80 research outputs found
Estimations of changes of the Sun's mass and the gravitation constant from the modern observations of planets and spacecraft
More than 635 000 positional observations (mostly radiotechnical) of planets
and spacecraft (1961-2010), have been used for estimating possible changes of
the gravitation constant, the solar mass, and semi-major axes of planets, as
well as the value of the astronomical unit, related to them. The analysis of
the observations has been performed on the basis of the EPM2010 ephemerides of
IAA RAS in post-newtonian approximation. The obtained results indicate on
decrease in the heliocentric gravitation constant per year at the level The positive secular
changes of semi-major axes have been obtained simultaneously
for the planets Mercury, Venus, Mars, Jupiter, Saturn, as expected if the
geliocentric gravitation constant is decreasing in century wise. The change of
the mass of the Sun due to the solar radiation and the solar wind and
the matter dropping on the Sun (comets, meteors, asteroids and dust) was
estimated. Taking into account the maximal limits of the possible
change, the value falls within the interval in year with the 95% probability. The
astronomical unit (au) is only connected with the geliocentric gravitation
constant by its definition. In the future, the connection between
and au should be fixed at the certain time moment, as it is inconvenient highly
to have the changing value of the astronomical unit.Comment: 20 pages, 4 tables, accepted for publication in Solar System
Research, 2011 (Astronomicheskii vestnik
A Counterexample to Claimed COBE Constraints on Compact Toroidal Universe Models
It has been suggested that if the Universe satisfies a flat, multiply
connected, perturbed Friedmann-Lema^itre model, then cosmic microwave
background data from the COBE satellite implies that the minimum size of the
injectivity diameter (shortest closed spatial geodesic) must be larger than
about two fifths of the horizon diameter. To show that this claim is
misleading, a simple universe model of injectivity diameter a
quarter of this size, i.e. a tenth of the horizon diameter, is shown to be
consistent with COBE four year observational maps of the cosmic microwave
background. This is done using the identified circles principle.Comment: 11 pages, 3 figures, accepted for Classical & Quantum Gravit
Cosmology with Interaction between Phantom Dark Energy and Dark Matter and the Coincidence Problem
We study a cosmological model in which phantom dark energy is coupled to dark
matter by phenomenologically introducing a coupled term to the equations of
motion of dark energy and dark matter. This term is parameterized by a
dimensionless coupling function , Hubble parameter and the energy
density of dark matter, and it describes an energy flow between the dark energy
and dark matter. We discuss two cases: one is the case where the
equation-of-state of the dark energy is a constant; the other is
that the dimensionless coupling function is a constant. We investigate
the effect of the interaction on the evolution of the universe, the total
lifetime of the universe, and the ratio of the period when the universe is in
the coincidence state to its total lifetime. It turns out that the interaction
will produce significant deviation from the case without the interaction.Comment: Latex, 17 pages including 14 figures, minor change
Suppressing CMB Quadrupole with a Bounce from Contracting Phase to Inflation
Recent released WMAP data show a low value of quadrupole in the CMB
temperature fluctuations, which confirms the early observations by COBE. In
this paper, a scenario, in which a contracting phase is followed by an
inflationary phase, is constructed. We calculate the perturbation spectrum and
show that this scenario can provide a reasonable explanation for lower CMB
anisotropies on large angular scales.Comment: 5 pages, 3 figure
Crossing the Phantom Divide: Theoretical Implications and Observational Status
If the dark energy equation of state parameter w(z) crosses the phantom
divide line w=-1 (or equivalently if the expression d(H^2(z))/dz - 3\Omega_m
H_0^2 (1+z)^2 changes sign) at recent redshifts, then there are two possible
cosmological implications: Either the dark energy consists of multiple
components with at least one non-canonical phantom component or general
relativity needs to be extended to a more general theory on cosmological
scales. The former possibility requires the existence of a phantom component
which has been shown to suffer from serious theoretical problems and
instabilities. Therefore, the later possibility is the simplest realistic
theoretical framework in which such a crossing can be realized. After providing
a pedagogical description of various dark energy observational probes, we use a
set of such probes (including the Gold SnIa sample, the first year SNLS
dataset, the 3-year WMAP CMB shift parameter, the SDSS baryon acoustic
oscillations peak (BAO), the X-ray gas mass fraction in clusters and the linear
growth rate of perturbations at z=0.15 as obtained from the 2dF galaxy redshift
survey) to investigate the priors required for cosmological observations to
