142 research outputs found
Gauge Coupling Variation in Brane Models
We consider the space-time variation of gauge couplings in brane-world models
induced by the coupling to a bulk scalar field. A variation is generated by the
running of the gauge couplings with energy and a conformal anomaly while going
from the Jordan to the Einstein frame. We indicate that the one-loop
corrections cancel implying that one obtains a variation of the fine structure
constant by either directly coupling the gauge fields to the bulk scalar field
or having bulk scalar field dependent Yukawa couplings. Taking into account the
cosmological dynamics of the bulk scalar field, we constrain the strength of
the gauge coupling dependence on the bulk scalar field and relate it to
modifications of gravity at low energy.Comment: 4 pages, 1 figur
Low Energy Branes, Effective Theory and Cosmology
The low energy regime of cosmological BPS-brane configurations with a bulk
scalar field is studied. We construct a systematic method to obtain
five-dimensional solutions to the full system of equations governing the
geometry and dynamics of the bulk. This is done for an arbitrary bulk scalar
field potential and taking into account the presence of matter on the branes.
The method, valid in the low energy regime, is a linear expansion of the system
about the static vacuum solution. Additionally, we develop a four-dimensional
effective theory describing the evolution of the system. At the lowest order in
the expansion, the effective theory is a bi-scalar tensor theory of gravity.
One of the main features of this theory is that the scalar fields can be
stabilized naturally without the introduction of additional mechanisms,
allowing satisfactory agreement between the model and current observational
constraints. The special case of the Randall-Sundrum model is discussed.Comment: 19 pages, 4 figure
Fluctuating brane in a dilatonic bulk
We consider a cosmological brane moving in a static five-dimensional bulk
spacetime endowed with a scalar field whose potential is exponential. After
studying various cosmological behaviours for the homogeneous background, we
investigate the fluctuations of the brane that leave spacetime unaffected. A
single mode embodies these fluctuations and obeys a wave equation which we
study for bouncing and ever-expanding branes.Comment: 17 pages, 7 figures, revte
Bulk scalar field in the braneworld can mimic the 4D inflaton dynamics
Based on the recently proposed scenario of inflation driven by a bulk scalar
field in the braneworld of the Randall-Sundrum (RS) type, we investigate the
dynamics of a bulk scalar field on the inflating braneworld. We derive the late
time behavior of the bulk scalar field by analyzing the property of the
retarded Green function. We find that the late time behavior is basically
dominated by a single (or a pair of) pole(s) in the Green function irrespective
of the initial condition and of the signature of , where
is the potential of the bulk scalar field. Including the lowest order
back-reaction to the geometry, this late time behavior can be well approximated
by an effective 4-dimensional scalar field with . The
mapping to the 4-dimensional effective theory is given by a simple scaling of
the potential with a redefinition of the field. Our result supports the picture
that the scenario of inflation driven by a bulk scalar field works in a quite
similar way to that in the standard 4-dimensional cosmology.Comment: 12 pages, no figures, final version to be published in PR
The SPHERE data center: a reference for high contrast imaging processing
The objective of the SPHERE Data Center is to optimize the scientific return
of SPHERE at the VLT, by providing optimized reduction procedures, services to
users and publicly available reduced data. This paper describes our motivation,
the implementation of the service (partners, infrastructure and developments),
services, description of the on-line data, and future developments. The SPHERE
Data Center is operational and has already provided reduced data with a good
reactivity to many observers. The first public reduced data have been made
available in 2017. The SPHERE Data Center is gathering a strong expertise on
SPHERE data and is in a very good position to propose new reduced data in the
future, as well as improved reduction procedures.Comment: SF2A proceeding
Cosmology and Brane Worlds: A Review
Cosmological consequences of the brane world scenario are reviewed in a
pedagogical manner. According to the brane world idea, the standard model
particles are confined on a hyper--surface (a so--called brane), which is
embedded in a higher--dimensional spacetime (the so--called bulk). We begin our
review with the simplest consistent brane world model: a single brane embedded
in a five--dimensional Anti-de Sitter space--time. Then we include a scalar
field in the bulk and discuss in detail the difference with the Anti-de Sitter
case. The geometry of the bulk space--time is also analysed in some depth.
