418 research outputs found
Holonomy corrections to the cosmological primordial tensor power spectrum
Loop quantum gravity is one of the leading candidate theory to
non-perturbatively quantize gravity. In this framework, holonomy corrections to
the equation of propagation of gravitons in a FLRW background have been
derived. We investigate the consequences of those corrections on the tensor
power spectrum in de-Sitter and slow-roll inflations, for n=-1/2. Depending on
the value of the Barbero-Immirzi parameter, several observational features
could be expected.Comment: 5 pages, Proc. of the 43rd Rencontres de Moriond "Cosmology 2008
Detecting chiral gravity with the pure pseudospectrum reconstruction of the cosmic microwave background polarized anisotropies
We consider the possible detection of parity violation at the linear level in
gravity using polarized anisotropies of the cosmic microwave background. Since
such a parity violation would lead to non-zero TB and EB correlations, this
makes those odd-parity angular power spectra a potential probe of parity
violation in the gravitational sector. These spectra are modeled incorporating
the impact of lensing and we explore their possible detection in the context of
small-scale (balloon-borne or ground-based) experiments and a future satellite
mission dedicated to B-mode detection. We assess the statistical uncertainties
on their reconstruction using mode-counting and a (more realistic) pure
pseudospectrum estimator approach. Those uncertainties are then translated into
constraints on the level of parity asymmetry. We found that detecting chiral
gravity is impossible for ongoing small-scale experiments. However, for a
satellite-like mission, a parity asymmetry of at least 50% could be detected at
68% of confidence level, and a parity asymmetry of 100% is measurable with at
least a confidence level of 95%. We also assess the impact of a possible
miscalibration of the orientation of the polarized detectors, leading to
spurious TB and EB cross-correlations. We show that in the context of
pseudospectrum estimation of the angular power spectra, self-calibration of
this angle could significantly reduce the statistical significance of the
measured level of parity asymmetry (by e.g. a factor ~2.4 for a miscalibration
angle of 1 degree). For chiral gravity and assuming a satellite mission
dedicated to primordial B-mode, a non detection of the TB and EB correlation
would translate into an upper bound on parity violation of 39% at 95%
confidence level for a tensor-to-scalar ratio of 0.2, excluding values of the
(imaginary) Barbero-Immirzi parameter comprised between 0.2 and 4.9 at 95% CL.Comment: 21 pages, 6 figures, accepted for publication in Phys. Rev. D (typos
and references corrected
Phenomenology of black hole evaporation with a cosmological constant
In this brief note, we investigate some possible experimental consequences of
the de-Sitter or Anti-de-Sitter background spacetime structure for
d-dimensional evaporating black holes. Possible observational signatures in
Large Hadron Collider (LHC) events are considered in the framework of the
Arkani-Hamed-Dimopoulos-Dvali (ADD) braneworld model. Lower bounds on the value
of the bulk cosmological constant required to produce visible effects are
derived thanks to a dynamical Monte-Carlo simulation. This preliminary study
has to be refined by the accurate computation of the greybody factors. It opens
a new way to investigate the structure of non-asymptotically flat
higher-dimensional spacetimes.Comment: Proceedings of the HEP2005 conference. Related greybody factors for
evaporating black holes available at : http://lpsc.in2p3.fr/ams/greybody
Very high energy gamma-rays and the Hubble parameter
A new method, based on the absorption of very high-energy gamma-rays by the
cosmic infrared background, is proposed to constrain the value of the Hubble
constant. As this value is both fundamental for cosmology and still not very
well measured, it is worth developing such alternative methods. Our lower limit
at the 68% confidence level is H0 > 74 km/s/Mpc, leading, when combined with
the HST results, to H0 ~ 76 km/s/Mpc. Interestingly, this value, which is
significantly higher than the usually considered one, is in exact agreement
with other independent approaches based on baryonic acoustic oscillations and
X-ray measurements. Forthcoming data from the experiments HESS-2 and CTA should
help improving those results. Finally, we briefly mention a plausible
correlation between absorption by the extragalactic background light and the
absence of observation of gamma-ray bursts (GRBs) at very high energies.Comment: Proc. of the 12th Marcel Grossmann meeting on general relativity. 3
pages, 1 figur
Astrophysical Production of Microscopic Black Holes in a Low Planck-scale World
In the framework of brane-world models lowering the Planck scale to the TeV
range, it has recently been pointed out that small black holes could be formed
at particle colliders or by neutrinos interactions in the atmosphere. This
article aims at reviewing other places and epochs where microscopic black holes
could be formed : the interstellar medium and the early Universe. The related
decay channels and the propagation of the emitted particles are studied to
conclude that, in spite of the large creation rate for such black holes, the
amount of produced particles do not conflict with experimental data. This
