458 research outputs found
Curvature corrections in DGP brane cosmology
We consider a DGP inspired brane scenario where the action on the brane is
augmented by a function of the Ricci scalar, . The cosmological
implications that such a scenario entails are examined for and shown to
be consistent with a universe expanding with power-law acceleration. It is
shown that two classes of solutions exist for the usual FRW metric and small
Hubble radii. When the Hubble radius becomes larger, we either have a
transition to a fully 5D regime or to a self-inflationary solution which
produces a late accelerated expansion such that the radius becomes a function
of .Comment: 11 pages, 2 figure
Graviton Mass or Cosmological Constant?
To describe a massive graviton in 4D Minkowski space-time one introduces a
quadratic term in the Lagrangian. This term, however, can lead to a
readjustment or instability of the background instead of describing a massive
graviton on flat space. We show that for all local Lorentz-invariant mass terms
Minkowski space is unstable. We start with the Pauli-Fierz (PF) term that is
the only local mass term with no ghosts in the linearized approximation. We
show that nonlinear completions of the PF Lagrangian give rise to instability
of Minkowski space. We continue with the mass terms that are not of a PF type.
Although these models are known to have ghosts in the linearized
approximations, nonlinear interactions can lead to background change due to
which the ghosts are eliminated. In the latter case, however, the graviton
perturbations on the new background are not massive. We argue that a consistent
theory of a massive graviton on flat space can be formulated in theories with
extra dimensions. They require an infinite number of fields or non-local
description from a 4D point of view.Comment: 16 pages; references and comments adde
Microbial communities of the Lemon Creek Glacier show subtle structural variation yet stable phylogenetic composition over space and time
Glaciers are geologically important yet transient ecosystems that support diverse, biogeochemically significant microbial communities. During the melt season glaciers undergo dramatic physical, geochemical and biological changes that exert great influence on downstream biogeochemical cycles. Thus, we sought to understand the temporal melt-season dynamics of microbial communities and associated geochemistry at the terminus of Lemon Creek Glacier (LCG) in coastal southern Alaska. Due to late season snowfall, sampling of LCG occurred in three interconnected areas: proglacial Lake Thomas, the lower glacial outflow stream and the glacier’s terminus. LCG associated microbial communities were phylogenetically diverse and varied by sampling location. However, Betaproteobacteria, Alphaproteobacteria and Bacteroidetes dominated communities at all sampling locations. Strict anaerobic groups such as methanogens, SR1, and OP11 were also recovered from glacier outflows, indicating anoxic conditions in at least some portions of the LCG subglacial environment. Microbial community structure was significantly correlated with sampling location and sodium concentrations. Microbial communities sampled from terminus outflow waters exhibited day-to-day fluctuation in taxonomy and phylogenetic similarity. However, these communities were not significantly different from randomly constructed communities from all three sites. These results indicate that glacial outflows share a large proportion of phylogenetic overlap with downstream environments and that the observed significant shifts in community structure are driven by changes in relative abundance of different taxa, and not complete restructuring of communities. We conclude that LCG glacial discharge hosts a diverse and relatively stable microbiome that shifts at fine taxonomic scales in response to geochemistry and likely water residence time
Error Cost Escalation Through the Project Life Cycle
It is well known that the costs to fix errors increase as the project matures, but how fast do those costs build? A study was performed to determine the relative cost of fixing errors discovered during various phases of a project life cycle. This study used three approaches to determine the relative costs: the bottom-up cost method, the total cost breakdown method, and the top-down hypothetical project method. The approaches and results described in this paper presume development of a hardware/software system having project characteristics similar to those used in the development of a large, complex spacecraft, a military aircraft, or a small communications satellite. The results show the degree to which costs escalate, as errors are discovered and fixed at later and later phases in the project life cycle. If the cost of fixing a requirements error discovered during the requirements phase is defined to be 1 unit, the cost to fix that error if found during the design phase increases to 3 - 8 units; at the manufacturing/build phase, the cost to fix the error is 7 - 16 units; at the integration and test phase, the cost to fix the error becomes 21 - 78 units; and at the operations phase, the cost to fix the requirements error ranged from 29 units to more than 1500 unit
Identification and analysis of the bacterial endosymbiont specialized for production of the chemotherapeutic natural product ET‐743
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/115956/1/emi12908.pd
DGP Cosmology with a Non-Minimally Coupled Scalar Field on the Brane
We construct a DGP inspired braneworld scenario where a scalar field
non-minimally coupled to the induced Ricci curvature is present on the brane.
