19 research outputs found
The Uncorrelated Universe: Statistical Anisotropy and the Vanishing Angular Correlation Function in WMAP Years 1-3
The large-angle (low-ell) correlations of the Cosmic Microwave Background
(CMB) as reported by the Wilkinson Microwave Anisotropy Probe (WMAP) after
their first year of observations exhibited statistically significant anomalies
compared to the predictions of the standard inflationary big-bang model. We
suggested then that these implied the presence of a solar system foreground, a
systematic correlated with solar system geometry, or both. We re-examine these
anomalies for the data from the first three years of WMAP's operation. We show
that, despite the identification by the WMAP team of a systematic correlated
with the equinoxes and the ecliptic, the anomalies in the first-year Internal
Linear Combination (ILC) map persist in the three-year ILC map, in all-but-one
case at similar statistical significance. The three-year ILC quadrupole and
octopole therefore remain inconsistent with statistical isotropy -- they are
correlated with each other (99.6% C.L.), and there are statistically
significant correlations with local geometry, especially that of the solar
system. The angular two-point correlation function at scales >60 degrees in the
regions outside the (kp0) galactic cut, where it is most reliably determined,
is approximately zero in all wavebands and is even more discrepant with the
best fit LambdaCDM inflationary model than in the first-year data - 99.97% C.L.
for the new ILC map. The full-sky ILC map, on the other hand, has a
non-vanishing angular two-point correlation function, apparently driven by the
region inside the cut, but which does not agree better with LambdaCDM. The role
of the newly identified low-ell systematics is more puzzling than reassuring.Comment: 18 pages, 7 figures, minor additions to refs and conclusions in v2.
High resolution figures, multipole vector code and other information can be
found at http://www.phys.cwru.edu/projects/mpvectors
CMB Alignment in Multi-Connected Universes
The low multipoles of the cosmic microwave background (CMB) anisotropy
possess some strange properties like the alignment of the quadrupole and the
octopole, and the extreme planarity or the extreme sphericity of some
multipoles, respectively. In this paper the CMB anisotropy of several
multi-connected space forms is investigated with respect to the maximal angular
momentum dispersion and the Maxwellian multipole vectors in order to settle the
question whether such spaces can explain the low multipole anomalies in the
CMB
Could dark energy be vector-like?
In this paper I explore whether a vector field can be the origin of the
present stage of cosmic acceleration. In order to avoid violations of isotropy,
the vector has be part of a ``cosmic triad'', that is, a set of three identical
vectors pointing in mutually orthogonal spatial directions. A triad is indeed
able to drive a stage of late accelerated expansion in the universe, and there
exist tracking attractors that render cosmic evolution insensitive to initial
conditions. However, as in most other models, the onset of cosmic acceleration
is determined by a parameter that has to be tuned to reproduce current
observations. The triad equation of state can be sufficiently close to minus
one today, and for tachyonic models it might be even less than that. I briefly
analyze linear cosmological perturbation theory in the presence of a triad. It
turns out that the existence of non-vanishing spatial vectors invalidates the
decomposition theorem, i.e. scalar, vector and tensor perturbations do not
decouple from each other. In a simplified case it is possible to analytically
study the stability of the triad along the different cosmological attractors.
The triad is classically stable during inflation, radiation and matter
domination, but it is unstable during (late-time) cosmic acceleration. I argue
that this instability is not likely to have a significant impact at present.Comment: 28 pages, 6 figures. Uses RevTeX4. v2: Discussion about relation to
phantoms added and additional references cite
Ellipsoidal universe in the brane world
We study a scenario of the ellipsoidal universe in the brane world cosmology
with a cosmological constant in the bulk . From the five-dimensional Einstein
equations we derive the evolution equations for the eccentricity and the scale
factor of the universe, which are coupled to each other. It is found that if
the anisotropy of our universe is originated from a uniform magnetic field
inside the brane, the eccentricity decays faster in the bulk in comparison with
a four-dimensional ellipsoidal universe. We also investigate the ellipsoidal
universe in the brane-induced gravity and find the evolution equation for the
eccentricity which has a contribution determined by the four- and
five-dimensional Newton's constants. The role of the eccentricity is discussed
in explaining the quadrupole problem of the cosmic microwave background.Comment: 15 pages, 1 figure, Version 3, references added, contents expande
Footprints of Statistical Anisotropies
We propose and develop a formalism to describe and constrain statistically
anisotropic primordial perturbations. Starting from a decomposition of the
primordial power spectrum in spherical harmonics, we find how the temperature
fluctuations observed in the CMB sky are directly related to the coefficients
in this harmonic expansion. Although the angular power spectrum does not
discriminate between statistically isotropic and anisotropic perturbations, it
is possible to define analogous quadratic estimators that are direct measures
of statistical anisotropy. As a simple illustration of our formalism we test
for the existence of a preferred direction in the primordial perturbations
using full-sky CMB maps. We do not find significant evidence supporting the
existence of a dipole component in the primordial spectrum.Comment: 26 pages, 5 double figures. Uses RevTeX
Cosmic Acceleration Driven by Mirage Inhomogeneities
A cosmological model based on an inhomogeneous D3-brane moving in an AdS_5 X
S_5 bulk is introduced. Although there is no special points in the bulk, the
brane Universe has a center and is isotropic around it. The model has an
accelerating expansion and its effective cosmological constant is inversely
proportional to the distance from the center, giving a possible geometrical
origin for the smallness of a present-day cosmological constant. Besides, if
our model is considered as an alternative of early time acceleration, it is
shown that the early stage accelerating phase ends in a dust dominated FRW
homogeneous Universe. Mirage-driven acceleration thus provides a dark matter
component for the brane Universe final state. We finally show that the model
fulfills the current constraints on inhomogeneities.Comment: 14 pages, 1 figure, IOP style. v2, changed style, minor corrections,
references added, version accepted in Class. Quant. Gra
Inflationary perturbations in anisotropic backgrounds and their imprint on the CMB
We extend the standard theory of cosmological perturbations to homogeneous
but anisotropic universes. We present an exhaustive computation for the case of
a Bianchi I model, with a residual isotropy between two spatial dimensions,
which is undergoing complete isotropization at the onset of inflation; we also
show how the computation can be further extended to more general backgrounds.
