7,554 research outputs found
Probing dark energy with the CMB: projected constraints from WMAP and Planck
We investigate the accuracy attainable by forthcoming space-based
observations of the cosmic microwave background (CMB) temperature and
polarization anisotropy in constraining the dark energy density parameter \Oq
and equation of state \wq=p_{\rm Q}/\rho_{\rm Q}. Despite degeneracies among
parameters, it is possible for high precision observations such as those from
WMAP and Planck to provide interesting information on the nature of the dark
energy. Furthermore, we show that imposing a flat universe constraint makes it
possible to obtain tight limits in the space of dark energy parameters even
from the CMB alone.Comment: 4 pages, 3 figures. Matches version accepted by ApJL. Figures may
also be downloaded at http://www.sissa.it/~bacci/de
Host lifestyle affects human microbiota on daily timescales
Background:
Disturbance to human microbiota may underlie several pathologies. Yet, we lack a comprehensive understanding of how lifestyle affects the dynamics of human-associated microbial communities.
Results:
Here, we link over 10,000 longitudinal measurements of human wellness and action to the daily gut and salivary microbiota dynamics of two individuals over the course of one year. These time series show overall microbial communities to be stable for months. However, rare events in each subjects’ life rapidly and broadly impacted microbiota dynamics. Travel from the developed to the developing world in one subject led to a nearly two-fold increase in the Bacteroidetes to Firmicutes ratio, which reversed upon return. Enteric infection in the other subject resulted in the permanent decline of most gut bacterial taxa, which were replaced by genetically similar species. Still, even during periods of overall community stability, the dynamics of select microbial taxa could be associated with specific host behaviors. Most prominently, changes in host fiber intake positively correlated with next-day abundance changes among 15% of gut microbiota members.
Conclusions:
Our findings suggest that although human-associated microbial communities are generally stable, they can be quickly and profoundly altered by common human actions and experiences.National Science Foundation (U.S.) (Grant 0821391
Reconstruction of a scalar-tensor theory of gravity in an accelerating universe
The present acceleration of the Universe strongly indicated by recent
observational data can be modeled in the scope of a scalar-tensor theory of
gravity. We show that it is possible to determine the structure of this theory
(the scalar field potential and the functional form of the scalar-gravity
coupling) along with the present density of dustlike matter from the following
two observable cosmological functions: the luminosity distance and the linear
density perturbation in the dustlike matter component as functions of redshift.
Explicit results are presented in the first order in the small inverse
Brans-Dicke parameter 1/omega.Comment: 4 pages, LaTeX 2.09, REVTeX 3.0, two-column forma
Neural networks and separation of Cosmic Microwave Background and astrophysical signals in sky maps
The Independent Component Analysis (ICA) algorithm is implemented as a neural
network for separating signals of different origin in astrophysical sky maps.
Due to its self-organizing capability, it works without prior assumptions on
the signals, neither on their frequency scaling, nor on the signal maps
themselves; instead, it learns directly from the input data how to separate the
physical components, making use of their statistical independence. To test the
capabilities of this approach, we apply the ICA algorithm on sky patches, taken
from simulations and observations, at the microwave frequencies, that are going
to be deeply explored in a few years on the whole sky, by the Microwave
Anisotropy Probe (MAP) and by the {\sc Planck} Surveyor Satellite. The maps are
at the frequencies of the Low Frequency Instrument (LFI) aboard the {\sc
Planck} satellite (30, 44, 70 and 100 GHz), and contain simulated astrophysical
radio sources, Cosmic Microwave Background (CMB) radiation, and Galactic
diffuse emissions from thermal dust and synchrotron. We show that the ICA
algorithm is able to recover each signal, with precision going from 10% for the
Galactic components to percent for CMB; radio sources are almost completely
recovered down to a flux limit corresponding to , where
is the rms level of CMB fluctuations. The signal recovering
possesses equal quality on all the scales larger then the pixel size. In
addition, we show that the frequency scalings of the input signals can be
partially inferred from the ICA outputs, at the percent precision for the
dominant components, radio sources and CMB.Comment: 15 pages; 6 jpg and 1 ps figures. Final version to be published in
