3,998 research outputs found
The formation of supermassive black holes in the first galaxies
We discuss the formation of supermassive black holes in the early universe,
and how to probe their subsequent evolution with the upcoming mm/sub-mm
telescope ALMA. We first focus on the chemical and radiative conditions for
black hole formation, in particular considering radiation trapping and
molecular dissociation effects. We then turn our attention towards the magnetic
properties in the halos where the first black holes form, and show that the
presence of turbulence may lead to a magnetic dynamo, which could support the
black hole formation process by providing an efficient means of transporting
the angular momentum. We finally focus on observable properties of
high-redshift black holes with respect to ALMA, and discuss how to distinguish
between chemistry driven by the starburst and chemistry driven by X-rays from
the black hole.Comment: Contribution to AIP conference proceedings "First Stars and Galaxies:
Challenges in the Next Decade". 4 pages, 3 figure
The universe dynamics in the tachyon cosmology with non-minimal coupling to matter
Recently, the tachyon cosmology has been represented as dark energy model to
support the current acceleration of the universe without phantom crossing. In
this paper, we study the dynamics of the tachyon cosmology in which the field
plays the role of tachyon field and also non--minimally coupled to the matter
lagrangian. The model shows current universe acceleration and also phantom
crossing in the future. Two cosmological tests are also performed to validate
the model; the difference in the distance modulus and the model independent
Cosmological Redshift Drift (CRD) test.Comment: 14 pages, 11 figure
Power-law corrections to entanglement entropy of horizons
We re-examine the idea that the origin of black-hole entropy may lie in the
entanglement of quantum fields between inside and outside of the horizon.
Motivated by the observation that certain modes of gravitational fluctuations
in a black-hole background behave as scalar fields, we compute the entanglement
entropy of such a field, by tracing over its degrees of freedom inside a
sphere. We show that while this entropy is proportional to the area of the
sphere when the field is in its ground state, a correction term proportional to
a fractional power of area results when the field is in a superposition of
ground and excited states. The area law is thus recovered for large areas.
Further, we identify location of the degrees of freedom that give rise to the
above entropy.Comment: 16 pages, 6 figures, to appear in Phys. Rev.
Primary trisomic of rice: orgin, morphology, cytology and use in linkage mapping
Twelve primary trisomics of Oryza sativa L. were isolated from the progenies of spontaneous triploids and were transferred by backcrossing to the genetic background of IR36, a widely grown high yielding rice variety. Eleven trisomics can be identified morphologically from one another and from diploids. However, triplo 11 is difficult to distinguish from diploid sibs. -The extra chromosome of each trisomic was identified cytologically at pachytene stage of meiosis, and the chromosomes were numbered according to their length at this stage. The major distinguishing features of each pachytene chromosome were redescribed. -The female transmission rates varied from 15.5% for triplo 1, the longest chromosome, to 43.9% for triplo 12, the shortest chromosome. Seven of the 12 primary trisomics transmitted the extra chromosome through the male. The low level of chromosomal imbalance tolerated by rice and other evidence are interpreted to indicate that this species is a basic diploid. -Genetic segregation for 22 marker genes in the trisomic progenies was studied. Of a possible 264 combinations, involving 22 genes and 12 trisomics, 120 were examined. Marker genes for each of the 12 chromosomes were identified. The results helped establish associations between linkage groups and cytologically identifiable chromosomes of rice for the first time. Relationships between various systems of numbering chromosomes, trisomics, linkage groups and marker genes are described, and a revised linkage map of rice is presented
Cross helicity and turbulent magnetic diffusivity in the solar convection zone
In a density-stratified turbulent medium the cross helicity is
considered as a result of the interaction of the velocity fluctuations and a
large-scale magnetic field. By means of a quasilinear theory and by numerical
simulations we find the cross helicity and the mean vertical magnetic field
anti-correlated. In the high-conductivity limit the ratio of the helicity and
the mean magnetic field equals the ratio of the magnetic eddy diffusivity and
the (known) density scale height. The result can be used to predict that the
cross helicity at the solar surface exceeds the value of 1 Gauss km/s. Its sign
is anti-correlated with that of the radial mean magnetic field. Alternatively,
we can use our result to determine the value of the turbulent magnetic
diffusivity from observations of the cross helicity.Comment: 9 pages, 2 figures, submitted to Solar Physic
Crossing the phantom divide with k-essence in brane-worlds
We study a flat 3-brane in presence of a linear field with nonzero
cosmological constant . In this model the crossing of the phantom
divide (PD) occurs when the -essence energy density becomes negative. We
show that in the high energy regime the effective equation of state has a
resemblance of a modified Chaplygin gas while in the low energy regime it
becomes linear. We find a scale factor that begins from a singularity and
evolves to a de Sitter stable stage while other solutions have a
super-accelerated regime and end with a big rip. We use the energy conditions
to show when the effective equation of state of the brane-universe crosses the
PD.Comment: 8 pages, 5 figures. The article was fully rewritten. References
added. Accepted for publication in MPLA (2010
Growth rate of small-scale dynamo at low magnetic Prandtl numbers
In this study we discuss two key issues related to a small-scale dynamo
instability at low magnetic Prandtl numbers and large magnetic Reynolds
numbers, namely: (i) the scaling for the growth rate of small-scale dynamo
instability in the vicinity of the dynamo threshold; (ii) the existence of the
Golitsyn spectrum of magnetic fluctuations in small-scale dynamos. There are
two different asymptotics for the small-scale dynamo growth rate: in the
vicinity of the threshold of the excitation of the small-scale dynamo
instability, , and when the
magnetic Reynolds number is much larger than the threshold of the excitation of
the small-scale dynamo instability, , where
is the small-scale dynamo instability threshold in the
magnetic Reynolds number . We demonstrated that the existence of the
Golitsyn spectrum of magnetic fluctuations requires a finite correlation time
of the random velocity field. On the other hand, the influence of the Golitsyn
spectrum on the small-scale dynamo instability is minor. This is the reason why
it is so difficult to observe this spectrum in direct numerical simulations for
the small-scale dynamo with low magnetic Prandtl numbers.Comment: 14 pages, 1 figure, revised versio
- …