1,010 research outputs found
Backreaction on the luminosity-redshift relation from gauge invariant light-cone averaging
Using a recently proposed gauge invariant formulation of light-cone
averaging, together with adapted "geodesic light-cone" coordinates, we show how
an "induced backreaction" effect emerges, in general, from correlated
fluctuations in the luminosity distance and covariant integration measure.
Considering a realistic stochastic spectrum of inhomogeneities of primordial
(inflationary) origin we find that both the induced backreaction on the
luminosity-redshift relation and the dispersion are larger than naively
expected. On the other hand the former, at least to leading order and in the
linear perturbative regime, cannot account by itself for the observed effects
of dark energy at large-redshifts. A full second-order calculation, or even
better a reliable estimate of contributions from the non-linear regime, appears
to be necessary before firm conclusions on the correct interpretation of the
data can be drawn.Comment: 22 pages, 4 figures. Comments and references added, Fig. 1 modified.
Version accepted for publication in JCA
Disintegrin and metalloproteinases (ADAMs) expression in gastroesophageal reflux disease and in esophageal adenocarcinoma
Clinically useful marker molecules for the progression of gastroesophageal reflux disease and Barrett's esophagus (BE) to esophageal adenocarcinoma (EAC) are lacking. Many adenocarcinomas and inflammatory conditions exhibit increased expression of ADAMs, 'a disintegrin and metalloproteinases'. We assessed the expression of five ADAMs (9, 10, 12, 17, 19) in three esophageal cell lines (Het-1A, OE19, OE33) by RT-PCR and Western blotting, and in human samples of normal esophagus, esophagitis, BE, Barrett's dysplasia, and EAC by RT-PCR, and in selected samples by immunohistochemistry. EAC patients showed increased mRNA expression of ADAMs 9, 12, 17 and 19, as compared to controls. At immunohistochemistry, ADAM9 and ADAM10 proteins were increased in EAC. Patient samples also showed increased mRNA expression of ADAM12 in esophagitis, of ADAM9 in BE, and of ADAMs 9, 12 and 19 in Barrett's dysplasia, as compared to controls. Two EAC cell lines showed increased ADAM9 mRNA. ADAM9 expression is increased in EAC. Its predecessors show increased ADAM9 mRNA expression. The importance of the alterations in ADAM expression for the development of EAC, and their use as marker molecules, warrant further studies.Peer reviewe
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
Chameleonic dilaton, nonequivalent frames, and the cosmological constant problem in quantum string theory
The chameleonic behaviour of the String theory dilaton is suggested. Some of
the possible consequences of the chameleonic string dilaton are analyzed in
detail. In particular, (1) we suggest a new stringy solution to the
cosmological constant problem and (2) we point out the non-equivalence of
different conformal frames at the quantum level. In order to obtain these
results, we start taking into account the (strong coupling) string loop
expansion in the string frame (S-frame), therefore the so-called form factors
are present in the effective action. The correct Dark Energy scale is recovered
in the Einstein frame (E-frame) without unnatural fine-tunings and this result
is robust against all quantum corrections, granted that we assume a proper
structure of the S-frame form factors in the strong coupling regime. At this
stage, the possibility still exists that a certain amount of fine-tuning may be
required to satisfy some phenomenological constraints. Moreover in the E-frame,
in our proposal, all the interactions are switched off on cosmological length
scales (i.e. the theory is IR-free), while higher derivative gravitational
terms might be present locally (on short distances) and it remains to be seen
whether these facts clash with phenomenology. A detailed phenomenological
analysis is definitely necessary to clarify these points
Is backreaction really small within concordance cosmology?
Smoothing over structures in general relativity leads to a renormalisation of
the background, and potentially many other effects which are poorly understood.
Observables such as the distance-redshift relation when averaged on the sky do
not necessarily yield the same smooth model which arises when performing
spatial averages. These issues are thought to be of technical interest only in
the standard model of cosmology, giving only tiny corrections. However, when we
try to calculate observable quantities such as the all-sky average of the
distance-redshift relation, we find that perturbation theory delivers divergent
answers in the UV and corrections to the background of order unity. There are
further problems. Second-order perturbations are the same size as first-order,
and fourth-order at least the same as second, and possibly much larger, owing
to the divergences. Much hinges on a coincidental balance of 2 numbers: the
primordial power, and the ratio between the comoving Hubble scales at
matter-radiation equality and today. Consequently, it is far from obvious that
backreaction is irrelevant even in the concordance model, however natural it
intuitively seems.Comment: 28 pages. Invited contribution to Classical and Quantum Gravity
special issue "Inhomogeneous Cosmological Models and Averaging in Cosmology
Photon Physics in Heavy Ion Collisions at the LHC
Various pion and photon production mechanisms in high-energy nuclear
collisions at RHIC and LHC are discussed. Comparison with RHIC data is done
whenever possible. The prospect of using electromagnetic probes to characterize
quark-gluon plasma formation is assessed.Comment: Writeup of the working group "Photon Physics" for the CERN Yellow
Report on "Hard Probes in Heavy Ion Collisions at the LHC", 134 pages. One
figure added in chapter 5 (comparison with PHENIX data). Some figures and
correponding text corrected in chapter 6 (off-chemical equilibrium thermal
photon rates). Some figures modified in chapter 7 (off-chemical equilibrium
photon rates) and comparison with PHENIX data adde
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
Cosmic Acceleration Driven by Mirage Inhomogeneities
A cosmological model based on an inhomogeneous D3-brane moving in an AdS_5 X
S_5 bulk is introduced. Although there is no special points in the bulk, the
brane Universe has a center and is isotropic around it. The model has an
accelerating expansion and its effective cosmological constant is inversely
proportional to the distance from the center, giving a possible geometrical
origin for the smallness of a present-day cosmological constant. Besides, if
our model is considered as an alternative of early time acceleration, it is
shown that the early stage accelerating phase ends in a dust dominated FRW
homogeneous Universe. Mirage-driven acceleration thus provides a dark matter
component for the brane Universe final state. We finally show that the model
fulfills the current constraints on inhomogeneities.Comment: 14 pages, 1 figure, IOP style. v2, changed style, minor corrections,
references added, version accepted in Class. Quant. Gra
Observational constraints on inhomogeneous cosmological models without dark energy
It has been proposed that the observed dark energy can be explained away by
the effect of large-scale nonlinear inhomogeneities. In the present paper we
discuss how observations constrain cosmological models featuring large voids.
We start by considering Copernican models, in which the observer is not
occupying a special position and homogeneity is preserved on a very large
scale. We show how these models, at least in their current realizations, are
constrained to give small, but perhaps not negligible in certain contexts,
corrections to the cosmological observables. We then examine non-Copernican
models, in which the observer is close to the center of a very large void.
These models can give large corrections to the observables which mimic an
accelerated FLRW model. We carefully discuss the main observables and tests
able to exclude them.Comment: 27 pages, 7 figures; invited contribution to CQG special issue
"Inhomogeneous Cosmological Models and Averaging in Cosmology". Replaced to
match the improved version accepted for publication. Appendix B and
references adde
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