275 research outputs found
Constraints on the cosmological density parameters and cosmic topology
A nontrivial topology of the spatial section of the universe is an
observable, which can be probed for all locally homogeneous and isotropic
universes, without any assumption on the cosmological density parameters. We
discuss how one can use this observable to set constraints on the density
parameters of the Universe by using a specific spatial topology along with type
Ia supenovae and X-ray gas mass fraction data sets.Comment: 11 pages, 4 figures. To appear in Int. J. Mod. Phys. D (2006).
Invited talk delivered at the 2nd International Workshop on Astronomy and
Relativistic Astrophysic
Non-Gaussianity in the HILC foreground-reduced three-year WMAP CMB map
A detection or nondetection of primordial non-Gaussianity in the CMB data is
essential not only to test alternative models of the physics of the early
universe but also to discriminate among classes of inflationary models. Given
this far reaching consequences of such a non-Gaussianity detection for our
understanding of the physics of the early universe, it is important to employ
alternative indicators in order to have further information about the
Gaussianity features of CMB that may be helpful for identifying their origins.
In this way, a considerable effort has recently gone into the design of
non-Gaussianity indicators, and in their application in the search for
deviation from Gaussianity in the CMB data. Recently we have proposed two new
large-angle non-Gaussianity indicators which provide measures of the departure
from Gaussianity on large angular scales. We have used these indicators to
carry out analyses of Gaussianity of the single frequency bands and of the
available foreground-reduced {\it five-year} maps with and without the KQ75
mask. Here we extend and complement these studies by performing a new analysis
of deviation from Gaussianity of the {\it three-year} harmonic ILC (HILC)
foreground-reduced full-sky and KQ75 masked maps obtained from WMAP data. We
show that this full-sky foreground-reduced maps presents a significant
deviation from Gaussianity, which is brought down to a level of consistency
with Gaussianity when the KQ75 mask is employed.Comment: 6 pages, 1 figure. To appear in IJMPD (2010). V2: Corrected ref.[8]
A note on the large-angle anisotropies in the WMAP cut-sky maps
Recent analyses of the WMAP data seem to indicate the possible presence of
large-angle anisotropy in the Universe. If confirmed, these can have important
consequences for our understanding of the Universe. A number of attempts have
recently been made to establish the reality and nature of such anisotropies in
the CMB data. Among these is a directional indicator recently proposed by the
authors. A distinctive feature of this indicator is that it can be used to
generate a sky map of the large-scale anisotropies of the CMB maps. Applying
this indicator to full-sky temperature maps we found a statistically
significant preferred direction. The full-sky maps used in these analyses are
known to have residual foreground contamination as well as complicated noise
properties. Thus, here we performed the same analysis for a map where regions
with high foreground contamination were removed. We find that the main feature
of the full-sky analysis, namely the presence of a significant axis of
asymmetry, is robust with respect to this masking procedure. Other subtler
anomalies of the full-sky are on the other hand no longer present.Comment: 10 pages, 3 figeres. We performed a similar analysis of
arXiv:astro-ph/0511666 by considering the LILC map with a Kp2 sky cut, and
find that the presence of a significant axis of asymmetry is robust with
respect to this masking procedur
Segre Types of Symmetric Two-tensors in n-Dimensional Spacetimes
Three propositions about Jordan matrices are proved and applied to
algebraically classify the Ricci tensor in n-dimensional Kaluza-Klein-type
spacetimes. We show that the possible Segre types are [1,1...1], [21...1],
[31\ldots 1], [z\bar{z}1...1] and degeneracies thereof. A set of canonical
forms for the Segre types is obtained in terms of semi-null bases of vectors.Comment: 14 pages, LaTeX, replaced due to a LaTex erro
Circles-in-the-sky searches and observable cosmic topology in the inflationary limit
While the topology of the Universe is at present not specified by any known
fundamental theory, it may in principle be determined through observations. In
particular, a non-trivial topology will generate pairs of matching circles of
temperature fluctuations in maps of the cosmic microwave background, the
so-called circles-in-the-sky. A general search for such pairs of circles would
be extremely costly and would therefore need to be confined to restricted
parameter ranges. To draw quantitative conclusions from the negative results of
such partial searches for the existence of circles we need a concrete
theoretical framework. Here we provide such a framework by obtaining
constraints on the angular parameters of these circles as a function of
cosmological density parameters and the observer's position. As an example of
the application of our results, we consider the recent search restricted to
pairs of nearly back-to-back circles with negative results. We show that
assuming the Universe to be very nearly flat, with its total matter-energy
density satisfying the bounds , compatible
with the predictions of typical inflationary models, this search, if confirmed,
could in principle be sufficient to exclude a detectable non-trivial cosmic
topology for most observers. We further relate explicitly the fraction of
observers for which this result holds to the cosmological density parameters.Comment: 8 pages, 4 figures. V2: Version to appear in Phys. Rev. D (2008),
typos corrected, references adde
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