428 research outputs found
"Swiss-Cheese" Inhomogeneous Cosmology & the Dark Energy Problem
We study an exact swiss-cheese model of the Universe, where inhomogeneous LTB
patches are embedded in a flat FLRW background, in order to see how
observations of distant sources are affected. We find negligible integrated
effect, suppressed by (L/R_{H})^3 (where L is the size of one patch, and R_{H}
is the Hubble radius), both perturbatively and non-perturbatively. We
disentangle this effect from the Doppler term (which is much larger and has
been used recently \cite{BMN} to try to fit the SN curve without dark energy)
by making contact with cosmological perturbation theory.Comment: 35 pages, 6 figure
Light Propagation and Large-Scale Inhomogeneities
We consider the effect on the propagation of light of inhomogeneities with
sizes of order 10 Mpc or larger. The Universe is approximated through a
variation of the Swiss-cheese model. The spherical inhomogeneities are
void-like, with central underdensities surrounded by compensating overdense
shells. We study the propagation of light in this background, assuming that the
source and the observer occupy random positions, so that each beam travels
through several inhomogeneities at random angles. The distribution of
luminosity distances for sources with the same redshift is asymmetric, with a
peak at a value larger than the average one. The width of the distribution and
the location of the maximum increase with increasing redshift and length scale
of the inhomogeneities. We compute the induced dispersion and bias on
cosmological parameters derived from the supernova data. They are too small to
explain the perceived acceleration without dark energy, even when the length
scale of the inhomogeneities is comparable to the horizon distance. Moreover,
the dispersion and bias induced by gravitational lensing at the scales of
galaxies or clusters of galaxies are larger by at least an order of magnitude.Comment: 27 pages, 9 figures, revised version to appear in JCAP, analytical
estimate included, typos correcte
Averaging anisotropic cosmologies
We examine the effects of spatial inhomogeneities on irrotational anisotropic
cosmologies by looking at the average properties of anisotropic pressure-free
models. Adopting the Buchert scheme, we recast the averaged scalar equations in
Bianchi-type form and close the standard system by introducing a propagation
formula for the average shear magnitude. We then investigate the evolution of
anisotropic average vacuum models and those filled with pressureless matter. In
the latter case we show that the backreaction effects can modify the familiar
Kasner-like singularity and potentially remove Mixmaster-type oscillations. The
presence of nonzero average shear in our equations also allows us to examine
the constraints that a phase of backreaction-driven accelerated expansion might
put on the anisotropy of the averaged domain. We close by assessing the status
of these and other attempts to define and calculate `average' spacetime
behaviour in general relativity.Comment: revised version, to appear in CQ
Cosmological Backreaction from Perturbations
We reformulate the averaged Einstein equations in a form suitable for use
with Newtonian gauge linear perturbation theory and track the size of the
modifications to standard Robertson-Walker evolution on the largest scales as a
function of redshift for both Einstein de-Sitter and Lambda CDM cosmologies. In
both cases the effective energy density arising from linear perturbations is of
the order of 10^-5 the matter density, as would be expected, with an effective
equation of state w ~ -1/19. Employing a modified Halofit code to extend our
results to quasilinear scales, we find that, while larger, the deviations from
Robertson-Walker behaviour remain of the order of 10^-5.Comment: 15 pages, 8 figures; replaced by version accepted by JCA
An inhomogeneous universe with thick shells and without cosmological constant
We build an exact inhomogeneous universe composed of a central flat Friedmann
zone up to a small redshift , a thick shell made of anisotropic matter, an
hyperbolic Friedmann metric up to the scale where dimming galaxies are observed
() that can be matched to a hyperbolic Lema\^{i}tre-Tolman-Bondi
spacetime to best fit the WMAP data at early epochs. We construct a general
framework which permits us to consider a non-uniform clock rate for the
universe. As a result, both for a uniform time and a uniform Hubble flow, the
deceleration parameter extrapolated by the central observer is always positive.
Nevertheless, by taking a non-uniform Hubble flow, it is possible to obtain a
negative central deceleration parameter, that, with certain parameter choices,
can be made the one observed currently. Finally, it is conjectured a possible
physical mechanism to justify a non-uniform time flow.Comment: Version published in Class. Quantum gra
LASE Measurements of Water Vapor, Aerosol, and Cloud Distributions in Saharan Air Layers and Tropical Disturbances
LASE (Lidar Atmospheric Sensing Experiment) onboard the NASA DC-8 was used to measure high resolution profiles of water vapor and aerosols, and cloud distributions in 14 flights over the eastern Atlantic region during the NAMMA (NASA African Monsoon Multidisciplinary Analyses) field experiment, which was conducted from August 15 to September 12, 2006. These measurements were made in conjunction with flights designed to study African Easterly Waves (AEW), Tropical Disturbances (TD), and Saharan Aerosol Layers (SALs) as well as flights performed in clear air and convective regions. As a consequence of their unique radiative properties and dynamics, SAL layers have a significant influence in the development of organized convection associated with TD. Interactions of the SAL with tropical air during early stages of the development of TD were observed. These LASE measurements represent the first simultaneous water vapor and aerosol lidar measurements to study the SAL and its impact on TDs and hurricanes. Seven AEWs were studied and four of these evolved into tropical storms and three did not. Three out of the four tropical storms evolved into hurricanes
LASE Observations of Interactions Between African Easterly Waves and the Saharan Air Layer
The Lidar Atmospheric Sensing Experiment (LASE) participated in the NASA African Monsoon Multidisciplinary Analyses (NAMMA) field experiment in 2006 that was conducted from Sal, Cape Verde to study the Saharan Air Layer (SAL) and its influence on the African Easterly Waves (AEWs) and Tropical Cyclones (TCs). During NAMMA, LASE collected simultaneous water vapor and aerosol lidar measurements from 14 flights onboard the NASA DC- 8. In this paper we present three examples of the interaction of the SAL and AEWs regarding: moistening of the SAL and transfer of latent heat; injection of dust in an updraft; and influence of dry air intrusion on an AEW. A brief discussion is also given on activities related to the refurbishment of LASE to enhance its operational performance and plans to participate in the next NASA hurricane field experiment in the summer of 2010
Can the Acceleration of Our Universe Be Explained by the Effects of Inhomogeneities?
