40 research outputs found
The Spatial Averaging Limit of Covariant Macroscopic Gravity - Scalar Corrections to the Cosmological Equations
It is known that any explicit averaging scheme of the type essential for
describing the large scale behaviour of the Universe, must necessarily yield
corrections to the Einstein equations applied in the Cosmological setting. The
question of whether or not the resulting corrections to the Einstein equations
are significant, is still a subject of debate, partly due to possible
ambiguities in the averaging schemes available. In particular, it has been
argued in the literature that the effects of averaging could be gauge
artifacts. We apply the formalism of Zalaletdinov's Macroscopic Gravity (MG)
which is a fully covariant and nonperturbative averaging scheme, in an attempt
to construct gauge independent corrections to the standard
Friedmann-Lemaitre-Robertson-Walker (FLRW) equations. We find that whereas one
cannot escape the problem of dependence on \emph{one} gauge choice -- which is
inherent in the assumption of large scale homogeneity and isotropy -- it is
however possible to construct \emph{spacetime scalar} corrections to the
standard FLRW equations. This partially addresses the criticism concerning the
corrections being gauge artifacts. For a particular initial choice of gauge
which simplifies the formalism, we explicitly construct these scalars in terms
of the underlying inhomogeneous geometry, and incidentally demonstrate that the
formal structure of the corrections with this gauge choice is identical to that
of analogous corrections derived by Buchert in the context of spatial averaging
of scalars.Comment: 18 pages, no figures, revtex4; v2 - minor clarifications added; v3 -
minor changes in presentation to improve clarity, reference added, to appear
in Phys. Rev.
Correspondence between kinematical backreaction and scalar field cosmologies - the `morphon field'
Spatially averaged inhomogeneous cosmologies in classical general relativity
can be written in the form of effective Friedmann equations with sources that
include backreaction terms. In this paper we propose to describe these
backreaction terms with the help of a homogeneous scalar field evolving in a
potential; we call it the `morphon field'. This new field links classical
inhomogeneous cosmologies to scalar field cosmologies, allowing to reinterpret,
e.g., quintessence scenarios by routing the physical origin of the scalar field
source to inhomogeneities in the Universe. We investigate a one-parameter
family of scaling solutions to the backreaction problem. Subcases of these
solutions (all without an assumed cosmological constant) include
scale-dependent models with Friedmannian kinematics that can mimic the presence
of a cosmological constant or a time-dependent cosmological term. We explicitly
reconstruct the scalar field potential for the scaling solutions, and discuss
those cases that provide a solution to the Dark Energy and coincidence
problems. In this approach, Dark Energy emerges from morphon fields, a
mechanism that can be understood through the proposed correspondence: the
averaged cosmology is characterized by a weak decay (quintessence) or growth
(phantom quintessence) of kinematical fluctuations, fed by `curvature energy'
that is stored in the averaged 3-Ricci curvature. We find that the late-time
trajectories of those models approach attractors that lie in the future of a
state that is predicted by observational constraints.Comment: 36 pages and 6 Figures, matches published version in Class.Quant.Gra
The Effect of Large-Scale Inhomogeneities on the Luminosity Distance
We study the form of the luminosity distance as a function of redshift in the
presence of large scale inhomogeneities, with sizes of order 10 Mpc or larger.
We approximate the Universe through the Swiss-cheese model, with each spherical
region described by the Tolman-Bondi metric. We study the propagation of light
beams in this background, assuming that the locations of the source and the
observer are random. We derive the optical equations for the evolution of the
beam area and shear. Through their integration we determine the configurations
that can lead to an increase of the luminosity distance relative to the
homogeneous cosmology. We find that this can be achieved if the Universe is
composed of spherical void-like regions, with matter concentrated near their
surface. For inhomogeneities consistent with the observed large scale
structure, the relative increase of the luminosity distance is of the order of
a few percent at redshifts near 1, and falls short of explaining the
substantial increase required by the supernova data. On the other hand, the
effect we describe is important for the correct determination of the energy
content of the Universe from observations.Comment: 27 pages, 5 figures Revised version. References added. Conclusions
clarifie
Dark energy as a mirage
Motivated by the observed cosmic matter distribution, we present the
following conjecture: due to the formation of voids and opaque structures, the
average matter density on the path of the light from the well-observed objects
changes from Omega_M ~ 1 in the homogeneous early universe to Omega_M ~ 0 in
the clumpy late universe, so that the average expansion rate increases along
our line of sight from EdS expansion Ht ~ 2/3 at high redshifts to free
expansion Ht ~ 1 at low redshifts. To calculate the modified observable
distance-redshift relations, we introduce a generalized Dyer-Roeder method that
allows for two crucial physical properties of the universe: inhomogeneities in
the expansion rate and the growth of the nonlinear structures. By treating the
transition redshift to the void-dominated era as a free parameter, we find a
