1,379 research outputs found
Isotropization of the universe during inflation
A primordial inflationary phase allows one to erase any possible anisotropic
expansion thanks to the cosmic no-hair theorem. If there is no global
anisotropic stress, then the anisotropic expansion rate tends to decrease. What
are the observational consequences of a possible early anisotropic phase? We
first review the dynamics of anisotropic universes and report analytic
approximations. We then discuss the structure of dynamical equations for
perturbations and the statistical properties of observables, as well as the
implication of a primordial anisotropy on the quantization of these
perturbations during inflation. Finally we briefly review models based on
primordial vector field which evade the cosmic no-hair theorem.Comment: 9 pages, 3 figures. Invited review article for the French Academy of
Scienc
Testing gaussianity, homogeneity and isotropy with the cosmic microwave background
We review the basic hypotheses which motivate the statistical framework used
to analyze the cosmic microwave background, and how that framework can be
enlarged as we relax those hypotheses. In particular, we try to separate as
much as possible the questions of gaussianity, homogeneity and isotropy from
each other. We focus both on isotropic estimators of non-gaussianity as well as
statistically anisotropic estimators of gaussianity, giving particular emphasis
on their signatures and the enhanced "cosmic variances" that become
increasingly important as our putative Universe becomes less symmetric. After
reviewing the formalism behind some simple model-independent tests, we discuss
how these tests can be applied to CMB data when searching for large scale
"anomalies"Comment: 52 pages, 22 pdf figures. Revised version of the invited review for
the special issue "Testing the Gaussianity and Statistical Isotropy of the
Universe" for Advances in Astronomy
CMB statistical isotropy confirmation at all scales using multipole vectors
We present an efficient numerical code and conduct, for the first time, a
null and model-independent CMB test of statistical isotropy using Multipole
Vectors (MVs) at all scales. Because MVs are insensitive to the angular power
spectrum , our results are independent from the assumed cosmological
model. We avoid a posteriori choices and use pre-defined ranges of scales
, and in our analyses. We
find that all four masked Planck maps, from both 2015 and 2018 releases, are in
agreement with statistical isotropy for , . For
we detect anisotropies but this is indicative of simply the
anisotropy in the noise: there is no anisotropy for and an
increasing level of anisotropy at higher multipoles. Our findings of no
large-scale anisotropies seem to be a consequence of avoiding \emph{a
posteriori} statistics. We also find that the degree of anisotropy in the full
sky (i.e. unmasked) maps vary enormously (between less than 5 and over 1000
standard deviations) among the different mapmaking procedures and data
releases.Comment: v4: additional analysis which increased statistical sensitivity,
including new plots and tables; extended discussion; 15 pages, 14 figures, 7
tables. Matches published versio
Weak-lensing -modes as a probe of the isotropy of the universe
We compute the angular power spectrum of the -modes of the weak-lensing
shear in a spatially anisotropic spacetime. We find that there must also exist
off-diagonal correlations between the -modes, -modes, and convergence
that allow one to reconstruct the eigendirections of expansion. Focusing on
future surveys such as Euclid and SKA, we show that observations can constrain
the geometrical shear in units of the Hubble rate at the percent level, or even
better, offering a new and powerful method to probe our cosmological model.Comment: 4 pages, 3 figures. This version matches the published on
Angular-planar CMB power spectrum
Gaussianity and statistical isotropy of the Universe are modern cosmology's
minimal set of hypotheses. In this work we introduce a new statistical test to
detect observational deviations from this minimal set. By defining the
temperature correlation function over the whole celestial sphere, we are able
to independently quantify both angular and planar dependence (modulations) of
the CMB temperature power spectrum over different slices of this sphere. Given
that planar dependence leads to further modulations of the usual angular power
spectrum , this test can potentially reveal richer structures in the
morphology of the primordial temperature field. We have also constructed an
unbiased estimator for this angular-planar power spectrum which naturally
generalizes the estimator for the usual 's. With the help of a chi-square
analysis, we have used this estimator to search for observational deviations of
statistical isotropy in WMAP's 5 year release data set (ILC5), where we found
only slight anomalies on the angular scales and . Since this
angular-planar statistic is model-independent, it is ideal to employ in
searches of statistical anisotropy (e.g., contaminations from the galactic
plane) and to characterize non-Gaussianities.Comment: Replaced to match the published version. Journal-ref: Phys.Rev. D80
063525 (2009
Cosmological Signatures of Anisotropic Spatial Curvature
If one is willing to give up the cherished hypothesis of spatial isotropy,
many interesting cosmological models can be developed beyond the simple
anisotropically expanding scenarios. One interesting possibility is presented
by shear-free models in which the anisotropy emerges at the level of the
curvature of the homogeneous spatial sections, whereas the expansion is
dictated by a single scale factor. We show that such models represent viable
alternatives to describe the large-scale structure of the inflationary
universe, leading to a kinematically equivalent Sachs-Wolfe effect. Through the
definition of a complete set of spatial eigenfunctions we compute the two-point
correlation function of scalar perturbations in these models. In addition, we
show how such scenarios would modify the spectrum of the CMB assuming that the
observations take place in a small patch of a universe with anisotropic
curvature.Comment: 21 pages, 1 figure. To appear in JCA
Inflationary Perturbations in Anisotropic, Shear-Free Universes
In this work, the linear and gauge-invariant theory of cosmological
perturbations in a class of anisotropic and shear-free spacetimes is developed.
After constructing an explicit set of complete eigenfunctions in terms of which
perturbations can be expanded, we identify the effective degrees of freedom
during a generic slow-roll inflationary phase. These correspond to the
anisotropic equivalent of the standard Mukhanov-Sasaki variables. The
associated equations of motion present a remarkable resemblance to those found
in perturbed Friedmann-Robertson-Walker spacetimes with curvature, apart from
the spectrum of the Laplacian, which exhibits the characteristic frequencies of
the underlying geometry. In particular, it is found that the perturbations
cannot develop arbitrarily large super-Hubble modes.Comment: 24 pages, 2 figure
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