9,868 research outputs found
The Cosmology of Massless String Modes
We consider the spacetime dynamics of a gas of closed strings in the context
of General Relativity in a background of arbitrary spatial dimensions. Our
motivation is primarily late time String Gas Cosmology, where such a spacetime
picture has to emerge after the dilaton has stabilized. We find that after
accounting for the thermodynamics of a gas of strings, only string modes which
are massless at the self-dual radius are relevant, and that they lead to a
dynamics which is qualitatively different from that induced by the modes
usually considered in the literature. In the context of an ansatz with three
large spatial dimensions and an arbitrary number of small extra dimensions, we
obtain isotropic stabilization of these extra dimensions at the self-dual
radius. This stabilization occurs for fixed dilaton, and is induced by the
special string states we focus on. The three large dimensions undergo a regular
Friedmann-Robertson-Walker expansion. We also show that this framework for
late-time cosmology is consistent with observational bounds.Comment: 15 pages, no figures, references added (again
Radion Stabilization by Stringy Effects in General Relativity
We consider the effects of a gas of closed strings (treated quantum
mechanically) on a background where one dimension is compactified on a circle.
After we address the effects of a time dependent background on aspects of the
string spectrum that concern us, we derive the energy-momentum tensor for a
string gas and investigate the resulting space-time dynamics. We show that a
variety of trajectories are possible for the radius of the compactified
dimension, depending on the nature of the string gas, including a demonstration
within the context of General Relativity (i.e. without a dilaton) of a solution
where the radius of the extra dimension oscillates about the self-dual radius,
without invoking matter that violates the various energy conditions. In
particular, we find that in the case where the string gas is in thermal
equilibrium, the radius of the compactified dimension dynamically stabilizes at
the self-dual radius, after which a period of usual Friedmann-Robertson-Walker
cosmology of the three uncompactified dimensions can set in. We show that our
radion stabilization mechanism requires a stringy realization of inflation as
scalar field driven inflation invalidates our mechanism. We also show that our
stabilization mechanism is consistent with observational bounds.Comment: New section on the phenomonological implications of and bound on the
model has been adde
Observation of pseudogap in MgB2
Pseudogap phase in superconductors continues to be an outstanding puzzle that
differentiates unconventional superconductors from the conventional ones
(BCS-superconductors). Employing high resolution photoemission spectroscopy on
a highly dense conventional superconductor, MgB2, we discover an interesting
scenario. While the spectral evolution close to the Fermi energy is
commensurate to BCS descriptions as expected, the spectra in the wider energy
range reveal emergence of a pseudogap much above the superconducting transition
temperature indicating apparent departure from the BCS scenario. The energy
scale of the pseudogap is comparable to the energy of E2g phonon mode
responsible for superconductivity in MgB2 and the pseudogap can be attributed
to the effect of electron-phonon coupling on the electronic structure. These
results reveal a scenario of the emergence of the superconducting gap within an
electron-phonon coupling induced pseudogap.Comment: 4 figure
Participatory cotton breeding for organic and low input farming in Central India
Up to 80% of world’s organic cotton is produced in India. However, involved producers are facing increased difficulties to find suitable cultivars. Few hybrids selected for high input farming and genetically-modified (GM) cotton, which is explicitly excluded in organic farming, are presently dominating the Indian seed market. In addition farmers have lost their traditional knowledge on seed production and hybrid seed needs to be purchased each season
Perturbations in a Bouncing Brane Model
The question of how perturbations evolve through a bounce in the Cyclic and
Ekpyrotic models of the Universe is still a matter of ongoing debate. In this
report we show that the collision between boundary branes is in most cases
singular even in the full 5-D formalism, and that first order perturbation
theory breaks down for at least one perturbation variable. Only in the case
that the boundary branes approach each other with constant velocity shortly
before the bounce, can a consistent, non singular solution be found. It is then
possible to follow the perturbations explicitly until the actual collision. In
this case, we find that if a scale invariant spectrum developed on the hidden
brane, it will get transferred to the visible brane during the bounce.Comment: 15 pages, minor modifications, a few typos correcte
Endophytic Mycoflora of Indian Medicinal Plant, Terminalia arjuna and their Biological Activities
Studies were undertaken to isolate, identify and evaluate the biological activities of endophytic mycoflora of Indian medicinal plant, Terminalia arjuna. A total of 20 isolates of endophytic fungi were obtained from the leaves, twigs and bark tissues of the Terminalia arjuna. Out of 20, six isolates exhibited promising antibacterial, antifungal and anti-inflammatory activities when cultivated at shake flask level. The selected isolates were identified on the basis of morphology and ITS gene sequencing. Three isolates, designated as TA BA 1, TA L1 and TA L2 were identified as Aspergillus flavus whereas; the remaining three endophytic fungi were identified as Diaporthe arengae (TA TW2), Alternaria Sp. (TA TW1) and Lasiodiplodia theobromae (TA BA2). Aspergillus flavus was found as the predominant endophyte in leaves and bark tissues of the plant. The crude extract of the test isolates showed considerable antimicrobial activity against common human bacterial (Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Proteus vulgaris, Salmonella abony and Bacillus subtilis) and fungal (Candida albicans, Aspergillus niger and Penicilium sp.) pathogens. The extract of Diaporthe arengae (TA TW2) significantly reduced the concentration of DPPH free radical as percent DPPH scavenging activity was found to be highest (69.56%) in comparison with other isolates. The % inhibition of hemolysis of RBCs was found to be highest (82.85%) with Diaporthe arengae (TA TW2) in comparison (83.26%) with standard drug (Ibuprofen). Among all, the extract of the Diaporthe arengae (TA TW2) showed excellent biological activities and hence was subjected to further characterization. The phytochemical investigation of the extract revealed the presence of terpenoids as the major phytoconstituents which was supported by TLC and UV spectroscopic studies. The results indicate that the isolated endophytes could be the valuable source of these bioactive molecules with diverse biological activities. The bioactivities may be attributed to the terpenoids present in the endophytic extract
Particle Size Dependent Gas Sensing Performance of Zno Nanorods based Thick Film Resistors
Pure bulk ZnO was observed to be less sensitive to hazardous, toxic and inflammable gases. As the particle size of the materials get reduced from bulk to nanoscale, the gas sensing performance of pure ZnO was observed to increase crucially. It was also observed that, further decrease in the particle size would leads to decrease in the gas response. The optimized average diameter of ZnO Nanorod / particle size is 12 nm, which exhibits crucial gas response. The disc type ultrasonicated microwave assisted centrifuge technique was used for the synthesis of materials. The dry powders of synthesized materials were transformed into thick films by screen printing technique. Various characterizations techniques were employed to study the surface topography, crystal structure, phase, particle size, etc. effect of the surface nanostructure, different gases, gas concentrations, particle size, long duration for ageing, etc. on the gas response of the samples were studied and discussed.
DOI: 10.17762/ijritcc2321-8169.15080
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