37,179 research outputs found
On choice of connection in loop quantum gravity
We investigate the quantum area operator in the loop approach based on the
Lorentz covariant hamiltonian formulation of general relativity. We show that
there exists a two-parameter family of Lorentz connections giving rise to
Wilson lines which are eigenstates of the area operator. For each connection
the area spectrum is evaluated. In particular, the results of the su(2)
approach turn out to be included in the formalism. However, only one connection
from the family is a spacetime connection ensuring that the 4d diffeomorphism
invariance is preserved under quantization. It leads to the area spectrum
independent of the Immirzi parameter. As a consequence, we conclude that the
su(2) approach must be modified accordingly to the results obtained since it
breaks one of the classical symmetries.Comment: 11 pages, RevTEX; minor changes; a sign mistake correcte
Reality conditions for Ashtekar gravity from Lorentz-covariant formulation
We show the equivalence of the Lorentz-covariant canonical formulation
considered for the Immirzi parameter to the selfdual Ashtekar
gravity. We also propose to deal with the reality conditions in terms of Dirac
brackets derived from the covariant formulation and defined on an extended
phase space which involves, besides the selfdual variables, also their
anti-selfdual counterparts.Comment: 14 page
The ELAIS deep X-ray survey - I. Chandra source catalogue and first results
We present an analysis of two deep (75 ks) Chandra observations of the European Large Area Infrared Space Observatory (ISO) Survey (ELAIS) fields N1 and N2 as the first results from the ELAIS deep X-ray survey. This survey is being conducted in well-studied regions with extensive multiwavelength coverage. Here we present the Chandra source catalogues along with an analysis of source counts, hardness ratios and optical classifications. A total of 233 X-ray point sources are detected in addition to two soft extended sources, which are found to be associated with galaxy clusters. An overdensity of sources is found in N1 with 30 per cent more sources than N2, which we attribute to large-scale structure. A similar variance is seen between other deep Chandra surveys. The source count statistics reveal an increasing fraction of hard sources at fainter fluxes. The number of galaxy-like counterparts also increases dramatically towards fainter fluxes, consistent with the emergence of a large population of obscured sources
Graviton propagator in loop quantum gravity
We compute some components of the graviton propagator in loop quantum
gravity, using the spinfoam formalism, up to some second order terms in the
expansion parameter.Comment: 41 pages, 6 figure
Three dimensional loop quantum gravity: physical scalar product and spin foam models
In this paper, we address the problem of the dynamics in three dimensional
loop quantum gravity with zero cosmological constant. We construct a rigorous
definition of Rovelli's generalized projection operator from the kinematical
Hilbert space--corresponding to the quantization of the infinite dimensional
kinematical configuration space of the theory--to the physical Hilbert space.
In particular, we provide the definition of the physical scalar product which
can be represented in terms of a sum over (finite) spin-foam amplitudes.
Therefore, we establish a clear-cut connection between the canonical
quantization of three dimensional gravity and spin-foam models. We emphasize
two main properties of the result: first that no cut-off in the kinematical
degrees of freedom of the theory is introduced (in contrast to standard
`lattice' methods), and second that no ill-defined sum over spins (`bubble'
divergences) are present in the spin foam representation.Comment: Typos corrected, version appearing in Class. Quant. Gra
NIKEL: Electronics and data acquisition for kilopixels kinetic inductance camera
A prototype of digital frequency multiplexing electronics allowing the real
time monitoring of microwave kinetic inductance detector (MKIDs) arrays for
mm-wave astronomy has been developed. Thanks to the frequency multiplexing, it
can monitor simultaneously 400 pixels over a 500 MHz bandwidth and requires
only two coaxial cables for instrumenting such a large array. The chosen
solution and the performances achieved are presented in this paper.Comment: 21 pages, 14 figure
Decoherence from internal degrees of freedom for cluster of mesoparticles : a hierarchy of master equations
A mesoscopic evolution equation for an ensemble of mesoparticles follows
after the elimination of internal degrees of freedom. If the system is composed
of a hierarchy of scales, the reduction procedure could be worked repeatedly
and the characterization of this iterating method is carried out. Namely, a
prescription describing a discrete hierarchy of master equations for the
density operator is obtained. Decoherence follows from the irreversible
coupling of the systems, defined by mesoscopic variables, to internal degrees
of freedom. We discuss briefly the existence of systems with the same dynamics
laws at different scales. We made an explicit calculation for an ensemble of
particles with internal harmonic interaction in an external anharmonic field.
New conditions related to the semiclassical limit for mesoscopic systems
(Wigner-function) are conjectured.Comment: 19 pages, 0 figures, late
Hilbert space structure of covariant loop quantum gravity
We investigate the Hilbert space in the Lorentz covariant approach to loop
quantum gravity. We restrict ourselves to the space where all area operators
are simultaneously diagonalizable, assuming that it exists. In this sector
quantum states are realized by a generalization of spin network states based on
Lorentz Wilson lines projected on irreducible representations of an SO(3)
subgroup. The problem of infinite dimensionality of the unitary Lorentz
representations is absent due to this projection. Nevertheless, the projection
preserves the Lorentz covariance of the Wilson lines so that the symmetry is
not broken. Under certain conditions the states can be thought as functions on
a homogeneous space. We define the inner product as an integral over this
space. With respect to this inner product the spin networks form an orthonormal
basis in the investigated sector. We argue that it is the only relevant part of
a larger state space arising in the approach. The problem of the
noncommutativity of the Lorentz connection is solved by restriction to the
simple representations. The resulting structure shows similarities with the
spin foam approach.Comment: 20 pages, RevTE
Beta-delayed-neutron studies of Sb and I performed with trapped ions
Beta-delayed-neutron (n) spectroscopy was performed using the
Beta-decay Paul Trap and an array of radiation detectors. The n
branching ratios and energy spectra for Sb and I were
obtained by measuring the time of flight of recoil ions emerging from the
trapped ion cloud. These nuclei are located at the edge of an isotopic region
identified as having n branching ratios that impact the r-process
abundance pattern around the A~130 peak. For Sb and I,
n branching ratios of 14.6(11)%, 17.6(28)%, and 7.6(28)% were
determined, respectively. The n energy spectra obtained for Sb
and I are compared with results from direct neutron measurements, and
the n energy spectrum for Sb has been measured for the first
time
- …