502 research outputs found
Cosmic acceleration in a model of scalar-tensor gravitation
In this paper we consider a model of scalar-tensor theory of gravitation in
which the scalar field, determines the gravitational coupling G and has
a Lagrangian of the form, . We study the cosmological consequence
of this theory in the matter dominated era and show that this leads to a
transition from an initial decelerated expansion to an accelerated expansion
phase at the present epoch. Using observational constraints, we see that the
effective equation of state today for the scalar field turns out to be
, with and that the transition
to an accelerated phase happened at a redshift of about 0.3.Comment: 12 pages, 2 figures, matches published versio
Dark matter and stable bound states of primordial black holes
We present three reasons for the formation of gravitational bound states of
primordial black holes,called holeums,in the early universe.Using Newtonian
gravity and nonrelativistic quantum mechanics we find a purely quantum
mechanical mass-dependant exclusion property for the nonoverlap of the
constituent black holes in a holeum.This ensures that the holeum occupies space
just like ordinary matter.A holeum emits only gravitational radiation whose
spectrum is an exact analogue of that of a hydrogen atom. A part of this
spectrum lies in the region accessible to the detectors being built.The holeums
would form haloes around the galaxies and would be an important component of
the dark matter in the universe today.They may also be the constituents of the
invisible domain walls in the universe.Comment: 13 pages,2tables,for wider circulation,PD
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%
Liquid droplet formation by HP1α suggests a role for phase separation in heterochromatin.
Gene silencing by heterochromatin is proposed to occur in part as a result of the ability of heterochromatin protein 1 (HP1) proteins to spread across large regions of the genome, compact the underlying chromatin and recruit diverse ligands. Here we identify a new property of the human HP1α protein: the ability to form phase-separated droplets. While unmodified HP1α is soluble, either phosphorylation of its N-terminal extension or DNA binding promotes the formation of phase-separated droplets. Phosphorylation-driven phase separation can be promoted or reversed by specific HP1α ligands. Known components of heterochromatin such as nucleosomes and DNA preferentially partition into the HP1α droplets, but molecules such as the transcription factor TFIIB show no preference. Using a single-molecule DNA curtain assay, we find that both unmodified and phosphorylated HP1α induce rapid compaction of DNA strands into puncta, although with different characteristics. We show by direct protein delivery into mammalian cells that an HP1α mutant incapable of phase separation in vitro forms smaller and fewer nuclear puncta than phosphorylated HP1α. These findings suggest that heterochromatin-mediated gene silencing may occur in part through sequestration of compacted chromatin in phase-separated HP1 droplets, which are dissolved or formed by specific ligands on the basis of nuclear context
Experimental study of magneto-superconductor RuSr2Eu1.5Ce0.5Cu2O10: Effect of Mo doping on magnetic behavior and Tc variation
Mo doped ruthenocuprates Ru1-xMoxSr2Eu1.5Ce0.5Cu2O10 are synthesized for x =
0.0, 0.2, 0.4, 0.6, 0.8 and 1.0, and their magnetic and superconducting
properties are studied. It has been found that the magnetic transition
temperature TZFCpeak, which corresponds to the appearance of weak ferromagnetic
effect, decreases from its value of 75 K for x = 0.0 to 22 K, 25 K and 18 K,
respectively for the x = 0.2, 0.4 and 0.6 samples. Another finding is that the
magnetic susceptibility reduces at TZFCpeak by a factor of about 6, 85 and 413
for x = 0.2, 0.4, and 0.6 respectively. The samples of x = 0.8 and 1.0 are
found to have no magnetic or superconducting effects. The values of the
superconducting transition temperature are obtained from the resistivity versus
temperature data. An important result is that Tc increases by 4.5 K and 7.0 K
for x = 0.2 and 0.4 respectively, and then decreases by 17 K for x = 0.6. The
observed variation of Tc with x has been explained in terms of a theory which
combines the effects of weakening magnetic behavior and reducing carrier
concentration in a phenomenological manner. The resulting theory is found to
provide a good agreement with the observed value of Tc.Comment: 14 pages with Text + Figs. To Appear in PHYS. REV. B, Ist Jan. 2006
issu
Anomalous thermoelectric power of Mg1-xAlxB2 system with x = 0.0 to 1.0
Thermoelectric power, S(T) of the Mg1-xAlxB2 system has been measured for x =
0.0, 0.1, 0.2, 0.4, 0.6, 0.8 and 1.0. XRD, resistivity and magnetization
measurements are also presented. It has been found that the thermoelectric
power is positive for x = 0.4 and is negative for x = 0.6 over the entire
temperature range studied up to 300 K. The thermoelectric power of x = 0.4
samples vanishes discontinuously below a certain temperature, implying
existence of superconductivity. In general, the magnitude of the thermoelectric
power increases with temperature up to a certain temperature, and then it
starts to decrease towards zero base line. In order to explain the observed
behavior of the thermoelectric power, we have used a model in which both
diffusion and phonon drag processes are combined by using a phenomenological
interpolation between the low and high temperature behaviors of the
thermoelectric power. The considered model provides an excellent fit to the
observed data. It is further found that Al doping enhances the Debye
temperature.Comment: 19 pages Text + Figs.
