7,319 research outputs found
Noise Kernel in Stochastic Gravity and Stress Energy Bi-Tensor of Quantum Fields in Curved Spacetimes
The noise kernel is the vacuum expectation value of the (operator-valued)
stress-energy bi-tensor which describes the fluctuations of a quantum field in
curved spacetimes. It plays the role in stochastic semiclassical gravity based
on the Einstein-Langevin equation similar to the expectation value of the
stress-energy tensor in semiclassical gravity based on the semiclassical
Einstein equation. According to the stochastic gravity program, this two point
function (and by extension the higher order correlations in a hierarchy) of the
stress energy tensor possesses precious statistical mechanical information of
quantum fields in curved spacetime and, by the self-consistency required of
Einstein's equation, provides a probe into the coherence properties of the
gravity sector (as measured by the higher order correlation functions of
gravitons) and the quantum nature of spacetime. It reflects the low and medium
energy (referring to Planck energy as high energy) behavior of any viable
theory of quantum gravity, including string theory. It is also useful for
calculating quantum fluctuations of fields in modern theories of structure
formation and for backreaction problems in cosmological and black holes
spacetimes.
We discuss the properties of this bi-tensor with the method of
point-separation, and derive a regularized expression of the noise-kernel for a
scalar field in general curved spacetimes. One collorary of our finding is that
for a massless conformal field the trace of the noise kernel identically
vanishes. We outline how the general framework and results derived here can be
used for the calculation of noise kernels for Robertson-Walker and
Schwarzschild spacetimes.Comment: 22 Pages, RevTeX; version accepted for publication in PR
Otimização de protocolos de extração de RNA em diferentes tecidos de milho.
Neste trabalho, foram testados cinco métodos de extração de RNA (CTAB microextração, Concert? Invitrogen, Concert? Adaptado, TRI Reagente® Sigma e TRI Reagente® Adaptado), em dois diferentes tecidos de milho (mesocótilo e raiz), com o objetivo de estabelecer um método eficiente de extração de RNA, visando posteriormente estudos de expressão gênica. Observou-se que o método Concert,utilizando o protocolo adaptado, foi o mais eficiente para a extração de RNA de ambos os tecidos de plântulas de milho, originando 2351,35 ?g por 100 mg de tecido para mesocótilo e 893,75 ?g por 100 mg de tecido para raiz, considerando tanto a quantidade como a qualidade das amostras, podendo ser submetidas às etapas posteriores de tratamento com DNAse e construção de cDNA para estudos de expressão gênica. Pôde-se observar que pequenas modificações nos protocolos, como, por exemplo, mudança no tempo e na posição dos tubos durante a incubação e o incremento de duas lavagens com clorofórmio, podem melhorar muito tanto a qualidade quanto a quantidade do RNA extraído
Vacuum Energy Density Fluctuations in Minkowski and Casimir States via Smeared Quantum Fields and Point Separation
We present calculations of the variance of fluctuations and of the mean of
the energy momentum tensor of a massless scalar field for the Minkowski and
Casimir vacua as a function of an intrinsic scale defined by a smeared field or
by point separation. We point out that contrary to prior claims, the ratio of
variance to mean-squared being of the order unity is not necessarily a good
criterion for measuring the invalidity of semiclassical gravity. For the
Casimir topology we obtain expressions for the variance to mean-squared ratio
as a function of the intrinsic scale (defined by a smeared field) compared to
the extrinsic scale (defined by the separation of the plates, or the
periodicity of space). Our results make it possible to identify the spatial
extent where negative energy density prevails which could be useful for
studying quantum field effects in worm holes and baby universe, and for
examining the design feasibility of real-life `time-machines'.