First we investigate the status of gravitational potential with non-minimal
coupling and observational constraints on this non-minimal model. Then we
further deepen the idea of embedding of FRW cosmology in this non-minimal
setup. Cosmological implications of this scenario are examined with details and
the quintessence and late-time expansion of the universe within this framework
are examined. Some observational constraints imposed on this non-minimal
scenario are studied and relation of this model with dark radiation formalism
is determined with details.Comment: 26 pages, 3 eps figure
Gravity induced over a smooth soliton
I consider gravity induced over a smooth (finite thickness) soliton. Graviton
kinetic term is coupled to bulk scalar that develops solitonic vacuum
expectation value. Couplings of Kaluza-Klein modes to soliton-localized matter
are suppressed, giving rise to crossover distance between
4D and 5D behavior. This system can be viewed as a finite thickness brane
regularization of the model of Dvali, Gabadadze and Porrati.Comment: 12 pages, 2 figure
Sulfur oxidation genes in diverse deep-sea viruses
Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of AAAS for personal use, not for redistribution. The definitive version was published in Science 344 (2014): 757-760, doi:10.1126/science.1252229.Viruses are the most abundant biological entities in the oceans and a pervasive cause of mortality of microorganisms that drive biogeochemical cycles. Although the ecological and evolutionary impacts of viruses on marine phototrophs are well-recognized, little is known about their impact on ubiquitous marine lithotrophs. Here we report 18 genome sequences of double-stranded DNA viruses that putatively infect widespread sulfur-oxidizing bacteria. Fifteen of these viral genomes contain auxiliary metabolic genes for the alpha and gamma subunits of reverse dissimilatory sulfite reductase (rdsr). This enzyme oxidizes elemental sulfur, which is abundant in the hydrothermal plumes studied here. Our findings implicate viruses as a key agent in the sulfur cycle and as a reservoir of genetic diversity for bacterial enzymes that underpin chemosynthesis in the deep oceans.This project is funded in part by the Gordon and Betty Moore Foundation Grant GBMF2609 and National Science Foundation Grant OCE1038006
Microbial iron uptake as a mechanism for dispersing iron from deep-sea hydrothermal vents
Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Nature Publishing Group for personal use, not for redistribution. The definitive version was published in Nature Communications 5 (2014): 3192, doi:10.1038/ncomms4192.Deep-sea hydrothermal vents are a significant source of oceanic iron. Although
hydrothermal iron rapidly precipitates as inorganic minerals upon mixing with
seawater, it can be stabilized by organic matter and dispersed more widely than
previously recognized. The nature and source of this organic matter is unknown.
Here we show that microbial genes involved in cellular iron uptake are highly
expressed in the Guaymas Basin deep-sea hydrothermal plume. The nature of
these microbial iron transporters, taken together with the low concentration of
dissolved iron and abundance of particulate iron in the plume, indicates that
iron minerals are the target for this microbial scavenging and uptake. Our
findings indicate that cellular iron uptake is a major process in plume microbial
communities and suggest new mechanisms for generating Fe-C complexes. This
“microbial iron pump” could represent an important mode of converting
hydrothermal iron into bioavailable forms that can be dispersed through the
oceans.This project is funded by the Gordon and Betty Moore
Foundation through grant GBMF 2609 to GJD/JAB/BMT and by the National
Science Foundation through grants OCE 1029242 to GJD, and R2K grant
OCE1038055 to JAB/BMT. We thank the University of Michigan Rackham Graduate
School Faculty Research Fellowship Program for their support.2014-08-0
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