favor crossing of the phantom divide. We find that a low \Omega_m prior
(0.2<\Omega_m <0.25) leads, for most observational probes (except of the SNLS
data), to an increased probability (mild trend) for phantom divide crossing. An
interesting degeneracy of the ISW effect in the CMB perturbation spectrum is
also pointed out.Comment: Accepted in JCAP (to appear). Comments added, typos corrected. 19
pages (revtex), 8 figures. The numerical analysis files (Mathematica +
Fortran) with instructions are available at
http://leandros.physics.uoi.gr/pdl-cross/pdl-cross.htm . The ppt file of a
relevant talk may be downloaded from
http://leandros.physics.uoi.gr/pdl-cross/pdl2006.pp
Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of
dark matter and dark energy in the universe. In this brief review we outline
how weak lensing helps determine the structure of dark matter halos, measure
the expansion rate of the universe, and distinguish between modified gravity
and dark energy explanations for the acceleration of the universe. We also
discuss requirements on the control of systematic errors so that the
systematics do not appreciably degrade the power of weak lensing as a
cosmological probe.Comment: Invited review article for the GRG special issue on gravitational
lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). V3: subsection on
three-point function and some references added. Matches the published versio
f(R) theories
Over the past decade, f(R) theories have been extensively studied as one of
the simplest modifications to General Relativity. In this article we review
various applications of f(R) theories to cosmology and gravity - such as
inflation, dark energy, local gravity constraints, cosmological perturbations,
and spherically symmetric solutions in weak and strong gravitational
backgrounds. We present a number of ways to distinguish those theories from
General Relativity observationally and experimentally. We also discuss the
extension to other modified gravity theories such as Brans-Dicke theory and
Gauss-Bonnet gravity, and address models that can satisfy both cosmological and
local gravity constraints.Comment: 156 pages, 14 figures, Invited review article in Living Reviews in
Relativity, Published version, Comments are welcom
The Cosmological Constant
This is a review of the physics and cosmology of the cosmological constant.
Focusing on recent developments, I present a pedagogical overview of cosmology
in the presence of a cosmological constant, observational constraints on its
magnitude, and the physics of a small (and potentially nonzero) vacuum energy.Comment: 50 pages. Submitted to Living Reviews in Relativity
(http://www.livingreviews.org/), December 199
Uso de leguminosas na dieta de ruminantes: adaptação às mudanças climåticas e mitigação da emissão de gases de efeito estufa.
Frente aos evidentes sinais de mudanças climĂĄticas, o uso de leguminosas forrageiras na dieta de ruminantes torna-se de grande relevância para a otimização da produção animal, o uso racional dos recursos naturais e a manutenção do meio ambiente. Ao longo deste capĂtulo serĂŁo abordadas as caracterĂsticas de cultura e de desempenho animal referente, principalmente, ao consĂłrcio de gramĂneas com as leguminosas macrotiloma ou feijĂŁo guandu. Observou-se que ambas as leguminosas, quando consorciadas, demonstraram serem eficientes em reduzir a emissĂŁo de metano quando levado em consideração o desempenho animal (dados preliminares), sendo, portanto, uma boa estratĂŠgia para mitigação das emissĂľes de gases de efeito estufa
Aberrant host immune response induced by highly virulent PRRSV identified by digital gene expression tag profiling
<p>Abstract</p> <p>Background</p> <p>There was a large scale outbreak of the highly pathogenic porcine reproductive and respiratory syndrome (PRRS) in China and Vietnam during 2006 and 2007 that resulted in unusually high morbidity and mortality among pigs of all ages. The mechanisms underlying the molecular pathogenesis of the highly virulent PRRS virus (H-PRRSV) remains unknown. Therefore, the relationship between pulmonary gene expression profiles after H-PRRSV infection and infection pathology were analyzed in this study using high-throughput deep sequencing and histopathology.</p> <p>Results</p> <p>H-PRRSV infection resulted in severe lung pathology. The results indicate that aberrant host innate immune responses to H-PRRSV and induction of an anti-apoptotic state could be responsible for the aggressive replication and dissemination of H-PRRSV. Prolific rapid replication of H-PRRSV could have triggered aberrant sustained expression of pro-inflammatory cytokines and chemokines leading to a markedly robust inflammatory response compounded by significant cell death and increased oxidative damage. The end result was severe tissue damage and high pathogenicity.</p> <p>Conclusions</p> <p>The systems analysis utilized in this study provides a comprehensive basis for better understanding the pathogenesis of H-PRRSV. Furthermore, it allows the genetic components involved in H-PRRSV resistance/susceptibility in swine populations to be identified.</p
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