Finally, we investigate the cosmology of a system with two branes and a bulk
scalar field. We comment on brane collisions and summarize some open problems
of brane world cosmology.Comment: 37 pages; invited topical review for Classical and Quantum Gravity;
to appea
Doubling of marine dinitrogen-fixation rates based on direct measurements
Biological dinitrogen fixation provides the largest input of nitrogen to the oceans, therefore exerting important control on the ocean’s nitrogen inventory and primary productivity. Nitrogen-isotope data fromocean sediments suggest that the marine-nitrogen inventory has been balanced for the past 3,000 years (ref. 4). Producing a balanced marine-nitrogenbudget based on direct measurements has proved difficult, however, with nitrogen loss exceeding the gain from dinitrogen fixation by approximately 200 TgNyr-1 (refs 5, 6). Here we present data from the Atlantic Ocean and show that the most widely used method of measuring oceanic N2-fixation rates underestimates the contribution of N2-fixing microorganisms (diazotrophs) relative to a newly developed method. Using molecular techniques to quantify the abundance of specific clades of diazotrophs in parallel with rates of 15N2 incorporation into particulate organic matter, we suggest that the difference between N2-fixation rates measured with the established method and those measured with the new method8 can be related to the composition of the diazotrophic community.
Our data show that in areas dominated by Trichodesmium, the established method underestimatesN2-fixation rates by an averageof 62%. We also find that the newly developed method yields N2-fixation rates more than six times higher than those from the established method when unicellular, symbiotic cyanobacteria and c-proteobacteria dominate the diazotrophic community. On the basis of average areal rates measured over the Atlantic Ocean, we calculated basin-wide N2-fixation rates of 14+/-1TgNyr-1 and 24+/-1TgNyr-1 for the established and new methods, respectively. If our findings can be extrapolated to other ocean basins, this suggests that the global marine N2-fixation rate derived from direct measurements may increase from 103+/-8TgNyr-1 to 177+/-8TgNyr-1, and that the contribution of N2 fixers other than Trichodesmium is much more significant than was
previously thought
Non-tachyonic brane inflation
We consider non-tachyonic hybrid inflation in the context of the braneworld
cosmology. When one considers models for brane inflation, hybrid inflation is a
natural consequence of the tachyon condensation if it appears at the end of
inflation. In this case, however, reheating is a difficult problem due to the
peculiar properties of the tachyon. In this paper we show some models for brane
inflation where a new type of hybrid inflation is realized due to the localized
matter fields. The obvious advantage of our scenario is successful reheating,
which is due to the potential that is localized on the brane. The serious
problem of the loop correction is also avoided.Comment: 14pages, latex2e, references added, final version to appear in PR
Four--dimensional Gravity from Singular Spaces
The modification to four--dimensional Einstein gravity at low energy in two
brane models is investigated within supergravity in singular spaces. Using
perturbation theory around a static BPS background, we study the effective
four--dimensional gravitational theory, a scalar--tensor theory, and derive the
Brans--Dicke parameter when matter is present on the positive tension brane
only. We show there is an attractor mechanism towards general relativity in the
matter dominated era. The dynamics of the interbrane distance are discussed.
Finally, when matter lives on both branes, we find that there is a violation of
the equivalence principle whose magnitude is governed by the warping of the
extra dimension.Comment: 9 pages, LaTe
Nitrogenase Gene Amplicons from Global Marine Surface Waters Are Dominated by Genes of Non-Cyanobacteria
Cyanobacteria are thought to be the main N2-fixing organisms
(diazotrophs) in marine pelagic waters, but recent molecular analyses indicate
that non-cyanobacterial diazotrophs are also present and active. Existing data
are, however, restricted geographically and by limited sequencing depths. Our
analysis of 79,090 nitrogenase (nifH) PCR amplicons encoding
7,468 unique proteins from surface samples (ten DNA samples and two RNA samples)
collected at ten marine locations world-wide provides the first in-depth survey
of a functional bacterial gene and yield insights into the composition and
diversity of the nifH gene pool in marine waters. Great
divergence in nifH composition was observed between sites.
Cyanobacteria-like genes were most frequent among amplicons from the warmest
waters, but overall the data set was dominated by nifH
sequences most closely related to non-cyanobacteria. Clusters related to Alpha-,
Beta-, Gamma-, and Delta-Proteobacteria were most common and showed distinct
geographic distributions. Sequences related to anaerobic bacteria
(nifH Cluster III) were generally rare, but preponderant in
cold waters, especially in the Arctic. Although the two transcript samples were
dominated by unicellular cyanobacteria, 42% of the identified
non-cyanobacterial nifH clusters from the corresponding DNA
samples were also detected in cDNA. The study indicates that non-cyanobacteria
account for a substantial part of the nifH gene pool in marine
surface waters and that these genes are at least occasionally expressed. The
contribution of non-cyanobacterial diazotrophs to the global N2
fixation budget cannot be inferred from sequence data alone, but the prevalence
of non-cyanobacterial nifH genes and transcripts suggest that
these bacteria are ecologically significant
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