6,804 research outputs found
Perturbations in the Ekpyrotic Scenarios
With the new cosmological data gathered over the last few years, the
inflationary paradigm has seen its predictions largely unchallenged. A recent
proposal, called the ekpyrotic scenario, was argued to be a viable competitor
as it was claimed that the spectrum of primordial perturbations it produces is
scale invariant. By investigating closely this scenario, we show that the
corresponding spectrum depends explicitly on an arbitrary function of
wavenumber and is therefore itself arbitrary. It can at will be set scale
invariant. We conclude that the scenario is not predictive at this stage.Comment: 4 pages, no figure, uses moriond.sty, to appear in the proceeding of
the Moriond cosmology meeting held at Les Arcs, France (March 16-23, 2002
Perturbations in Bouncing Cosmological Models
I describe the features and general properties of bouncing models and the
evolution of cosmological perturbations on such backgrounds. I will outline
possible observational consequences of the existence of a bounce in the
primordial Universe and I will make a comparison of these models with standard
long inflationary scenarios.Comment: 9 pages, no figure
Concept for a Time-of-Flight Small Angle Neutron Scattering Instrument at the European Spallation Source
A new Small Angle Neutron Scattering instrument is proposed for the European
Spallation Source. The pulsed source requires a time-of-flight analysis of the
gathered neutrons at the detector. The optimal instrument length is found to be
rather large, which allows for a polarizer and a versatile collimation. The
polarizer allows for studying magnetic samples and incoherent background
subtraction. The wide collimation will host VSANS and SESANS options that
increase the resolution of the instrument towards um and tens of um,
respectively. Two 1m2 area detectors will cover a large solid angle
simultaneously. The expected gains for this new instrument will lie in the
range between 20 and 36, depending on the assessment criteria, when compared to
up-to-date reactor based instruments. This will open new perspectives for fast
kinetics, weakly scattering samples, and multi-dimensional contrast variation
studies.Comment: 18 pages, 10 figure
Adiabatic and entropy perturbations propagation in a bouncing Universe
By studying some bouncing universe models dominated by a specific class of
hydrodynamical fluids, we show that the primordial cosmological perturbations
may propagate smoothly through a general relativistic bounce. We also find that
the purely adiabatic modes, although almost always fruitfully investigated in
all other contexts in cosmology, are meaningless in the bounce or null energy
condition (NEC) violation cases since the entropy modes can never be neglected
in these situations: the adiabatic modes exhibit a fake divergence that is
compensated in the total Bardeen gravitational potential by inclusion of the
entropy perturbations.Comment: 25 pages, no figure, LaTe
Crustal evolution between 2.0 and 3.5 Ga in the southern Gaviao block (Umburanas-Brumado-Aracatu region), Sao Francisco Craton, Brazil: A 3.5-3.8 Ga photo-crust in the Gaviao block?
The main evolution of the Gavião block in the Umburanas-Brumado-Aracatu region, in the state of Bahia, is defined by several sets of tonalitic-trondhjemitic and granodioritic gneisses emplaced during the Paleoarchean. The juvenile Bernada gneisses are e
Passing through the bounce in the ekpyrotic models
By considering a simplified but exact model for realizing the ekpyrotic
scenario, we clarify various assumptions that have been used in the literature.
In particular, we discuss the new ekpyrotic prescription for passing the
perturbations through the singularity which we show to provide a spectrum
depending on a non physical normalization function. We also show that this
prescription does not reproduce the exact result for a sharp transition. Then,
more generally, we demonstrate that, in the only case where a bounce can be
obtained in Einstein General Relativity without facing singularities and/or
violation of the standard energy conditions, the bounce cannot be made
arbitrarily short. This contrasts with the standard (inflationary) situation
where the transition between two eras with different values of the equation of
state can be considered as instantaneous. We then argue that the usually
conserved quantities are not constant on a typical bounce time scale. Finally,
we also examine the case of a test scalar field (or gravitational waves) where
similar results are obtained. We conclude that the full dynamical equations of
the underlying theory should be solved in a non singular case before any
conclusion can be drawn.Comment: 17 pages, ReVTeX 4, 13 figures, minor corrections, conclusions
unchange
Probing the Halo From the Solar Vicinity to the Outer Galaxy: Connecting Stars in Local Velocity Structures to Large-Scale Clouds
(Abridged) This paper presents the first connections made between two local
features in velocity-space found in a survey of M giant stars and stellar
spatial inhomogeneities on global scales. Comparison to cosmological,
chemodynamical stellar halo models confirm that the M giant population is
particularly sensitive to rare, recent and massive accretion events. These
events can give rise to local observed velocity sequences - a signature of a
small fraction of debris from a common progenitor, passing at high velocity
through the survey volume, near the pericenters of their eccentric orbits. The
majority of the debris is found in much larger structures, whose morphologies
are more cloud-like than stream-like and which lie at the orbital apocenters.
