13 research outputs found
Timelike and null focusing singularities in spherical symmetry: a solution to the cosmological horizon problem and a challenge to the cosmic censorship hypothesis
Extending the study of spherically symmetric metrics satisfying the dominant
energy condition and exhibiting singularities of power-law type initiated in
SI93, we identify two classes of peculiar interest: focusing timelike
singularity solutions with the stress-energy tensor of a radiative perfect
fluid (equation of state: ) and a set of null singularity
classes verifying identical properties. We consider two important applications
of these results: to cosmology, as regards the possibility of solving the
horizon problem with no need to resort to any inflationary scenario, and to the
Strong Cosmic Censorship Hypothesis to which we propose a class of physically
consistent counter-examples.Comment: 26 pages, 2 figures, LaTeX file. Submitted to Phys. Rev.
Testing the Copernican and Cosmological Principles in the local universe with galaxy surveys
Cosmological density fields are assumed to be translational and rotational
invariant, avoiding any special point or direction, thus satisfying the
Copernican Principle. A spatially inhomogeneous matter distribution can be
compatible with the Copernican Principle but not with the stronger version of
it, the Cosmological Principle which requires the additional hypothesis of
spatial homogeneity. We establish criteria for testing that a given density
field, in a finite sample at low redshifts, is statistically and/or spatially
homogeneous. The basic question to be considered is whether a distribution is,
at different spatial scales, self-averaging. This can be achieved by studying
the probability density function of conditional fluctuations. We find that
galaxy structures in the SDSS samples, the largest currently available, are
spatially inhomogeneous but statistically homogeneous and isotropic up to ~ 100
Mpc/h. Evidences for the breaking of self-averaging are found up to the largest
scales probed by the SDSS data. The comparison between the results obtained in
volumes of different size allows us to unambiguously conclude that the lack of
elf-averaging is induced by finite-size effects due to long-range correlated
fluctuations. We finally discuss the relevance of these results from the point
of view of cosmological modeling.Comment: 12 pages, 3 figures, accepted for publication in JCA
How close can an Inhomogeneous Universe mimic the Concordance Model?
Recently, spatially inhomogeneous cosmological models have been proposed as
an alternative to the LCDM model, with the aim of reproducing the late time
dynamics of the Universe without introducing a cosmological constant or dark
energy. This paper investigates the possibility of distinguishing such models
from the standard LCDM using background or large scale structure data. It also
illustrates and emphasizes the necessity of testing the Copernican principle in
order to confront the tests of general relativity with the large scale
structure.Comment: 15 pages, 7 figure
The Scale of Cosmic Isotropy
The most fundamental premise to the standard model of the universe, the
Cosmological Principle (CP), states that the large-scale properties of the
universe are the same in all directions and at all comoving positions.
Demonstrating this theoretical hypothesis has proven to be a formidable
challenge. The cross-over scale R_{iso} above which the galaxy distribution
becomes statistically isotropic is vaguely defined and poorly (if not at all)
quantified. Here we report on a formalism that allows us to provide an
unambiguous operational definition and an estimate of R_{iso}. We apply the
method to galaxies in the Sloan Digital Sky Survey (SDSS) Data Release 7,
finding that R_{iso}\sim 150h^{-1} Mpc. Besides providing a consistency test of
the Copernican principle, this result is in agreement with predictions based on
numerical simulations of the spatial distribution of galaxies in cold dark
matter dominated cosmological models.Comment: 15 pages, 4 figures, accepted by JCAP. The text matches the published
versio
An Inhomogeneous Model Universe Behaving Homogeneously
We present a new model universe based on the junction of FRW to flat
Lemaitre-Tolman-Bondi (LTB) solutions of Einstein equations along our past
light cone, bringing structures within the FRW models. The model is assumed
globally to be homogeneous, i.e. the cosmological principle is valid. Local
inhomogeneities within the past light cone are modeled as a flat LTB, whereas
those outside the light cone are assumed to be smoothed out and represented by
a FRW model. The model is singularity free, always FRW far from the observer
along the past light cone, gives way to a different luminosity distance
relation as for the CDM/FRW models, a negative deceleration parameter near the
observer, and correct linear and non-linear density contrast. As a whole, the
model behaves like a FRW model on the past light cone with a special behavior
of the scale factor, Hubble and deceleration parameter, mimicking dark energy.Comment: 23 pages, 19 figures, published version in GR
Testing the Copernican Principle via Cosmological Observations
Observations of distances to Type-Ia supernovae can be explained by
cosmological models that include either a gigaparsec-scale void, or a cosmic
flow, without the need for Dark Energy. Instead of invoking dark energy, these
inhomogeneous models instead violate the Copernican Principle. we show that
current cosmological observations (Supernovae, Baryon Acoustic Oscillations and
estimates of the Hubble parameters based on the age of the oldest stars) are
not able to rule out inhomogeneous anti-Copernican models. The next generation
of surveys for baryonic acoustic oscillations will be sufficiently precise to
either validate the Copernican Principle or determine the existence of a local
Gpc scale inhomogeneity.Comment: 16 pages, 9 figures; accepted for publication in JCA
Imitating accelerated expansion of the Universe by matter inhomogeneities - corrections of some misunderstandings
A number of misunderstandings about modeling the apparent accelerated
expansion of the Universe, and about the `weak singularity' are clarified: 1.
Of the five definitions of the deceleration parameter given by Hirata and
Seljak (HS), only is a correct invariant measure of
acceleration/deceleration of expansion. The and are unrelated to
acceleration in an inhomogeneous model. 2. The averaging over directions
involved in the definition of does not correspond to what is done in
observational astronomy. 3. HS's equation (38) connecting to the flow
invariants gives self-contradictory results when applied at the centre of
symmetry of the Lema\^{\i}tre-Tolman (L-T) model. The intermediate equation
(31) that determines is correct, but approximate, so it cannot be used
for determining the sign of the deceleration parameter. Even so, at the centre
of symmetry of the L-T model, it puts no limitation on the sign of .
4. The `weak singularity' of Vanderveld {\it et al.} is a conical profile of
mass density at the centre - a perfectly acceptable configuration. 5. The
so-called `critical point' in the equations of the `inverse problem' for a
central observer in an L-T model is a manifestation of the apparent horizon - a
common property of the past light cones in zero-lambda L-T models, perfectly
manageable if the equations are correctly integrated.Comment: 15 pages. Completely rewritten to match the published version. We
added discussion of 2 key papers cited by VFW and identified more clearly the
assumptions, approximations and mistakes that led to certain misconceptions
New exact stationary cylindrical anisotropic fluid solution of GR
International audienceIn a previous paper, the properties of interior spacetimes sourced by stationary cylindrical anisotropic fluids have been analytically studied for both nonrigid and rigid rotation. The gravito-electromagnetic features of different classes of such GR solutions have been described. Their regularity conditions and those for their junction to a vacuum exterior have also been provided. A new class of rigidly rotating exact solutions to Einsteinâs field equations satisfying a physically consistent equation of state for anisotropic fluids is displayed here. Its physical properties are discussed