1,338 research outputs found
Spectra of primordial fluctuations in two-perfect-fluid regular bounces
We introduce analytic solutions for a class of two components bouncing
models, where the bounce is triggered by a negative energy density perfect
fluid. The equation of state of the two components are constant in time, but
otherwise unrelated. By numerically integrating regular equations for scalar
cosmological perturbations, we find that the (would be) growing mode of the
Newtonian potential before the bounce never matches with the the growing mode
in the expanding stage. For the particular case of a negative energy density
component with a stiff equation of state we give a detailed analytic study,
which is in complete agreement with the numerical results. We also perform
analytic and numerical calculations for long wavelength tensor perturbations,
obtaining that, in most cases of interest, the tensor spectral index is
independent of the negative energy fluid and given by the spectral index of the
growing mode in the contracting stage. We compare our results with previous
investigations in the literature.Comment: 11 pages, 5 figure
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
Tensor Perturbations in Quantum Cosmological Backgrounds
In the description of the dynamics of tensor perturbations on a homogeneous
and isotropic background cosmological model, it is well known that a simple
Hamiltonian can be obtained if one assumes that the background metric satisfies
Einstein classical field equations. This makes it possible to analyze the
quantum evolution of the perturbations since their dynamics depends only on
this classical background. In this paper, we show that this simple Hamiltonian
can also be obtained from the Einstein-Hilbert lagrangian without making use of
any assumption about the dynamics of the background metric. In particular, it
can be used in situations where the background metric is also quantized, hence
providing a substantial simplification over the direct approach originally
developed by Halliwell and Hawking.Comment: 24 pages, JHEP forma
Cosmology without inflation
We propose a new cosmological paradigm in which our observed expanding phase
is originated from an initially large contracting Universe that subsequently
experienced a bounce. This category of models, being geodesically complete, is
non-singular and horizon-free, and can be made to prevent any relevant scale to
ever have been smaller than the Planck length. In this scenario, one can find
new ways to solve the standard cosmological puzzles. One can also obtain scale
invariant spectra for both scalar and tensor perturbations: this will be the
case, for instance, if the contracting Universe is dust-dominated at the time
at which large wavelength perturbations get larger than the curvature scale. We
present a particular example based on a dust fluid classically contracting
model, where a bounce occurs due to quantum effects, in which these features
are explicit.Comment: 8 pages, no figur
A non inflationary model with scale invariant cosmological perturbations
We show that a contracting universe which bounces due to quantum cosmological
effects and connects to the hot big-bang expansion phase, can produce an almost
scale invariant spectrum of perturbations provided the perturbations are
produced during an almost matter dominated era in the contraction phase. This
is achieved using Bohmian solutions of the canonical Wheeler-de Witt equation,
thus treating both the background and the perturbations in a fully quantum
manner. We find a very slightly blue spectrum (). Taking
into account the spectral index constraint as well as the CMB normalization
measure yields an equation of state that should be less than , implying , and
that the characteristic size of the Universe at the bounce is , a region where one expects that the Wheeler-DeWitt
equation should be valid without being spoiled by string or loop quantum
gravity effects.Comment: 9 pages, 4 figure
Wake losses optimization of offshore wind farms with moveable floating wind turbines
In the future, floating wind turbines could be used to harvest energy in deep offshore areas where higher
wind mean speeds are observed. Currently, several floating turbine concepts are being designed and
tested in small scale projects; in particular, one concept allows the turbine to move after installation. This
article presents a novel layout optimization framework for wind farms composed of moveable floating
turbines. The proposed framework uses an evolutionary optimization strategy in a nested configuration
which simultaneously optimizes the anchoring locations and the wind turbine position within the mooring
lines for each individual wind direction. The results show that maximum energy production is
obtained when moveable wind turbines are deployed in an optimized layout. In conclusion, the framework
represents a new design optimization tool for future offshore wind farms composed of moveable
floating turbines
Designing for value: Structuring voluntary certification programs to increase stakeholder acceptance
Voluntary certification programs are one type of intervention used to incentivize the commodity agricultural sector in tropical forest landscapes to reduce deforestation and improve sustainability. These programs encourage supply-chain actors to produce and source products according to agreed standards. We review the cases of the Roundtable on Sustainable Palm Oil (RSPO) voluntary certification program in Indonesia, and the Sustainable Agriculture Network (SAN) voluntary certification program for cattle in Brazil. Based on field interviews, we explore the challenges faced by these programs to simultaneously sustain the rigor of their standards and boost producer participation. Taken together, we consider that rigor and participation are the principle components of a program’s sustainability impact. Given the high level of contention that often surrounds certification standards, we suggest that the other core activities (including adoption, implementation, and monitoring and enforcement) of certification programs are under-utilized places for generating benefits to producers that bypass the complex politics surrounding standards setting. We further identify a common progression from capacity building to full compliance for most producers. This trend suggests a need to design programs to maintain or increase the rigor of program standards in tandem with deliberate efforts to provide producers with additional benefits. In particular, providing benefits to producers at earlier points in their progression towards full compliance may attract additional producers to the program. Clear and objective expectations of producers at each stage in their progression towards full compliance also may benefit external stakeholders interested in tracking more granularly the progress of producers and the overall impact of certification programs.