For the Minkowski vacuum we find that the ratio of the variance to the
mean-squared, calculated from the coincidence limit, is identical to the value
of the Casimir case at the same limit for spatial point separation while
identical to the value of a hot flat space result with a temporal
point-separation. We analyze the origin of divergences in the fluctuations of
the energy density and discuss choices in formulating a procedure for their
removal, thus raising new questions into the uniqueness and even the very
meaning of regularization of the energy momentum tensor for quantum fields in
curved or even flat spacetimes when spacetime is viewed as having an extended
structure.Comment: 41 pages, 2 figure
Stochastic semiclassical fluctuations in Minkowski spacetime
The semiclassical Einstein-Langevin equations which describe the dynamics of
stochastic perturbations of the metric induced by quantum stress-energy
fluctuations of matter fields in a given state are considered on the background
of the ground state of semiclassical gravity, namely, Minkowski spacetime and a
scalar field in its vacuum state. The relevant equations are explicitly derived
for massless and massive fields arbitrarily coupled to the curvature. In doing
so, some semiclassical results, such as the expectation value of the
stress-energy tensor to linear order in the metric perturbations and particle
creation effects, are obtained. We then solve the equations and compute the
two-point correlation functions for the linearized Einstein tensor and for the
metric perturbations. In the conformal field case, explicit results are
obtained. These results hint that gravitational fluctuations in stochastic
semiclassical gravity have a ``non-perturbative'' behavior in some
characteristic correlation lengths.Comment: 28 pages, RevTeX, no figure
Critical phenomena of thick branes in warped spacetimes
We have investigated the effects of a generic bulk first-order phase
transition on thick Minkowski branes in warped geometries. As occurs in
Euclidean space, when the system is brought near the phase transition an
interface separating two ordered phases splits into two interfaces with a
disordered phase in between. A remarkable and distinctive feature is that the
critical temperature of the phase transition is lowered due to pure geometrical
effects. We have studied a variety of critical exponents and the evolution of
the transverse-traceless sector of the metric fluctuations.Comment: revtex4, 4 pages, 4 figures, some comments added, typos corrected,
published in PR
Anisotropic brane cosmologies with exponential potentials
We study Bianchi I type brane cosmologies with scalar matter self-interacting
through combinations of exponential potentials. Such models correspond in some
cases to inflationary universes. We discuss the conditions for accelerated
expansion to occur, and pay particular attention to the influence of extra
dimensions and anisotropy. Our results show that the associated effects evolve
in such a way that they become negligible in the late time limit, those related
to the anisotropy disappearing earlier. This study focuses mainly on single
field models, but we also consider a generalization yielding models with
multiple non-interacting fields and examine its features briefly. We conclude
that in the brane scenario, as happens in general relativity, an increase in
the number of fields assists inflation.Comment: 11 pages, 1 figur
Galaxy number counts- IV. surveying the Herschel deep field in the near-infrared
(abridged) We present results from two new near-infrared imaging surveys. One
covers 47.2 arcmin^2 to K(3\sigma)<20 whilst a second, deeper survey covers a
sub-area of 1.8 arcmin^2 to K(3\sigma)<22.75. Over the entire area we have
extremely deep UBRI photometry. Our K- counts are consistent with the
predictions of non-evolving models with 0 < q0 <0.5. The K-selected (B-K)
galaxy colour distributions move sharply bluewards fainter than K~20 and at at
brighter magnitudes (K<20) our observed colour distributions indicate a
deficiency of red, early-type galaxies at z~1 in comparison with passively
evolving models. This implies either a pure luminosity evolution (PLE) model
with a low level of continuing star-formation following an an initial burst, or
dynamical merging. At fainter magnitudes, the continuing bluewards trend
observed in (B-K) can be explained purely in terms of passively evolving PLE
models. Our observed numbers of (I-K)>4 galaxies at K<20 exhibit the same
deficiency, suggesting that at least part of the larger deficit observed in
(B-K) at K<20 may be due to star-formation rather than dynamical merging.
Finally, as we and others have noted, the number-redshift distribution at
18<K<19 of recent, deep K- selected redshift surveys is well fitted by
non-evolving models; passively evolving models with a Salpeter or Scalo initial
mass functions overpredict the numbers of galaxies with z>1. Dynamical merging
is one possible solution to reduce the numbers of these galaxies but a
dwarf-dominated IMF for early-type galaxies could offer an alternative
explanation; we show that this model reproduces both the optical-infrared
colour distributions and the K- band galaxy counts.Comment: 15 pages, 9 figures, revised version, requires astrobib.sty,
mn-abs.sty, submitted to MNRA
Stochastic semiclassical cosmological models
We consider the classical stochastic fluctuations of spacetime geometry
induced by quantum fluctuations of massless non-conformal matter fields in the
Early Universe. To this end, we supplement the stress-energy tensor of these
fields with a stochastic part, which is computed along the lines of the
Feynman-Vernon and Schwinger-Keldysh techniques; the Einstein equation is
therefore upgraded to a so called Einstein-Langevin equation. We consider in
some detail the conformal fluctuations of flat spacetime and the fluctuations
of the scale factor in a simple cosmological modelintroduced by Hartle, which
consists of a spatially flat isotropic cosmology driven by radiation and dust.Comment: 29 pages, no figures, ReVTeX fil
Wrapped fivebranes and N=2 super Yang-Mills theory
We construct D=10 supergravity solutions corresponding to type IIB fivebranes
wrapping a two-sphere in a Calabi-Yau two-fold. These are related in the IR to
the large N limit of pure N=2 SU(N) super Yang-Mills theory. We show that the
singularities in the IR correspond to the wrapped branes being distributed on a
ring. We analyse the dynamics of a probe fivebrane and show that it
incorporates the full perturbative structure of the gauge theory. For a class
of solutions the two-dimensional moduli space is non-singular and we match the
result for the corresponding slice of the Coulomb branch of the gauge theory.Comment: 24 Latex pages, two figures;v2 typos corrected, references adde
- …