8,090 research outputs found
A New Redshift Interpretation
A nonhomogeneous universe with vacuum energy, but without spacetime
expansion, is utilized together with gravitational and Doppler redshifts as the
basis for proposing a new interpretation of the Hubble relation and the 2.7K
Cosmic Blackbody Radiation.Comment: 9 pages LaTeX, no figure
Conditions for low-redshift positive apparent acceleration in smooth inhomogeneous models
It is known that a smooth LTB model cannot have a positive apparent central
acceleration. Using a local Taylor expansion method we study the low-redshift
conditions to obtain an apparent negative deceleration parameter
derived from the luminosity distance for a central observer in a LTB
space, confirming that central smoothness implies a positive central
deceleration. Since observational data is only available at redshift greater
than zero we find the critical values of the parameters defining a centrally
smooth LTB model which give a positive apparent acceleration at ,
providing a graphical representation of the conditions in the
plane, which are respectively the zero and first order
terms of the central Taylor expansion of . We finally derive a
coordinate independent expression for the apparent deceleration parameter based
on the expansion of the relevant functions in red-shift rather than in the
radial coordinate.
We calculate with two different methods to solve the null
geodesic equations, one based on a local central expansion of the solution in
terms of cosmic time and the other one using the exact analytical solution in
terms of generalized conformal time. %The expansion of the solution in terms of
cosmic time is quite useful also for other applications requiring foliation %of
space-time in space-like hyper-surfaces, such as spatial averaging, which is
much more difficult to study using the %analytical solution in terms of the
generalized conformal time coordinate.Comment: 18 pages, 3 figures, abstract, added section with coordinate
independent conditions, version accepted for publication in GR
The spherical symmetry Black hole collapse in expanding universe
The spherical symmetry Black holes are considered in expanding background.
The singularity line and the marginally trapped tube surface behavior are
discussed. In particular, we address the conditions whether dynamical horizon
forms for these cosmological black holes. We also discuss about the
cosmological constant effect on these black hole and the redshift of the light
which comes from the marginally trapped tube surface.Comment: 7 pages, 3 figures. Accepted for publication in International Journal
of Modern Physics D (IJMPD). arXiv admin note: text overlap with
arXiv:gr-qc/0308033 and arXiv:gr-qc/030611
Big Bang Nucleosynthesis Constraints on the Self-Gravity of Pressure
Using big bang nucleosynthesis and present, high-precision measurements of
light element abundances, we constrain the self-gravity of radiation pressure
in the early universe. The self-gravity of pressure is strictly non-Newtonian,
and thus the constraints we set provide a direct test of this prediction of
general relativity and of the standard, Robertson-Walker-Friedmann cosmology.Comment: 5 pages, 1 figure. This paper was developed from an earlier version
which was posted as arXiv:0707.358
On cosmological observables in a swiss-cheese universe
Photon geodesics are calculated in a swiss-cheese model, where the cheese is
made of the usual Friedmann-Robertson-Walker solution and the holes are
constructed from a Lemaitre-Tolman-Bondi solution of Einstein's equations. The
observables on which we focus are the changes in the redshift, in the
angular-diameter--distance relation, in the luminosity-distance--redshift
relation, and in the corresponding distance modulus. We find that redshift
effects are suppressed when the hole is small because of a compensation effect
acting on the scale of half a hole resulting from the special case of spherical
symmetry. However, we find interesting effects in the calculation of the
angular distance: strong evolution of the inhomogeneities (as in the approach
to caustic formation) causes the photon path to deviate from that of the FRW
case. Therefore, the inhomogeneities are able to partly mimic the effects of a
dark-energy component. Our results also suggest that the nonlinear effects of
caustic formation in cold dark matter models may lead to interesting effects on
photon trajectories.Comment: 25 pages, 21 figures; replaced to fit the version accepted for
publication in Phys. Rev.
Interpretations of the Accelerating Universe
It is generally argued that the present cosmological observations support the
accelerating models of the universe, as driven by the cosmological constant or
`dark energy'. We argue here that an alternative model of the universe is
possible which explains the current observations of the universe. We
demonstrate this with a reinterpretation of the magnitude-redshift relation for
Type Ia supernovae, since this was the test that gave a spurt to the current
trend in favour of the cosmological constant.Comment: 12 pages including 2 figures, minor revision, references added, a
paragraph on the interpretation of the CMB anisotropy in the QSSC added in
conclusion, general results unchanged. To appear in the October 2002 issue of
the "Publications of the Astronmical Society of the Pacific
Formation of cosmological mass condensation within a FRW universe: exact general relativistic solutions
Within the framework of an exact general relativistic formulation of gluing
manifolds, we consider the problem of matching an inhomogeneous overdense
region to a Friedmann-Robertson-Walker background universe in the general
spherical symmetric case of pressure-free models. It is shown that, in general,
the matching is only possible through a thin shell, a fact ignored in the
literature. In addition to this, in subhorizon cases where the matching is
possible, an intermediate underdense region will necessarily arise.Comment: 6 page
On matching LTB and Vaidya spacetimes through a null hypersurface
In this work the matching of a LTB interior solution representing dust matter
to the Vaidya exterior solution describing null fluid through a null
hypersurface is studied. Different cases in which one is able to smoothly match
these two solutions to Einstein equations along a null hypesurface are
discussed.Comment: 5 pages, to appear in GR
Redshift spherical shell energy in isotropic Universes
We introduce the redshift spherical shell energy (RSSE), which can be used to
test in the redshift space the radial inhomogeneity of an isotropic universe,
providing additional constraints for LTB models, and a more general test of
cosmic homogeneity.Comment: 11 pages, 2 figures, Accepted by Physical Review D1
Spatial averaging and apparent acceleration in inhomogeneous spaces
As an alternative to dark energy that explains the observed acceleration of
the universe, it has been suggested that we may be at the center of an
inhomogeneous isotropic universe described by a Lemaitre-Tolman-Bondi (LTB)
solution of Einstein's field equations. To test this possibility, it is
necessary to solve the null geodesics. In this paper we first give a detailed
derivation of a fully analytical set of differential equations for the radial
null geodesics as functions of the redshift in LTB models. As an application we
use these equaions to show that a positive averaged acceleration obtained
in LTB models through spatial averaging can be incompatible with cosmological
observations. We provide examples of LTB models with positive which fail
to reproduce the observed luminosity distance . Since the apparent
cosmic acceleration is obtained from fitting the observed luminosity
distance to a FLRW model we conclude that in general a positive in LTB
models does not imply a positive .Comment: 16 pages, 12 figures. Explicit derivation of the fully analytical
null geodesic equations has been added. Published in GR
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