612 research outputs found
Corrections to the apparent value of the cosmological constant due to local inhomogeneities
Supernovae observations strongly support the presence of a cosmological
constant, but its value, which we will call apparent, is normally determined
assuming that the Universe can be accurately described by a homogeneous model.
Even in the presence of a cosmological constant we cannot exclude nevertheless
the presence of a small local inhomogeneity which could affect the apparent
value of the cosmological constant. Neglecting the presence of the
inhomogeneity can in fact introduce a systematic misinterpretation of
cosmological data, leading to the distinction between an apparent and true
value of the cosmological constant. We establish the theoretical framework to
calculate the corrections to the apparent value of the cosmological constant by
modeling the local inhomogeneity with a solution. Our assumption
to be at the center of a spherically symmetric inhomogeneous matter
distribution correspond to effectively calculate the monopole contribution of
the large scale inhomogeneities surrounding us, which we expect to be the
dominant one, because of other observations supporting a high level of isotropy
of the Universe around us.
By performing a local Taylor expansion we analyze the number of independent
degrees of freedom which determine the local shape of the inhomogeneity, and
consider the issue of central smoothness, showing how the same correction can
correspond to different inhomogeneity profiles. Contrary to previous attempts
to fit data using large void models our approach is quite general. The
correction to the apparent value of the cosmological constant is in fact
present for local inhomogeneities of any size, and should always be taken
appropriately into account both theoretically and observationally.Comment: 16 pages,new sections added analyzing central smoothness and accuracy
of the Taylor expansion approach, Accepted for publication by JCAP. An essay
based on this paper received honorable mention in the 2011 Essay Context of
the Gravity Research Foundatio
Can the cosmological constant be mimicked by smooth large-scale inhomogeneities for more than one observable?
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 such an hypothesis we calculate the low redshift expansion of the
luminosity distance and the redshift spherical shell mass density
for a central observer in a LTB space without cosmological constant and
show how they cannot fit the observations implied by a model if
the conditions to avoid a weak central singularity are imposed, i.e. if the
matter distribution is smooth everywhere. Our conclusions are valid for any
value of the cosmological constant, not only for as
implied by previous proofs that has to be positive in a smooth LTB
space, based on considering only the luminosity distance.
The observational signatures of smooth LTB matter dominated models are
fundamentally different from the ones of models not only because
it is not possible to reproduce a negative apparent central deceleration
, but because of deeper differences in their space-time geometry
which make impossible the inversion problem when more than one observable is
considered, and emerge at any redshift, not only for .Comment: 18 pages, corrected a typo in the definition of the energy density
which doesn't change the conclusion, references adde
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
Effects of inhomogeneities on apparent cosmological observables: "fake" evolving dark energy
Using the exact Lemaitre-Bondi-Tolman solution with a non-vanishing
cosmological constant , we investigate how the presence of a local
spherically-symmetric inhomogeneity can affect apparent cosmological
observables, such as the deceleration parameter or the effective equation of
state of dark energy (DE), derived from the luminosity distance under the
assumption that the real space-time is exactly homogeneous and isotropic. The
presence of a local underdensity is found to produce apparent phantom behavior
of DE, while a locally overdense region leads to apparent quintessence
behavior. We consider relatively small large scale inhomogeneities which today
are not linear and could be seeded by primordial curvature perturbations
compatible with CMB bounds. Our study shows how observations in an
inhomogeneous CDM universe with initial conditions compatible with the
inflationary beginning, if interpreted under the wrong assumption of
homogeneity, can lead to the wrong conclusion about the presence of "fake"
evolving dark energy instead of .Comment: 22 pages, 19 figures,Final version to appear in European Physical
Journal
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
CMB observations in LTB universes: Part I: Matching peak positions in the CMB spectrum
Acoustic peaks in the spectrum of the cosmic microwave background in
spherically symmetric inhomogeneous cosmological models are studied. At the
photon-baryon decoupling epoch, the universe may be assumed to be dominated by
non-relativistic matter, and thus we may treat radiation as a test field in the
universe filled with dust which is described by the Lema\^itre-Tolman-Bondi
(LTB) solution. First, we give an LTB model whose distance-redshift relation
agrees with that of the concordance CDM model in the whole redshift
domain and which is well approximated by the Einstein-de Sitter universe at and
before decoupling. We determine the decoupling epoch in this LTB universe by
Gamow's criterion and then calculate the positions of acoustic peaks. Thus
obtained results are not consistent with the WMAP data. However, we find that
one can fit the peak positions by appropriately modifying the LTB model,
namely, by allowing the deviation of the distance-redshift relation from that
of the concordance CDM model at where no observational data are
available at present. Thus there is still a possibility of explaining the
apparent accelerated expansion of the universe by inhomogeneity without
resorting to dark energy if we abandon the Copernican principle. Even if we do
not take this extreme attitude, it also suggests that local, isotropic
inhomogeneities around us may seriously affect the determination of the density
contents of the universe unless the possible existence of such inhomogeneities
is properly taken into account.Comment: 20 pages, 5 figure
Testing the Void against Cosmological data: fitting CMB, BAO, SN and H0
