159 research outputs found
Do stochastic inhomogeneities affect dark-energy precision measurements?
The effect of a stochastic background of cosmological perturbations on the
luminosity-redshift relation is computed to second order through a recently
proposed covariant and gauge-invariant light-cone averaging procedure. The
resulting expressions are free from both ultraviolet and infrared divergences,
implying that such perturbations cannot mimic a sizable fraction of dark
energy. Different averages are estimated and depend on the particular function
of the luminosity distance being averaged. The energy flux, being minimally
affected by perturbations at large z, is proposed as the best choice for
precision estimates of dark-energy parameters. Nonetheless, its irreducible
(stochastic) variance induces statistical errors on \Omega_{\Lambda}(z)
typically lying in the few-percent range.Comment: 5 pages, 3 figures. Comments and references added. Typos corrected.
Version accepted for publication in Phys. Rev. Let
Can we avoid dark energy?
The idea that we live near the centre of a large, nonlinear void has
attracted attention recently as an alternative to dark energy or modified
gravity. We show that an appropriate void profile can fit both the latest
cosmic microwave background and supernova data. However, this requires either a
fine-tuned primordial spectrum or a Hubble rate so low as to rule these models
out. We also show that measurements of the radial baryon acoustic scale can
provide very strong constraints. Our results present a serious challenge to
void models of acceleration.Comment: 5 pages, 4 figures; minor changes; version published in Phys. Rev.
Let
A Test of the Copernican Principle
The blackbody nature of the cosmic microwave background (CMB) radiation
spectrum is used in a modern test of the Copernican Principle. The reionized
universe serves as a mirror to reflect CMB photons, thereby permitting a view
of ourselves and the local gravitational potential. By comparing with
measurements of the CMB spectrum, a limit is placed on the possibility that we
occupy a privileged location, residing at the center of a large void. The
Hubble diagram inferred from lines-of-sight originating at the center of the
void may be misinterpreted to indicate cosmic acceleration. Current limits on
spectral distortions are shown to exclude the largest voids which mimic cosmic
acceleration. More sensitive measurements of the CMB spectrum could prove the
existence of such a void or confirm the validity of the Copernican Principle.Comment: 4 pages, 3 figure
Large scale structure simulations of inhomogeneous LTB void models
We perform numerical simulations of large scale structure evolution in an
inhomogeneous Lemaitre-Tolman-Bondi (LTB) model of the Universe. We follow the
gravitational collapse of a large underdense region (a void) in an otherwise
flat matter-dominated Einstein-deSitter model. We observe how the (background)
density contrast at the centre of the void grows to be of order one, and show
that the density and velocity profiles follow the exact non-linear LTB solution
to the full Einstein equations for all but the most extreme voids. This result
seems to contradict previous claims that fully relativistic codes are needed to
properly handle the non-linear evolution of large scale structures, and that
local Newtonian dynamics with an explicit expansion term is not adequate. We
also find that the (local) matter density contrast grows with the scale factor
in a way analogous to that of an open universe with a value of the matter
density OmegaM(r) corresponding to the appropriate location within the void.Comment: 7 pages, 6 figures, published in Physical Review
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
Testing homogeneity with galaxy number counts : light-cone metric and general low-redshift expansion for a central observer in a matter dominated isotropic universe without cosmological constant
As an alternative to dark energy 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. In order to
test this hypothesis we calculate the general analytical formula to fifth order
for the redshift spherical shell mass. Using the same analytical method we
write the metric in the light-cone by introducing a gauge invariant quantity
which together with the luminosity distance completely
determine the light-cone geometry of a LTB model.Comment: 13 page
Redshift Drift in LTB Void Universes
We study the redshift drift, i.e., the time derivative of the cosmological
redshift in the Lema\^itre-Tolman-Bondi (LTB) solution in which the observer is
assumed to be located at the symmetry center. This solution has often been
studied as an anti-Copernican universe model to explain the acceleration of
cosmic volume expansion without introducing the concept of dark energy. One of
decisive differences between LTB universe models and Copernican universe models
with dark energy is believed to be the redshift drift. The redshift drift is
negative in all known LTB universe models, whereas it is positive in the
redshift domain in Copernican models with dark energy. However,
there have been no detailed studies on this subject. In the present paper, we
prove that the redshift drift of an off-center source is always negative in the
case of LTB void models. We also show that the redshift drift can be positive
with an extremely large hump-type inhomogeneity. Our results suggest that we
can determine whether we live near the center of a large void without dark
energy by observing the redshift drift.Comment: 16 pages, 2 figure
Precision cosmology defeats void models for acceleration
The suggestion that we occupy a privileged position near the centre of a
large, nonlinear, and nearly spherical void has recently attracted much
attention as an alternative to dark energy. Putting aside the philosophical
problems with this scenario, we perform the most complete and up-to-date
comparison with cosmological data. We use supernovae and the full cosmic
microwave background spectrum as the basis of our analysis. We also include
constraints from radial baryonic acoustic oscillations, the local Hubble rate,
age, big bang nucleosynthesis, the Compton y-distortion, and for the first time
include the local amplitude of matter fluctuations, \sigma_8. These all paint a
consistent picture in which voids are in severe tension with the data. In
particular, void models predict a very low local Hubble rate, suffer from an
"old age problem", and predict much less local structure than is observed.Comment: 22 pages, 12 figures; v2 adds models in closed backgrounds;
conclusions strengthened; version accepted to Phys. Rev.
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