1,804 research outputs found
A unified approach to scaling solutions in a general cosmological background
Our ignorance about the source of cosmic acceleration has stimulated study of
a wide range of models and modifications to gravity. Cosmological scaling
solutions in any of these theories are privileged because they represent
natural backgrounds relevant to dark energy. We study scaling solutions in a
generalized background in the presence of a scalar field
\vp and a barotropic perfect fluid, where is a Hubble rate and
is a total energy density. The condition for the existence of scaling solutions
restricts the form of Lagrangian to be p=X^{1/n}g(Xe^{n\lambda \vp}), where
X=-g^{\mu\nu} \partial_\mu \vp \partial_\nu \vp /2 and is an arbitrary
function. This is very useful to find out scaling solutions and corresponding
scalar-field potentials in a broad class of dark energy models including
(coupled)-quintessence, ghost-type scalar field, tachyon and k-essence. We
analytically derive the scalar-field equation of state w_\vp and the
fractional density \Omega_\vp and apply it to a number of dark energy models.Comment: 7 pages, no figures, references updated; final version to appear in
PL
Gravity and Crossing the Phantom Divide Barrier
The gravity models formulated in Einstein conformal frame are
equivalent to Einstein gravity together with a minimally coupled scalar field.
We shall explore phantom behavior of models in this frame and compare
the results with those of the usual notion of phantom scalar field.Comment: 13 Pages, 9 figures. To appear in Physics Letters
Observational constraints on thawing quintessence models
We use a dynamical systems approach to study thawing quintessence models,
using a multi-parameter extension of the exponential potential which can
approximate the form of typical thawing potentials. We impose observational
constraints using a compilation of current data, and forecast the tightening of
constraints expected from future dark energy surveys, as well as discussing the
relation of our results to analytical constraints already in the literature.Comment: 6 pages MNRAS style with 8 figures included. Minor updates to match
MNRAS accepted versio
2-loop Quantum Yang-Mills Condensate as Dark Energy
In seeking a model solving the coincidence problem, the effective Yang-Mills
condensate (YMC) is an alternative candidate for dark energy. A study is made
for the model up to the 2-loop order of quantum corrections. It is found that,
like in the 1-loop model, for generic initial conditions during the radiation
era, there is always a desired tracking solution, yielding the current status
and . As the time the dynamics is a stable attractor. Thus the model naturally solves the
coincidence problem of dark energy. Moreover, if YMC decays into matter, its
equation of state (EoS) crosses -1 and takes , as indicated by the
recent observations.Comment: 9 pages, 4 figures. Phys. Lett. B accepte
Recognition-mediated hydrogel swelling controlled by interaction with a negative thermoresponsive LCST polymer
Most polymeric thermoresponsive hydrogels contract upon heating beyond the lower critical solution temperature (LCST) of the polymers used. Herein, we report a supramolecular hydrogel system that shows the opposite temperature dependence. When the non-thermosesponsive hydrogel NaphtGel, containing dialkoxynaphthalene guest molecules, becomes complexed with the tetra cationic macrocyclic host CBPQT4+, swelling occurred as a result of hostâguest complex formation leading to charge repulsion between the host units, as well as an osmotic contribution of chloride counter-ions embedded in the network. The immersion of NaphtGel in a solution of poly(N-isopropylacrylamide) with tetrathiafulvalene (TTF) end groups complexed with CBPQT4+ induced positive thermoresponsive behaviour. The LCST-induced dethreading of the polymer-based pseudorotaxane upon heating led to transfer of the CBPQT4+ host and a concomitant swelling of NaphtGel. Subsequent cooling led to reformation of the TTF-based hostâguest complexes in solution and contraction of the hydrogel
Asymptotic behavior of w in general quintom model
For the quintom models with arbitrary potential , the
asymptotic value of equation of state parameter w is obtained by a new method.
In this method, w of stable attractors are calculated by using the ratio (d ln
V)/(d ln a) in asymptotic region. All the known results, have been obtained by
other methods, are reproduced by this method as specific examples.Comment: 8 pages, one example is added, accepted for publication in Gen. Rel.
Gra
A structural and dynamic model for the assembly of Replication Protein A on single-stranded DNA
Replication Protein A (RPA), the major eukaryotic single stranded DNA-binding protein, binds to exposed ssDNA to protect it from nucleases, participates in a myriad of nucleic acid transactions and coordinates the recruitment of other important players. RPA is a heterotrimer and coats long stretches of single-stranded DNA (ssDNA). The precise molecular architecture of the RPA subunits and its DNA binding domains (DBDs) during assembly is poorly understood. Using cryo electron microscopy we obtained a 3D reconstruction of the RPA trimerisation core bound with ssDNA (âŒ55âkDa) at âŒ4.7âĂ
resolution and a dimeric RPA assembly on ssDNA. FRET-based solution studies reveal dynamic rearrangements of DBDs during coordinated RPA binding and this activity is regulated by phosphorylation at S178 in RPA70. We present a structural model on how dynamic DBDs promote the cooperative assembly of multiple RPAs on long ssDNA
Optical (VRI) Photometry in the Field of the Galaxy Cluster AC 118 at z=0.31
We present new photometric V-, R- and I-band data for the rich galaxy cluster
AC 118 at z=0.31. The new photometry covers an area of 8.6x8.6 arcmin,
corresponding to 2.92.9 Mpc (H km s Mpc,
q and ). The data have been collected for a project aimed
at studying galaxy evolution through the color-magnitude relation and the
fundamental plane. We provide a catalogue including all the sources (N = 1206)
detected in the cluster field. The galaxy sample is complete to V=22.8 mag
(N_gal=574), R=22.3 mag (N_gal=649) and I=20.8 mag (N_gal=419). We give
aperture magnitudes within a fixed aperture of 4.4 arcsec and Kron magnitudes.
We also give photometric redshifts for 459 sources for which additional U- and
K-band photometry is available. We derive and discuss the V- and R-band
luminosity functions. The catalogue, which is distributed in electronic form,
is intended as a tool for studies in galaxy evolution.Comment: 15 pages, 6 figures, A&A in pres
High growth rate 4H-SiC epitaxial growth using dichlorosilane in a hot-wall CVD reactor
Thick, high quality 4H-SiC epilayers have been grown in a vertical hot-wall
chemical vapor deposition system at a high growth rate on (0001) 80 off-axis
substrates. We discuss the use of dichlorosilane as the Si-precursor for 4H-SiC
epitaxial growth as it provides the most direct decomposition route into SiCl2,
which is the predominant growth species in chlorinated chemistries. A specular
surface morphology was attained by limiting the hydrogen etch rate until the
system was equilibrated at the desired growth temperature. The RMS roughness of
the grown films ranged from 0.5-2.0 nm with very few morphological defects
(carrots, triangular defects, etc.) being introduced, while enabling growth
rates of 30-100 \mum/hr, 5-15 times higher than most conventional growths.
Site-competition epitaxy was observed over a wide range of C/Si ratios, with
doping concentrations < 1x1014 cm-3 being recorded. X-ray rocking curves
indicated that the epilayers were of high crystallinity, with linewidths as
narrow as 7.8 arcsec being observed, while microwave photoconductive decay
(\muPCD) measurements indicated that these films had high injection (ambipolar)
carrier lifetimes in the range of 2 \mus
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