219 research outputs found
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Loop Quantum Cosmology yields two kinds of quantum corrections to the
effective equations of motion for cosmological perturbations. Here we focus on
the holonomy kind and we study the problem of the closure of the resulting
algebra of constraints. Up to now, tensor, vector and scalar perturbations were
studied independently, leading to different algebras of constraints. The
structures of the related algebras were imposed by the requirement of anomaly
freedom. In this article we show that the algebra can be modified by a very
simple quantum correction, holding for all types of perturbations. This
demonstrates the consistency of the theory and shows that lessons from the
study of scalar perturbations should be taken into account when studying tensor
modes. The Mukhanov-Sasaki equations of motion are similarly modified by a
simple term.Comment: 5 page
Bulk and Brane Decay of a (4+n)-Dimensional Schwarzschild-De-Sitter Black Hole: Scalar Radiation
In this paper, we extend the idea that the spectrum of Hawking radiation can
reveal valuable information on a number of parameters that characterize a
particular black hole background - such as the dimensionality of spacetime and
the value of coupling constants - to gain information on another important
aspect: the curvature of spacetime. We investigate the emission of Hawking
radiation from a D-dimensional Schwarzschild-de-Sitter black hole emitted in
the form of scalar fields, and employ both analytical and numerical techniques
to calculate greybody factors and differential energy emission rates on the
brane and in the bulk. The energy emission rate of the black hole is
significantly enhanced in the high-energy regime with the number of spacelike
dimensions. On the other hand, in the low-energy part of the spectrum, it is
the cosmological constant that leaves a clear footprint, through a
characteristic, constant emission rate of ultrasoft quanta determined by the
values of black hole and cosmological horizons. Our results are applicable to
"small" black holes arising in theories with an arbitrary number and size of
extra dimensions, as well as to pure 4-dimensional primordial black holes,
embedded in a de Sitter spacetime.Comment: 31 pages, latex file, data files available at
http://lpsc.in2p3.fr/ams/greybody/ some clarifying comments and references
added, typos corrected, version to appear in Phys. Rev.
Kerr-Gauss-Bonnet Black Holes: An Analytical Approximation
Gauss-Bonnet gravity provides one of the most promising frameworks to study
curvature corrections to the Einstein action in supersymmetric string theories,
while avoiding ghosts and keeping second order field equations. Although
Schwarzschild-type solutions for Gauss-Bonnet black holes have been known for
long, the Kerr-Gauss-Bonnet metric is missing. In this paper, a five
dimensional Gauss-Bonnet approximation is analytically derived for spinning
black holes and the related thermodynamical properties are briefly outlined.Comment: 5 pages, 1 figur
Maximum likelihood, parametric component separation and CMB B-mode detection in suborbital experiments
We investigate the performance of the parametric Maximum Likelihood component
separation method in the context of the CMB B-mode signal detection and its
characterization by small-scale CMB suborbital experiments. We consider
high-resolution (FWHM=8') balloon-borne and ground-based observatories mapping
low dust-contrast sky areas of 400 and 1000 square degrees, in three frequency
channels, 150, 250, 410 GHz, and 90, 150, 220 GHz, with sensitivity of order 1
to 10 micro-K per beam-size pixel. These are chosen to be representative of
some of the proposed, next-generation, bolometric experiments. We study the
residual foreground contributions left in the recovered CMB maps in the pixel
and harmonic domain and discuss their impact on a determination of the
tensor-to-scalar ratio, r. In particular, we find that the residuals derived
from the simulated data of the considered balloon-borne observatories are
sufficiently low not to be relevant for the B-mode science. However, the
ground-based observatories are in need of some external information to permit
satisfactory cleaning. We find that if such information is indeed available in
the latter case, both the ground-based and balloon-borne experiments can detect
the values of r as low as ~0.04 at 95% confidence level. The contribution of
the foreground residuals to these limits is found to be then subdominant and
these are driven by the statistical uncertainty due to CMB, including E-to-B
leakage, and noise. We emphasize that reaching such levels will require a
sufficient control of the level of systematic effects present in the data.Comment: 18 pages, 12 figures, 6 table
Probing Loop Quantum Gravity with Evaporating Black Holes
This letter aims at showing that the observation of evaporating black holes
should allow distinguishing between the usual Hawking behavior and Loop Quantum
Gravity (LQG) expectations. We present a full Monte-Carlo simulation of the
evaporation in LQG and statistical tests that discriminate between competing
models. We conclude that contrarily to what was commonly thought, the
discreteness of the area in LQG leads to characteristic features that qualify
evaporating black holes as objects that could reveal quantum gravity
footprints.Comment: 5 pages, 3 figures. Version accpeted by Phys. Rev. Let
Spherically symmetric Einstein-Maxwell theory and loop quantum gravity corrections
Effects of inverse triad corrections and (point) holonomy corrections,
occuring in loop quantum gravity, are considered on the properties of
Reissner-Nordstr\"om black holes. The version of inverse triad corrections with
unmodified constraint algebra reveals the possibility of occurrence of three
horizons (over a finite range of mass) and also shows a mass threshold beyond
which the inner horizon disappears. For the version with modified constraint
algebra, coordinate transformations are no longer a good symmetry. The
covariance property of spacetime is regained by using a \emph{quantum} notion
of mapping from phase space to spacetime. The resulting quantum effects in both
versions of these corrections can be associated with renormalization of either
mass, charge or wave function. In neither of the versions, Newton's constant is
renormalized. (Point) Holonomy corrections are shown to preclude the undeformed
version of constraint algebra as also a static solution, though
time-independent solutions exist. A possible reason for difficulty in
constructing a covariant metric for these corrections is highlighted.