No. It is simply not plausible that cosmic acceleration could arise within
the context of general relativity from a back-reaction effect of
inhomogeneities in our universe, without the presence of a cosmological
constant or ``dark energy.'' We point out that our universe appears to be
described very accurately on all scales by a Newtonianly perturbed FLRW metric.
(This assertion is entirely consistent with the fact that we commonly encounter
.) If the universe is accurately described by a
Newtonianly perturbed FLRW metric, then the back-reaction of inhomogeneities on
the dynamics of the universe is negligible. If not, then it is the burden of an
alternative model to account for the observed properties of our universe. We
emphasize with concrete examples that it is {\it not} adequate to attempt to
justify a model by merely showing that some spatially averaged quantities
behave the same way as in FLRW models with acceleration. A quantity
representing the ``scale factor'' may ``accelerate'' without there being any
physically observable consequences of this acceleration. It also is {\it not}
adequate to calculate the second-order stress energy tensor and show that it
has a form similar to that of a cosmological constant of the appropriate
magnitude. The second-order stress energy tensor is gauge dependent, and if it
were large, contributions of higher perturbative order could not be neglected.
We attempt to clear up the apparent confusion between the second-order stress
energy tensor arising in perturbation theory and the ``effective stress energy
tensor'' arising in the ``shortwave approximation.''Comment: 20 pages, 1 figure, several footnotes and references added, version
accepted for publication in CQG;some clarifying comments adde
LTB solutions in Newtonian gauge: from strong to weak fields
Lemaitre-Tolman-Bondi (LTB) solutions are used frequently to describe the
collapse or expansion of spherically symmetric inhomogeneous mass distributions
in the Universe. These exact solutions are obtained in the synchronous gauge
where nonlinear dynamics (with respect to the FLRW background) induce large
deviations from the FLRW metric. In this paper we show explicitly that this is
a gauge artefact (for realistic sub-horizon inhomogeneities). We write down the
nonlinear gauge transformation from synchronous to Newtonian gauge for a
general LTB solution using the fact that the peculiar velocities are small. In
the latter gauge we recover the solution in the form of a weakly perturbed FLRW
metric that is assumed in standard cosmology. Furthermore we show how to obtain
the LTB solutions directly in Newtonian gauge and illustrate how the Newtonian
approximation remains valid in the nonlinear regime where cosmological
perturbation theory breaks down. Finally we discuss the implications of our
results for the backreaction scenario.Comment: 17 page
Testing the Void against Cosmological data: fitting CMB, BAO, SN and H0
In this paper, instead of invoking Dark Energy, we try and fit various
cosmological observations with a large Gpc scale under-dense region (Void)
which is modeled by a Lemaitre-Tolman-Bondi metric that at large distances
becomes a homogeneous FLRW metric. We improve on previous analyses by allowing
for nonzero overall curvature, accurately computing the distance to the
last-scattering surface and the observed scale of the Baryon Acoustic peaks,
and investigating important effects that could arise from having nontrivial
Void density profiles. We mainly focus on the WMAP 7-yr data (TT and TE),
Supernova data (SDSS SN), Hubble constant measurements (HST) and Baryon
Acoustic Oscillation data (SDSS and LRG). We find that the inclusion of a
nonzero overall curvature drastically improves the goodness of fit of the Void
model, bringing it very close to that of a homogeneous universe containing Dark
Energy, while by varying the profile one can increase the value of the local
Hubble parameter which has been a challenge for these models. We also try to
gauge how well our model can fit the large-scale-structure data, but a
comprehensive analysis will require the knowledge of perturbations on LTB
metrics. The model is consistent with the CMB dipole if the observer is about
15 Mpc off the centre of the Void. Remarkably, such an off-center position may
be able to account for the recent anomalous measurements of a large bulk flow
from kSZ data. Finally we provide several analytical approximations in
different regimes for the LTB metric, and a numerical module for CosmoMC, thus
allowing for a MCMC exploration of the full parameter space.Comment: 70 pages, 12 figures, matches version accepted for publication in
JCAP. References added, numerical values in tables changed due to minor bug,
conclusions unaltered. Numerical module available at
http://web.physik.rwth-aachen.de/download/valkenburg
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