Adopting this interpretation, the full-space motions represented by the
observed velocity features are derived under the assumption that the members
within each sequence share a common velocity. Orbit integrations are then used
to trace the past and future trajectories of these stars across the sky
revealing plausible associations with large, previously-discovered, cloud-like
structures. The connections made between nearby velocity structures and these
distant clouds represent preliminary steps towards developing coherent maps of
such giant debris systems. These maps promise to provide new insights into the
origin of debris clouds, new probes of Galactic history and structure, and new
constraints on the high-velocity tails of the local dark matter distribution
that are essential for interpreting direct detection experiments.Comment: submitted to the Astrophysical Journal, 40 pages, 13 figure
A dwarf galaxy remnant in Canis Major: the fossil of an in-plane accretion onto the Milky Way
We present an analysis of the asymmetries in the population of Galactic
M-giant stars present in the 2MASS All Sky catalogue. Several large-scale
asymmetries are detected, the most significant of which is a strong
elliptical-shaped stellar over-density, close to the Galactic plane at (l=240,
b=-8), in the constellation of Canis Major. A small grouping of globular
clusters (NGC 1851, NGC 1904, NGC 2298, and NGC 2808), coincident in position
and radial velocity, surround this structure, as do a number of open clusters.
The population of M-giant stars in this over-density is similar in number to
that in the core of the Sagittarius dwarf galaxy. We argue that this object is
the likely dwarf galaxy progenitor of the ring-like structure that has recently
been found at the edge of the Galactic disk. A numerical study of the tidal
disruption of an accreted dwarf galaxy is presented. The simulated debris fits
well the extant position, distance and velocity information on the ``Galactic
Ring'', as well as that of the M-giant over-densities, suggesting that all
these structures are the consequence of a single accretion event. The disrupted
dwarf galaxy stream orbits close to the Galactic Plane, with a pericentre at
approximately the Solar circle, an orbital eccentricity similar to that of
stars in the Galactic thick disk, as well as a vertical scale height similar to
that of the thick disk. This finding strongly suggests that the Canis Major
dwarf galaxy is a building block of the Galactic thick disk, that the thick
disk is continually growing, even up to the present time, and that thick disk
globular clusters were accreted onto the Milky Way from dwarf galaxies in
co-planar orbits.Comment: 13 pages, 18 figures (2 in colour), accepted for publication in MNRA
Radar climatology of the COPS region
A climatology of convection initiation (CI) and convective enhancements (CE) has been developed using radar reflectivity data in southwestern Germany and eastern
France over the period of May�August of 2000�2006 and 2008. The study region included the Vosges Mountains of France, the Rhine Valley which straddles France and Germany, the Black ForestMountains and the SwabianMountains of Germany.
Convection occurred frequently during the summer months throughout the study region. The CI density (number of initiations per square km) illustrates preferential formation in the mountain regions while the CE events spanned both mountains and valleys nearly equally. There is a strongmid-day peak of the CI events suggesting that diurnal heating is critical for CI in the region. The very strong thunderstorms (>46 dBZ) first occurred in the mountains and �2 h later in the Rhine Valley.
During the summer of 2007, the Convective and Orographically-induced Precipitation Study (COPS) field campaign was conducted with the objective
of obtaining improved understanding of convective processes and short-term quantitative precipitation forecasting in low-mountain regions. Comparisons were
made between the radar climatology results and the COPS summer. The COPS summer exhibited preferential CI density in the mountainous regions but not as pronounced as the climatology. The COPS summer had a similar diurnal peak of CI events as climatology but the ratio of daytime to nighttime CI (1.7), or amplitude of the diurnal cycle, was less than that of climatology (3.0). While both the 8-year climatology and COPS summer were dominated by daytime, locally-forced CI
occurrences, the broad distribution of daytime CI events and increase in nighttime events observed during COPS indicate a more active synoptic pattern in 2007
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