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Artemisinin inhibits neutrophil and macrophage chemotaxis, cytokine production and NET release
Immune cell chemotaxis to the sites of pathogen invasion is critical for fighting infection, but in life-threatening conditions such as sepsis and Covid-19, excess activation of the innate immune system is thought to cause a damaging invasion of immune cells into tissues and a consequent excessive release of cytokines, chemokines and neutrophil extracellular traps (NETs). In these circumstances, tempering excessive activation of the innate immune system may, paradoxically, promote recovery. Here we identify the antimalarial compound artemisinin as a potent and selective inhibitor of neutrophil and macrophage chemotaxis induced by a range of chemotactic agents. Artemisinin released calcium from intracellular stores in a similar way to thapsigargin, a known inhibitor of the Sarco/Endoplasmic Reticulum Calcium ATPase pump (SERCA), but unlike thapsigargin, artemisinin blocks only the SERCA3 isoform. Inhibition of SERCA3 by artemisinin was irreversible and was inhibited by iron chelation, suggesting iron-catalysed alkylation of a specific cysteine residue in SERCA3 as the mechanism by which artemisinin inhibits neutrophil motility. In murine infection models, artemisinin potently suppressed neutrophil invasion into both peritoneum and lung in vivo and inhibited the release of cytokines/chemokines and NETs. This work suggests that artemisinin may have value as a therapy in conditions such as sepsis and Covid-19 in which over-activation of the innate immune system causes tissue injury that can lead to death
Understanding the Mechanisms through Which an Influential Early Childhood Program Boosted Adult Outcomes
A growing literature establishes that high quality early childhood interventions targeted toward disadvantaged children have substantial impacts on later life outcomes. Little is known about the mechanisms producing these impacts. This paper uses longitudinal data on cognitive and personality skills from an experimental evaluation of the influential Perry Preschool program to analyze the channels through which the program boosted both male and female participant outcomes. Experimentally induced changes in personality skills explain a sizable portion of adult treatment effects
Near-Ultraviolet Properties of a Large Sample of Type Ia Supernovae as Observed with the Swift UVOT
We present ultraviolet (UV) and optical photometry of 26 Type Ia supernovae
(SNe~Ia) observed from March 2005 to March 2008 with the NASA {\it Swift}
Ultraviolet and Optical Telescope (UVOT). The dataset consists of 2133
individual observations, making it by far the most complete study of the UV
emission from SNe~Ia to date. Grouping the SNe into three subclasses as derived
from optical observations, we investigate the evolution of the colors of these
SNe, finding a high degree of homogeneity within the normal subclass, but
dramatic differences between that group and the subluminous and SN 2002cx-like
groups. For the normal events, the redder UV filters on UVOT (, ) show
more homogeneity than do the bluer UV filters (, ). Searching for
purely UV characteristics to determine existing optically based groupings, we
find the peak width to be a poor discriminant, but we do see a variation in the
time delay between peak emission and the late, flat phase of the light curves.
The UV light curves peak a few days before the band for most subclasses (as
was previously reported by Jha et al. 2006a), although the SN 2002cx-like
objects peak at a very early epoch in the UV. That group also features the
bluest emission observed among SNe~Ia. As the observational campaign is
ongoing, we discuss the critical times to observe, as determined by this study,
in order to maximize the scientific output of future observations.Comment: Accepted to Astrophysical Journa
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