536 research outputs found
Inverse approach to Einstein's equations for fluids with vanishing anisotropic stress tensor
We expand previous work on an inverse approach to Einstein Field Equations
where we include fluids with energy flux and consider the vanishing of the
anisotropic stress tensor. We consider the approach using warped product
spacetimes of class . Although restricted, these spacetimes include many
exact solutions of interest to compact object studies and to cosmological
models studies. The question explored here is as follows: given a spacetime
metric, what fluid flow (timelike congruence), if any, could generate the
spacetime via Einstein's equations. We calculate the flow from the condition of
a vanishing anisotropic stress tensor and give results in terms of the metric
functions in the three canonical types of coordinates. A condition for perfect
fluid sources is also provided. The framework developed is algorithmic and
suited for the study and validation of exact solutions using computer algebra
systems. The framework can be applied to solutions in comoving and non-comoving
frames of reference, and examples in different types of coordinates are worked
out.Comment: 15 pages, matches version to appear in Phys.Rev.
A Doppler theory of quasars
We examine a Doppler theory of quasars in which it is assumed that a fraction of the total population of quasars are fired from centres of explosion with moderate cosmological redshifts. It is argued that the substantial part of the redshift of a typical high redshift quasar could be of Doppler origin. If Hoyle's recent hypothesis that quasars emit the bulk of their radiation in a narrow backward cone is given a quantitative form, it is shown that the kinematic and emission parameters of this model can explain the observed features of the four aligned triplets of quasars discovered by Arp and Hazard (1980) and by Saslaw (personal communication). The model predicts a small but nonzero fraction of quasars with blueshifts. Further observational tests of the model are discussed
Modeling Repulsive Gravity with Creation
There is a growing interest in the cosmologists for theories with negative
energy scalar fields and creation, in order to model a repulsive gravity. The
classical steady state cosmology proposed by Bondi, Gold and Hoyle in 1948, was
the first such theory which used a negative kinetic energy creation field to
invoke creation of matter. We emphasize that creation plays very crucial role
in cosmology and provides a natural explanation to the various explosive
phenomena occurring in local (z<0.1) and extra galactic universe. We exemplify
this point of view by considering the resurrected version of this theory - the
quasi-steady state theory, which tries to relate creation events directly to
the large scale dynamics of the universe and supplies more natural explanations
of the observed phenomena. Although the theory predicts a decelerating universe
at the present era, it explains successfully the recent SNe Ia observations
(which require an accelerating universe in the standard cosmology), as we show
in this paper by performing a Bayesian analysis of the data.Comment: The paper uses an old SNeIa dataset. With the new improved data, for
example the updated gold sample (Riess et al, astro-ph/0611572), the fit
improves considerably (\chi^2/DoF=197/180 and a probability of
goodness-of-fit=18%
Microwave properties of DyBa_2Cu_3O_(7-x) monodomains and related compounds in magnetic fields
We present a microwave characterization of a DyBaCuO
single domain, grown by the top-seeded melt-textured technique. We report the
(a,b) plane field-induced surface resistance, , at 48.3 GHz,
measured by means of a cylindrical metal cavity in the end-wall-replacement
configuration. Changes in the cavity quality factor Q against the applied
magnetic field yield at fixed temperatures. The temperature
range [70 K ; T_c] was explored. The magnetic field 0.8 T was
applied along the c axis. The field dependence of does not
exhibit the steep, step-like increase at low fields typical of weak-links. This
result indicates the single-domain character of the sample under investigation.
exhibits a nearly square-root dependence on H, as expected for
fluxon motion. From the analysis of the data in terms of motion of Abrikosov
vortices we estimate the temperature dependences of the London penetration
depth and the vortex viscosity , and their zero-temperature
values 165 nm and 3 10 Nsm, which are
found in excellent agreement with reported data in YBaCuO
single crystals. Comparison of microwave properties with those of related
samples indicate the need for reporting data as a function of T/T_c in order to
obtain universal laws.Comment: 6 pages, 4 figures, LaTeX, submitted to Journal of Applied Physic
Is the present expansion of the universe really accelerating?
The current observations are usually explained by an accelerating expansion
of the present universe. However, with the present quality of the supernovae Ia
data, the allowed parameter space is wide enough to accommodate the
decelerating models as well. This is shown by considering a particular example
of the dark energy equation-of-state ,
which is equivalent to modifying the \emph{geometrical curvature} index of
the standard cosmology by shifting it to where is a
constant. The resulting decelerating model is consistent with the recent CMB
observations made by WMAP, as well as, with the high redshift supernovae Ia
data including SN 1997ff at . It is also consistent with the newly
discovered supernovae SN 2002dc at and SN 2002dd at which
have a general tendency to improve the fit.Comment: Replaced with the accepted version to appear in MNRA
Carrier Transport in Magnesium Diboride: Role of Nano-inclusions
Anisotropic-gap and two-band effects smear out the superconducting transition
(Tc) in literature reported thermal conductivity of MgB2, where large
electronic contributions also suppress anomaly-manifestation in their
negligible phononic-parts. Present thermal transport results on scarcely
explored specimens featuring nano-inclusions exhibit a small but clear
Tc-signature, traced to relatively appreciable phononic conduction, and its
dominant electronic-scattering. The self-formed MgO as extended defects
strongly scatter the charge carriers and minutely the phonons with their
longer-mean-free-path near Tc. Conversely, near room temperature, the
shorter-dominant-wavelength phonon's transport is hugely affected by these
nanoparticles, undergoing ballistic to diffusive crossover and eventually
entering the Ioffe-Regel mobility threshold regime.Comment: 14 pages, 4 figures, 28 reference
Interpretations of the Accelerating Universe
It is generally argued that the present cosmological observations support the
accelerating models of the universe, as driven by the cosmological constant or
`dark energy'. We argue here that an alternative model of the universe is
possible which explains the current observations of the universe. We
demonstrate this with a reinterpretation of the magnitude-redshift relation for
Type Ia supernovae, since this was the test that gave a spurt to the current
trend in favour of the cosmological constant.Comment: 12 pages including 2 figures, minor revision, references added, a
paragraph on the interpretation of the CMB anisotropy in the QSSC added in
conclusion, general results unchanged. To appear in the October 2002 issue of
the "Publications of the Astronmical Society of the Pacific
The chromatin remodeller ACF acts as a dimeric motor to space nucleosomes.
Evenly spaced nucleosomes directly correlate with condensed chromatin and gene silencing. The ATP-dependent chromatin assembly factor (ACF) forms such structures in vitro and is required for silencing in vivo. ACF generates and maintains nucleosome spacing by constantly moving a nucleosome towards the longer flanking DNA faster than the shorter flanking DNA. How the enzyme rapidly moves back and forth between both sides of a nucleosome to accomplish bidirectional movement is unknown. Here we show that nucleosome movement depends cooperatively on two ACF molecules, indicating that ACF functions as a dimer of ATPases. Further, the nucleotide state determines whether the dimer closely engages one or both sides of the nucleosome. Three-dimensional reconstruction by single-particle electron microscopy of the ATPase-nucleosome complex in an activated ATP state reveals a dimer architecture in which the two ATPases face each other. Our results indicate a model in which the two ATPases work in a coordinated manner, taking turns to engage either side of a nucleosome, thereby allowing processive bidirectional movement. This novel dimeric motor mechanism differs from that of dimeric motors such as kinesin and dimeric helicases that processively translocate unidirectionally and reflects the unique challenges faced by motors that move nucleosomes
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