shows, from the astronomical viewpoint, that models with large extra dimensions
making the gravity scale much lower are compatible with observations.Comment: To appear in Ap
Observational issues in loop quantum cosmology
Quantum gravity is sometimes considered as a kind of metaphysical
speculation. In this review, we show that, although still extremely difficult
to reach, observational signatures can in fact be expected. The early universe
is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop
Quantum Gravity as one of the best candidate for a non-perturbative and
background-independant quantization of gravity, we detail some expected
features.Comment: 75 pages, invited topical review for Classical and Quantum Gravit
Modular multilevel converter with modified half-bridge submodule and arm filter for dc transmission systems with DC fault blocking capability
Although a modular multilevel converter (MMC) is universally accepted as a suitable converter topology for the high voltage dc transmission systems, its dc fault ride performance requires substantial improvement in order to be used in critical infrastructures such as transnational multi-terminal dc (MTDC) networks. Therefore, this paper proposes a modified submodule circuit for modular multilevel converter that offers an improved dc fault ride through performance with reduced semiconductor losses and enhanced control flexibility compared to that achievable with full-bridge submodules. The use of the proposed submodules allows MMC to retain its modularity; with semiconductor loss similar to that of the mixed submodules MMC, but higher than that of the half-bridge submodules. Besides dc fault blocking, the proposed submodule offers the possibility of controlling ac current in-feed during pole-to-pole dc short circuit fault, and this makes such submodule increasingly attractive and useful for continued operation of MTDC networks during dc faults. The aforesaid attributes are validated using simulations performed in MATLAB/SIMULINK, and substantiated experimentally using the proposed submodule topology on a 4-level small-scale MMC prototype
Quantum Bound States Around Black Holes
Quantum mechanics around black holes has shown to be one of the most fascinating fields of theoretical physics. It involves both general relativity and particle physics, opening new eras to establish the groundings of unified theories. In this article, we show that quantum bound states with no classical equivalent -- as it can easily be seen at the dominant monopolar order -- should be formed around black holes for massive scalar particles. We qualitatively investigate some important physical consequences, in particular for the Hawking evaporation mechanism and the associated greybody factors
Comparison of primordial tensor power spectra from the deformed algebra and dressed metric approaches in loop quantum cosmology
Loop quantum cosmology tries to capture the main ideas of loop quantum
gravity and to apply them to the Universe as a whole. Two main approaches
within this framework have been considered to date for the study of
cosmological perturbations: the dressed metric approach and the deformed
algebra approach. They both have advantages and drawbacks. In this article, we
accurately compare their predictions. In particular, we compute the associated
primordial tensor power spectra. We show -- numerically and analytically --
that the large scale behavior is similar for both approaches and compatible
with the usual prediction of general relativity. The small scale behavior is,
the other way round, drastically different. Most importantly, we show that in a
range of wavenumbers explicitly calculated, both approaches do agree on
predictions that, in addition, differ from standard general relativity and do
not depend on unknown parameters. These features of the power spectrum at
intermediate scales might constitute a universal loop quantum cosmology
prediction that can hopefully lead to observational tests and constraints. We
also present a complete analytical study of the background evolution for the
bouncing universe that can be used for other purposes.Comment: 15 pages, 7 figure
Semiclassical scalar propagators in curved backgrounds: formalism and ambiguities
The phenomenology of quantum systems in curved space-times is among the most
fascinating fields of physics, allowing --often at the gedankenexperiment
level-- constraints on tentative theories of quantum gravity. Determining the
dynamics of fields in curved backgrounds remains however a complicated task
because of the highly intricate partial differential equations involved,
especially when the space metric exhibits no symmetry. In this article, we
provide --in a pedagogical way-- a general formalism to determine this dynamics
at the semiclassical order. To this purpose, a generic expression for the
semiclassical propagator is computed and the equation of motion for the
probability four-current is derived. Those results underline a direct analogy
between the computation of the propagator in general relativistic quantum
mechanics and the computation of the propagator for stationary systems in
non-relativistic quantum mechanics. A possible application of this formalism to
curvature-induced quantum interferences is also discussed.Comment: New materials on gravitationally-induced quantum interferences has
been adde
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