Furthermore, the deformed algebra with holonomy corrections is shown to imply
signature change.Comment: 38 pages, 9 figures, matches published versio
Estimating the tensor-to-scalar ratio and the effect of residual foreground contamination
We consider future balloon-borne and ground-based suborbital experiments
designed to search for inflationary gravitational waves, and investigate the
impact of residual foregrounds that remain in the estimated cosmic microwave
background maps. This is achieved by propagating foreground modelling
uncertainties from the component separation, under the assumption of a
spatially uniform foreground frequency scaling, through to the power spectrum
estimates, and up to measurement of the tensor to scalar ratio in the parameter
estimation step. We characterize the error covariance due to subtracted
foregrounds, and find it to be subdominant compared to instrumental noise and
sample variance in our simulated data analysis. We model the unsubtracted
residual foreground contribution using a two-parameter power law and show that
marginalization over these foreground parameters is effective in accounting for
a bias due to excess foreground power at low . We conclude that, at least
in the suborbital experimental setups we have simulated, foreground errors may
be modeled and propagated up to parameter estimation with only a slight
degradation of the target sensitivity of these experiments derived neglecting
the presence of the foregrounds.Comment: 19 pages, 12 figures, accepted for publication in JCA
Loop quantum gravity: the first twenty five years
This is a review paper invited by the journal "Classical ad Quantum Gravity"
for a "Cluster Issue" on approaches to quantum gravity. I give a synthetic
presentation of loop gravity. I spell-out the aims of the theory and compare
the results obtained with the initial hopes that motivated the early interest
in this research direction. I give my own perspective on the status of the
program and attempt of a critical evaluation of its successes and limits.Comment: 24 pages, 3 figure
Relative energetics and structural properties of zirconia using a self-consistent tight-binding model
We describe an empirical, self-consistent, orthogonal tight-binding model for
zirconia, which allows for the polarizability of the anions at dipole and
quadrupole levels and for crystal field splitting of the cation d orbitals.
This is achieved by mixing the orbitals of different symmetry on a site with
coupling coefficients driven by the Coulomb potentials up to octapole level.
The additional forces on atoms due to the self-consistency and polarizabilities
are exactly obtained by straightforward electrostatics, by analogy with the
Hellmann-Feynman theorem as applied in first-principles calculations. The model
correctly orders the zero temperature energies of all zirconia polymorphs. The
Zr-O matrix elements of the Hamiltonian, which measure covalency, make a
greater contribution than the polarizability to the energy differences between
phases. Results for elastic constants of the cubic and tetragonal phases and
phonon frequencies of the cubic phase are also presented and compared with some
experimental data and first-principles calculations. We suggest that the model
will be useful for studying finite temperature effects by means of molecular
dynamics.Comment: to be published in Physical Review B (1 march 2000
Big sugar in southern Africa : rural development and the perverted potential of sugar/ethanol exports
This paper asks how investment in large-scale sugar cane production has contributed, and will contribute, to rural development in southern Africa. Taking a case study of the South African company Illovo in Zambia, the argument is made that the potential for greater tax revenue, domestic competition, access to resources and wealth distribution from sugar/ethanol production have all been perverted and with relatively little payoff in wage labour opportunities in return. If the benefits of agro-exports cannot be so easily assumed, then the prospective 'balance sheet' of biofuels needs to be re-examined. In this light, the paper advocates smaller-scale agrarian initiatives
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