In presence of a single inflaton field, there are three physical perturbations
(precisely as in the isotropic case), which are obtained (i) by removing gauge
and nondynamical degrees of freedom, and (ii) by finding the combinations of
the remaining modes in terms of which the quadratic action of the perturbations
is canonical. The three perturbations, which later in the isotropic regime
become a scalar mode and two tensor polarizations (gravitational wave), are
coupled to each other already at the linearized level during the anisotropic
phase. This generates nonvanishing correlations between different modes of the
CMB anisotropies, which can be particularly relevant at large scales (and,
potentially, be related to the large scale anomalies in the WMAP data). As an
example, we compute the spectrum of the perturbations in this Bianchi I
geometry, assuming that the inflaton is in a slow roll regime also in the
anisotropic phase. For this simple set-up, fixing the initial conditions for
the perturbations appears more difficult than in the standard case, and
additional assumptions seem to be needed to provide predictions for the CMB
anisotropies.Comment: 31 pages, 3 figure
Hemispherical power asymmetry: parameter estimation from CMB WMAP5 data
We reexamine the evidence of the hemispherical power asymmetry, detected in
the CMB WMAP data using a new method. At first, we analyze the hemispherical
variance ratios and compare these with simulated distributions. Secondly,
working within a previously-proposed CMB bipolar modulation model, we constrain
model parameters: the amplitude and the orientation of the modulation field as
a function of various multipole bins. Finally, we select three ranges of
multipoles leading to the most anomalous signals, and we process corresponding
100 Gaussian, random field (GRF) simulations, treated as observational data, to
further test the statistical significance and robustness of the hemispherical
power asymmetry. For our analysis we use the Internally-Linearly-Coadded (ILC)
full sky map, and KQ75 cut-sky V channel, foregrounds reduced map of the WMAP
five year data (V5). We constrain the modulation parameters using a generic
maximum a posteriori method.
In particular, we find differences in hemispherical power distribution, which
when described in terms of a model with bipolar modulation field, exclude the
field amplitude value of the isotropic model A=0 at confidence level of ~99.5%
(~99.4%) in the multipole range l=[7,19] (l=[7,79]) in the V5 data, and at the
confidence level ~99.9% in the multipole range l=[7,39] in the ILC5 data, with
the best fit (modal PDF) values in these particular multipole ranges of A=0.21
(A=0.21) and A=0.15 respectively. However, we also point out that similar or
larger significances (in terms of rejecting the isotropic model), and large
best-fit modulation amplitudes are obtained in GRF simulations as well, which
reduces the overall significance of the CMB power asymmetry down to only about
94% (95%) in the V5 data, in the range l=[7,19] (l=[7,79]).Comment: 24 pages, 10 figures; few typos corrected; published in JCA
Cosmic Microwave Background, Accelerating Universe and Inhomogeneous Cosmology
We consider a cosmology in which a spherically symmetric large scale
inhomogeneous enhancement or a void are described by an inhomogeneous metric
and Einstein's gravitational equations. For a flat matter dominated universe
the inhomogeneous equations lead to luminosity distance and Hubble constant
formulas that depend on the location of the observer. For a general
inhomogeneous solution, it is possible for the deceleration parameter to differ
significantly from the FLRW result. The deceleration parameter can be
interpreted as ( for a flat matter dominated universe) in a
FLRW universe and be as inferred from the inhomogeneous enhancement
that is embedded in a FLRW universe. A spatial volume averaging of local
regions in the backward light cone has to be performed for the inhomogeneous
solution at late times to decide whether the decelerating parameter can be
negative for a positive energy condition. The CMB temperature fluctuations
across the sky can be unevenly distributed in the northern and southern
hemispheres in the inhomogeneous matter dominated solution, in agreement with
the analysis of the WMAP power spectrum data by several authors. The model can
possibly explain the anomalous alignment of the quadrupole and octopole moments
observed in the WMAP data.Comment: 20 pages, no figures, LaTex file. Equations and typos corrected and
references added. Additional material and some conclusions changed. Final
published versio
Accelerated expansion from structure formation
We discuss the physics of backreaction-driven accelerated expansion. Using
the exact equations for the behaviour of averages in dust universes, we explain
how large-scale smoothness does not imply that the effect of inhomogeneity and
anisotropy on the expansion rate is small. We demonstrate with an analytical
toy model how gravitational collapse can lead to acceleration. We find that the
conjecture of the accelerated expansion being due to structure formation is in
agreement with the general observational picture of structures in the universe,
and more quantitative work is needed to make a detailed comparison.Comment: 44 pages, 1 figure. Expanded treatment of topics from the Gravity
Research Foundation contest essay astro-ph/0605632. v2: Added references,
clarified wordings. v3: Published version. Minor changes and corrections,
added a referenc