MNRA
Nature and statistical properties of quasar associated absorption systems in the XQ-100 Legacy Survey
We statistically study the physical properties of a sample of narrow
absorption line (NAL) systems looking for empirical evidences to distinguish
between intrinsic and intervening NALs without taking into account any a priori
definition or velocity cut-off. We analyze the spectra of 100 quasars with 3.5
< z < 4.5, observed with X-shooter/VLT in the context of the XQ-100
Legacy Survey. We detect a 8 excess in the number density of
absorbers within 10,000 km/s of the quasar emission redshift with respect to
the random occurrence of NALs. This excess does not show a dependence on the
quasar bolometric luminosity and it is not due to the redshift evolution of
NALs. It extends far beyond the standard 5000 km/s cut-off traditionally
defined for associated absorption lines. We propose to modify this definition,
extending the threshold to 10,000 km/s when also weak absorbers (equivalent
width < 0.2 \AA) are considered. We infer NV is the ion that better traces the
effects of the quasar ionization field, offering the best statistical tool to
identify intrinsic systems. Following this criterion we estimate that the
fraction of quasars in our sample hosting an intrinsic NAL system is 33
percent. Lastly, we compare the properties of the material along the quasar
line of sight, derived from our sample, with results based on close quasar
pairs investigating the transverse direction. We find a deficiency of cool gas
(traced by CII) along the line of sight associated with the quasar host galaxy,
in contrast with what is observed in the transverse direction.Comment: 18 pages, 13 figures, 5 table
Non-chaotic dynamics in general-relativistic and scalar-tensor cosmology
In the context of scalar-tensor models of dark energy and inflation, the
dynamics of vacuum scalar-tensor cosmology are analysed without specifying the
coupling function or the scalar field potential. A conformal transformation to
the Einstein frame is used and the dynamics of general relativity with a
minimally coupled scalar field are derived for a generic potential. It is shown
that the dynamics are non-chaotic, thus settling an existing debate.Comment: 20 pages, LaTeX, to appear in Class. Quantum Gra
Phenomenology of a realistic accelerating universe using only Planck-scale physics
Modern data is showing increasing evidence that the Universe is accelerating.
So far, all attempts to account for the acceleration have required some
fundamental dimensionless quantities to be extremely small. We show how a class
of scalar field models (which may emerge naturally from superstring theory) can
account for acceleration which starts in the present epoch with all the
potential parameters O(1) in Planck units.Comment: 4 pages including 4 figures. Final version accepted for publication
in PRL with expanded discussion of the relationship to other quintessence
research. No changes to our own wor
Tracker fields from nonminimally coupled theory
We extend the concept of quintessence to a flat nonminimally coupled scalar -
tensor theories of gravity. By means of Noether's symmetries for the
cosmological pointlike Lagrangian L, it is possible to exhibit exact solutions
for a class of models depending on a free parameter s. This parameter comes out
in the relationship existing between the coupling F(\phi) and the potential
V(\phi) because of such a symmetry for L. When inverse power law potentials are
taken in account, a whole family of exact solutions parametrized by such an s
is proposed as a class of tracker fields, and some considerations are made
about them.Comment: 17 pages, submitted to Physical Review
A two-scalar model for a small but nonzero cosmological constant
We revisit a model of the two-scalar system proposed previously for
understanding a small but nonzero cosmological constant. The model provides
solutions of the scalar-fields energy which behaves truly constant for
a limited time interval rather than in the way of tracker- or scaling-type
variations. This causes a mini-inflation, as indicated by recent observations.
As another novel feature, and the ordinary matter density
fall off always side by side, but interlacing, also like (time) as an
overall behavior in conformity with the scenario of a decaying cosmological
constant. A mini-inflation occurs whenever overtakes , which
may happen more than once, shedding a new light on the coincidence problem. We
present a new example of the solution, and offer an intuitive interpretation of
the mechanism of the nonlinear dynamics. We also discuss a chaos-like nature of
the solution.Comment: 9 pages plus 7 figure
Influence of patient-reported outcomes on the treatment effect of deferasirox film-coated and dispersible tablet formulations in the ECLIPSE trial: A post hoc mediation analysis
- …