In this paper, instead of invoking Dark Energy, we try and fit various
cosmological observations with a large Gpc scale under-dense region (Void)
which is modeled by a Lemaitre-Tolman-Bondi metric that at large distances
becomes a homogeneous FLRW metric. We improve on previous analyses by allowing
for nonzero overall curvature, accurately computing the distance to the
last-scattering surface and the observed scale of the Baryon Acoustic peaks,
and investigating important effects that could arise from having nontrivial
Void density profiles. We mainly focus on the WMAP 7-yr data (TT and TE),
Supernova data (SDSS SN), Hubble constant measurements (HST) and Baryon
Acoustic Oscillation data (SDSS and LRG). We find that the inclusion of a
nonzero overall curvature drastically improves the goodness of fit of the Void
model, bringing it very close to that of a homogeneous universe containing Dark
Energy, while by varying the profile one can increase the value of the local
Hubble parameter which has been a challenge for these models. We also try to
gauge how well our model can fit the large-scale-structure data, but a
comprehensive analysis will require the knowledge of perturbations on LTB
metrics. The model is consistent with the CMB dipole if the observer is about
15 Mpc off the centre of the Void. Remarkably, such an off-center position may
be able to account for the recent anomalous measurements of a large bulk flow
from kSZ data. Finally we provide several analytical approximations in
different regimes for the LTB metric, and a numerical module for CosmoMC, thus
allowing for a MCMC exploration of the full parameter space.Comment: 70 pages, 12 figures, matches version accepted for publication in
JCAP. References added, numerical values in tables changed due to minor bug,
conclusions unaltered. Numerical module available at
http://web.physik.rwth-aachen.de/download/valkenburg
PAMELA, DAMA, INTEGRAL and Signatures of Metastable Excited WIMPs
Models of dark matter with ~ GeV scale force mediators provide attractive
explanations of many high energy anomalies, including PAMELA, ATIC, and the
WMAP haze. At the same time, by exploiting the ~ MeV scale excited states that
are automatically present in such theories, these models naturally explain the
DAMA/LIBRA and INTEGRAL signals through the inelastic dark matter (iDM) and
exciting dark matter (XDM) scenarios, respectively. Interestingly, with only
weak kinetic mixing to hypercharge to mediate decays, the lifetime of excited
states with delta < 2 m_e is longer than the age of the universe. The
fractional relic abundance of these excited states depends on the temperature
of kinetic decoupling, but can be appreciable. There could easily be other
mechanisms for rapid decay, but the consequences of such long-lived states are
intriguing. We find that CDMS constrains the fractional relic population of
~100 keV states to be <~ 10^-2, for a 1 TeV WIMP with sigma_n = 10^-40 cm^2.
Upcoming searches at CDMS, as well as xenon, silicon, and argon targets, can
push this limit significantly lower. We also consider the possibility that the
DAMA excitation occurs from a metastable state into the XDM state, which decays
via e+e- emission, which allows lighter states to explain the INTEGRAL signal
due to the small kinetic energies required. Such models yield dramatic signals
from down-scattering, with spectra peaking at high energies, sometimes as high
as ~1 MeV, well outside the usual search windows. Such signals would be visible
at future Ar and Si experiments, and may be visible at Ge and Xe experiments.
We also consider other XDM models involving ~ 500 keV metastable states, and
find they can allow lighter WIMPs to explain INTEGRAL as well.Comment: 22 pages, 7 figure
The Cosmic Microwave Background in an Inhomogeneous Universe - why void models of dark energy are only weakly constrained by the CMB
The dimming of Type Ia supernovae could be the result of Hubble-scale
inhomogeneity in the matter and spatial curvature, rather than signaling the
presence of a dark energy component. A key challenge for such models is to fit
the detailed spectrum of the cosmic microwave background (CMB). We present a
detailed discussion of the small-scale CMB in an inhomogeneous universe,
focusing on spherically symmetric `void' models. We allow for the dynamical
effects of radiation while analyzing the problem, in contrast to other work
which inadvertently fine tunes its spatial profile. This is a surprisingly
important effect and we reach substantially different conclusions. Models which
are open at CMB distances fit the CMB power spectrum without fine tuning; these
models also fit the supernovae and local Hubble rate data which favours a high
expansion rate. Asymptotically flat models may fit the CMB, but require some
extra assumptions. We argue that a full treatment of the radiation in these
models is necessary if we are to understand the correct constraints from the
CMB, as well as other observations which rely on it, such as spectral
distortions of the black body spectrum, the kinematic Sunyaev-Zeldovich effect
or the Baryon Acoustic Oscillations.Comment: 23 pages with 14 figures. v2 has considerably extended discussion and
analysis, but the basic results are unchanged. v3 is the final versio
Accelerating Universe from an Evolving Lambda in Higher Dimension
We find exact solutions in five dimensional inhomogeneous matter dominated
model with a varying cosmological constant. Adjusting arbitrary constants of
integration one can also achieve acceleration in our model. Aside from an
initial singularity our spacetime is regular everywhere including the centre of
the inhomogeneous distribution. We also study the analogous homogeneous
universe in (4+d) dimensions. Here an initially decelerating model is found to
give late acceleration in conformity with the current observational demands. We
also find that both anisotropy and number of dimensions have a role to play in
determining the time of flip, in fact the flip is delayed in multidimensional
models. Some astrophysical parameters like the age, luminosity distance etc are
also calculated and the influence of extra dimensions is briefly discussed.
Interestingly our model yields a larger age of the universe compared to many
other quintessential models.Comment: 18 pages, 9 figure
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