SPSS 19.0). Results: Median age was 62 (34-82) years. Gender: female 73/44%; male: 93/56%. Observed blood group proportions: 0: 43 (25.9%)/A: 94 (56.6%)/B: 16 (9.6%)/AB: 13 (7.8%)/Rhesus positive: 131 (78.9%)/negative: 35 (21.1%). We detected a significant difference to the German reference distribution of the AB0 system (Chi(2) 19.34, df 3, p < 0.001). Rhesus factor has no impact on AB0-distribution (Chi(2) 4.13, df 3, p = 0.25), but differs significantly from reference distribution-probably due to initial AB0-variation (Chi(2) 4.82, df 1, p = 0.028). The odds ratio for blood group A is 2.01 and for blood group 0 is 0.5. Conclusions: The incidence of PC in the German cohort is highly associated with the AB0-system as well. More patients with blood group A suffer from PC (p < 0.001) whereas blood group 0 was less frequent in patients with PC (p < 0.001). Thus, our findings support the results from other non-German surveys. The causal trigger points of this carcinogenesis correlation are still not known
Intensified chemotherapy and simultaneous treatment with heparin in outpatients with pancreatic cancer - the CONKO 004 pilot trial
BACKGROUND: Advanced pancreatic cancer (APC), beside its high mortality, causes the highest rates of venous thromboembolic events (VTE). Enoxaparin, a low molecular weight heparin (LMWH), is effective in prevention and treatment of VTE. Some small studies indicated that this benefit might extend to patients with cancer and probably prolong survival due to independent mechanisms. We initiated this safety investigation to get feasibility information on intensified chemotherapy combined with LMWH in outpatients with APC treated in 1st line. METHODS: The trial was a prospective, open-label, single center investigation in outpatients with inoperable pancreatic cancer who were treated with intensified first-line chemotherapy along with concomitant application of subcutaneous LMWH. The combined chemotherapy consisted of gemcitabine 1 g/m2 (30 min), 5-FU 750 mg/m2 (24 h), folinic acid 200 mg/m2 (30 min), and Cisplatin 30 mg/m2 (90 min) on day 1 and 8; q3w for the first 12 weeks (GFFC) followed by gemcitabine alone in patients without cancer progression. The simultaneous application of prophylactic enoxaparin started on day 1 of chemotherapy with a fixed dose of 40 mg daily. Statistical analyses were performed using R 3.01 with software package CMPRSK and SPSS software v19.0. RESULTS: The investigation was stopped after recruitment of 19 patients. At this time 15 patients had completed the required 12 weeks of treatment. Based on 71 cycles of GFFC + enoxaparin (median 4/pt [range: 2-4]) and 108 cycles of single-agent gemcitabine + enoxaparin (median 4/pt [range: 0-18]) the cumulative frequency of NCI-CTC toxicities grade 3/4 was below 10%. One case (5%) of a symptomatic non-lethal thromboembolic event was observed while receiving LMWH treatment. No severe bleeding event as defined in the protocol has been observed. The median overall survival was 10.05 [95%CI: 8.67-18.14] months. CONCLUSIONS: The addition of enoxaparin to GFFC chemotherapy is feasible, safe and does not appear to affect the efficacy or the toxicity profile of the chemotherapy regimen in patients with advanced pancreatic adenocarcinoma. Based on these findings we have initiated the randomized CONKO-004 trial to examine whether enoxaparin reduces the incidence of thromboembolic events or increases overall outcome. Trial registration: Clinical Trials NCT01945879
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