suggestions/comments([email protected]
Broadening of Spectral Lines due to Dynamic Multiple Scattering and the Tully-Fisher Relation
The frequency shift of spectral lines is most often explained by the Doppler
Effect in terms of relative motion, whereas the Doppler broadening of a
particular line mainly depends on the absolute temperature. The Wolf effect on
the other hand deals with the correlation induced spectral change and explains
both the broadening and shift of the spectral lines. In this framework a
relation between the width of the spectral line is related to the redshift z
for the line and hence with the distance. For smaller values of z a relation
similar to the Tully-Fisher relation can be obtained and for larger values of z
a more general relation can be constructed. The derivation of this kind of
relation based on dynamic multiple scattering theory may play a significant
role in explaining the overall spectra of quasi stellar objects. We emphasize
that this mechanism is not applicable for nearby galaxies, .Comment: 18 pages, 5 figures, revised Version has been submitted to Physical
Review A. (2nd author's affiliation corrected
A Machian Model of Dark Energy
Einstein believed that Mach's principle should play a major role in finding a
meaningful spacetime geometry, though it was discovered later that his field
equations gave some solutions which were not Machian. It is shown, in this
essay, that the kinematical models, which are invoked to solve the
cosmological constant problem, are in fact consistent with Mach's ideas. One
particular model in this category is described which results from the
microstructure of spacetime and seems to explain the current observations
successfully and also has some benefits over the conventional models. This
forces one to think whether the Mach's ideas and the cosmological constant are
interrelated in some way.Comment: Received an Honorable mention in the Essay Contest-2002 sponsored by
the Gravity Research Foundation; A paragraph added on how the model can
explain the CMB anisotropy observations; To appear in the Classical and
Quantum Gravit
High Field Performance of Nano-Diamond Doped MgB2 Superconductor
Polycrystalline MgB2-nDx (x= 0 to 0.1) samples are synthesized by solid-state
route with ingredients of Mg, B and n-Diamond. The results from
magneto-transport and magnetization of nano-diamond doped MgB2-nDx are
reported. Superconducting transition temperature (Tc) is not affected
significantly by x up to x = 0.05 and latter decreases slightly for higher x >
0.05. R(T) vs H measurements show higher Tc values under same applied magnetic
fields for the nano-diamond added samples, resulting in higher estimated Hc2
values. From the magnetization measurements it was found that irreversibility
field value Hirr for the pristine sample is 7.5 Tesla at 4 K and the same is
increased to 13.5 Tesla for 3-wt% nD added sample at the same temperature. The
Jc(H) plots at all temperatures show that Jc value is lowest at all applied
fields for pristine MgB2 and the sample doped with 3-wt% nD gives the best Jc
values at all fields. For the pure sample the value of Jc is of the order of
105 A/cm2 at lower fields but it decreases very fast as the magnetic field is
applied and becomes negligible above 7 Tesla. The Jc is 40 times higher than
pure MgB2 at 10 K at 6 Tesla field in case of 3%nD doped sample and its value
is still of the order of 103 A/cm2 at 10 Tesla for the same sample. On the
other hand at 20K the 5%nD sample shows the best performance at higher fields.
These results are discussed in terms of extrinsic pinning due to dispersed
n-Diamond in the host MgB2 matrix along with the intrinsic pinning due to
possible substitution of C at Boron site and increased inter-band scattering
for highly doped samples resulting in extraordinary performance of the doped
system.Comment: 12 PAGES (TEXT+FIGS). ACCEPTED: J. APPL.PHYS. (MMM-2007 Proceedings
Path integral duality modified propagators in spacetimes with constant curvature
The hypothesis of path integral duality provides a prescription to evaluate
the propagator of a free, quantum scalar field in a given classical background,
taking into account the existence of a fundamental length, say, the Planck
length, \lp, in a {\it locally Lorentz invariant manner}. We use this
prescription to evaluate the duality modified propagators in spacetimes with
{\it constant curvature} (exactly in the case of one spacetime, and in the
Gaussian approximation for another two), and show that: (i) the modified
propagators are ultra violet finite, (ii) the modifications are {\it
non-perturbative} in \lp, and (iii) \lp seems to behave like a `zero point
length' of spacetime intervals such that \l =
\l[\sigma^{2}(x,x')+ {\cal O}(1) \lp^2 \r], where is the
geodesic distance between the two spacetime points and , and the
angular brackets denote (a suitable) average over the quantum gravitational
fluctuations. We briefly discuss the implications of our results.Comment: v1. 10 pages, no figures; v2. 11 pages, acknowledgments adde
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