phenomenological fit to the observations from the CMB anisotropy, the position
of the baryon oscillation peak, the magnitude-redshift relations of type Ia
supernovae, the local Hubble flow and the nucleosynthesis, resulting in a
concordant model of the universe with 90% dark matter, 10% baryons, no dark
energy, 15 Gyr as the age of the universe and a natural value for the
transition redshift z_0=0.35. Unlike a large local void, the model respects the
cosmological principle, further offering an explanation for the late onset of
the perceived acceleration as a consequence of the forming nonlinear
structures. Additional tests, such as quantitative predictions for angular
deviations due to an anisotropic void distribution and a theoretical derivation
of the model, can vindicate or falsify the interpretation that light
propagation in voids is responsible for the perceived acceleration.Comment: 33 pages, 2 figs; v2: minor clarifications, results unchanged; v3:
matches the version published in General Relativity and Gravitatio
Dark Energy and Gravity
I review the problem of dark energy focusing on the cosmological constant as
the candidate and discuss its implications for the nature of gravity. Part 1
briefly overviews the currently popular `concordance cosmology' and summarises
the evidence for dark energy. It also provides the observational and
theoretical arguments in favour of the cosmological constant as the candidate
and emphasises why no other approach really solves the conceptual problems
usually attributed to the cosmological constant. Part 2 describes some of the
approaches to understand the nature of the cosmological constant and attempts
to extract the key ingredients which must be present in any viable solution. I
argue that (i)the cosmological constant problem cannot be satisfactorily solved
until gravitational action is made invariant under the shift of the matter
lagrangian by a constant and (ii) this cannot happen if the metric is the
dynamical variable. Hence the cosmological constant problem essentially has to
do with our (mis)understanding of the nature of gravity. Part 3 discusses an
alternative perspective on gravity in which the action is explicitly invariant
under the above transformation. Extremizing this action leads to an equation
determining the background geometry which gives Einstein's theory at the lowest
order with Lanczos-Lovelock type corrections. (Condensed abstract).Comment: Invited Review for a special Gen.Rel.Grav. issue on Dark Energy,
edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figure
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc
Apparent and average acceleration of the Universe
In this paper we consider the relation between the volume deceleration
parameter obtained within the Buchert averaging scheme and the deceleration
parameter derived from the supernova observation. This work was motivated by
recent findings that showed that there are models which despite
have volume deceleration parameter . This opens the possibility
that backreaction and averaging effects may be used as an interesting
alternative explanation to the dark energy phenomenon.
We have calculated in some Lema\^itre--Tolman models. For those
models which are chosen to be realistic and which fit the supernova data, we
find that , while those models which we have been able to find
which exhibit turn out to be unrealistic. This indicates that
care must be exercised in relating the deceleration parameter to observations.Comment: 15 pages, 5 figures; matches published versio
Expression of Mir-21 and Mir-143 in Cervical Specimens Ranging from Histologically Normal through to Invasive Cervical Cancer
MicroRNA expression is severely disrupted in carcinogenesis, however limited evidence is available validating results from cell-line models in human clinical cancer specimens. MicroRNA-21 (mir-21) and microRNA-143 (mir-143) have previously been identified as significantly deregulated in a range of cancers including cervical cancer. Our goal was to investigate the expression patterns of several well-studied microRNA species in cervical samples and compare the results to cell line samples.We measured the expression of mir-21 and mir-143 in 142 formalin-fixed, paraffin embedded (FFPE) cervical biopsy tissue blocks, collected from Dantec Oncology Clinic, Dakar, Senegal. MicroRNA expression analysis was performed using Taqman-based real-time PCR assays. Protein immunohistochemical staining was also performed to investigate target protein expression on 72 samples. We found that mir-21 expression increased with worsening clinical diagnosis but that mir-143 was not correlated with histology. These observations were in stark contrast to previous reports involving cervical cancer cell lines in which mir-143 was consistently down-regulated but mir-21 largely unaffected. We also identified, for the first time, that cytoplasmic expression of Programmed Cell Death Protein 4 PDCD4; a known target of mir-21) was significantly lower in women with invasive cervical carcinoma (ICC) in comparison to those with cervical intraepithelial neoplasia (2-3) or carcinoma in situ (CIN2-3/CIS), although there was no significant correlation between mir-21 and PDCD4 expression, despite previous studies identifying PDCD4 transcript as a known mir-21 target.Whilst microRNA biomarkers have a number of promising features, more studies on expression levels in histologically defined clinical specimens are required to investigate clinical relevance of discovery-based studies. Mir-21 may be of some utility in predictive screening, given that we observed a significant correlation between mir-21 expression level and worsening histological diagnosis of cervical cancer