253 research outputs found
Influence of Small-Scale Inhomogeneities on the Cosmological Consistency Tests
The current cosmological dark sector (dark matter plus dark energy) is
challenging our comprehension about the physical processes taking place in the
Universe. Recently, some authors tried to falsify the basic underlying
assumptions of such dark matter-dark energy paradigm. In this Letter, we show
that oversimplifications of the measurement process may produce false positives
to any consistency test based on the globally homogeneous and isotropic LCDM
model and its expansion history based on distance measurements. In particular,
when local inhomogeneity effects due to clumped matter or voids are taken into
account, an apparent violation of the basic assumptions ("Copernican
Principle") seems to be present. Conversely, the amplitude of the deviations
also probes the degree of reliability underlying the phenomenological
Dyer-Roeder procedure by confronting its predictions with the accuracy of the
weak lensing approach. Finally, a new method is devised to reconstruct the
effects of the inhomogeneities in a LCDM model, and some suggestions of how to
distinguish between clumpiness (or void) effects from different cosmologies are
discussed.Comment: 18 pages, 2 figures. Improved version accepted for publication as a
Letter in MNRA
Studying light propagation in a locally homogeneous universe through an extended Dyer-Roeder approach
Light is affected by local inhomogeneities in its propagation, which may
alter distances and so cosmological parameter estimation. In the era of
precision cosmology, the presence of inhomogeneities may induce systematic
errors if not properly accounted. In this vein, a new interpretation of the
conventional Dyer-Roeder (DR) approach by allowing light received from distant
sources to travel in regions denser than average is proposed. It is argued that
the existence of a distribution of small and moderate cosmic voids (or "black
regions") implies that its matter content was redistributed to the homogeneous
and clustered matter components with the former becoming denser than the cosmic
average in the absence of voids. Phenomenologically, this means that the DR
smoothness parameter (denoted here by ) can be greater than unity,
and, therefore, all previous analyses constraining it should be rediscussed
with a free upper limit. Accordingly, by performing a statistical analysis
involving 557 type Ia supernovae (SNe Ia) from Union2 compilation data in a
flat CDM model we obtain for the extended parameter,
(). The effects of are also
analyzed for generic CDM models and flat XCDM cosmologies. For both
models, we find that a value of greater than unity is able to
harmonize SNe Ia and cosmic microwave background observations thereby
alleviating the well-known tension between low and high redshift data. Finally,
a simple toy model based on the existence of cosmic voids is proposed in order
to justify why can be greater than unity as required by supernovae
data.Comment: 5 pages, 2 figures. Title modified, results unchanged. It matches
version published as a Brief Report in Phys. Rev.
Galaxy clusters, type Ia supernovae and the fine structure constant
As is well known, measurements of the Sunyaev-Zeldovich effect can be
combined with observations of the X-ray surface brightness of galaxy clusters
to estimate the angular diameter distance to these structures. In this paper,
we show that this technique depends on the fine structure constant, .
Therefore, if is a time-dependent quantity, e.g., , where is a function of redshift, we argue that current data do
not provide the real angular diameter distance, , to the cluster
but instead . We use this result to
derive constraints on a possible variation of for a class of dilaton
runaway models considering a sample of 25 measurements of in
redshift range and estimates of from
current type Ia supernovae observations. We find no significant indication of
variation of with the present data.Comment: 8 pages, 4 figures, To appear in JCA
On tidal forces in f(R) theories of gravity
Despite the extraordinary attention that modified gravity theories have
attracted over the past decade, the geodesic deviation equation in this context
has not received proper formulation thus far. This equation provides an elegant
way to investigate the timelike, null and spacelike structure of spacetime
geometries. In this investigation we provide the full derivation of this
equation in situations where General Relativity has been extended in
Robertson-Walker background spacetimes. We find that for null geodesics the
contribution arising from the geometrical new terms is in general non-zero.
Finally we apply the results to a well known class of f(R) theories, compare
the results with General Relativity predictions and obtain the equivalent area
distance relation.Comment: 9 pages, 2 figure
Constraints on a possible variation of the fine structure constant from galaxy cluster data
We propose a new method to probe a possible time evolution of the fine
structure constant from X-ray and Sunyaev-Zeldovich measurements of
the gas mass fraction () in galaxy clusters. Taking into account a
direct relation between variations of and violations of the
distance-duality relation, we discuss constraints on for a class of
dilaton runaway models. Although not yet competitive with bounds from high-
quasar absorption systems, our constraints, considering a sample of 29
measurements of , in the redshift interval , provide
an independent estimate of variation at low and intermediate
redshifts. Furthermore, current and planned surveys will provide a larger
amount of data and thus allow to improve the limits on variation
obtained in the present analysis.Comment: 7 pages, 2 figures, accepted for publication in JCA
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