966 research outputs found
Generation of seed perturbations from Quantum Cosmology
The origin of seed perturbations in the Universe is studied within the
framework of a specific minisuperspace model. It is shown that the `creation'
of the Universe as a result of a quantum transition from a flat empty spacetime
would lead to a flat FLRW (Friedmann Lema\^\i tre Robertson-Walker) Universe
with weak inhomogeneous perturbations at large wavelengths. The power spectrum
of these perturbations is found to be scale invariant at horizon crossing
(i.e., the Harrison-Zeldovich spectrum). It is also recognised that the seed
perturbations generated in our model would be generically of the isocurvature
kind.Comment: Plain Tex 24 Page
Statistical Isotropy of CMB Polarization Maps
We formulate statistical isotropy of CMB anisotropy maps in its most general
form. We also present a fast and orientation independent statistical method to
determine deviations from statistical isotropy in CMB polarization maps.
Importance of having statistical tests of departures from SI for CMB
polarization maps lies not only in interesting theoretical motivations but also
in testing cleaned CMB polarization maps for observational artifacts such as
residuals from polarized foreground emission. We propose a generalization of
the Bipolar Power Spectrum (BiPS) to polarization maps. Application to the
observed CMB polarization maps will be soon possible after the release of WMAP
three year data. As a demonstration we show that for E-polarization this test
can detect breakdown of statistical isotropy due to polarized synchrotron
foreground.Comment: 6 pages, 2 figures, Conclusions & results unchanged; Extension to
cutsky included (discussion & references added); Matches version accepted to
Phys. Rev. D Rapid Com
Odd-Parity Bipolar Spherical Harmonics
Bipolar spherical harmonics (BiPoSHs) provide a general formalism for
quantifying departures in the cosmic microwave background (CMB) from
statistical isotropy (SI) and from Gaussianity. However, prior work has focused
only on BiPoSHs with even parity. Here we show that there is another set of
BiPoSHs with odd parity, and we explore their cosmological applications. We
describe systematic artifacts in a CMB map that could be sought by measurement
of these odd-parity BiPoSH modes. These BiPoSH modes may also be produced
cosmologically through lensing by gravitational waves (GWs), among other
sources. We derive expressions for the BiPoSH modes induced by the weak lensing
of both scalar and tensor perturbations. We then investigate the possibility of
detecting parity-breaking physics, such as chiral GWs, by cross-correlating
opposite parity BiPoSH modes with multipole moments of the CMB polarization. We
find that the expected signal-to-noise of such a detection is modest.Comment: 19 pages, 4 figures, Accepted to PR
Gaussian-Mixture-Model-based Cluster Analysis Finds Five Kinds of Gamma Ray Bursts in the BATSE Catalog
Clustering methods are an important tool to enumerate and describe the
different coherent kinds of Gamma Ray Bursts (GRBs). But their performance can
be affected by a number of factors such as the choice of clustering algorithm
and inherent associated assumptions, the inclusion of variables in clustering,
nature of initialization methods used or the iterative algorithm or the
criterion used to judge the optimal number of groups supported by the data. We
analyzed GRBs from the BATSE 4Br catalog using -means and Gaussian Mixture
Models-based clustering methods and found that after accounting for all the
above factors, all six variables -- different subsets of which have been used
in the literature -- and that are, namely, the flux duration variables
(, ), the peak flux () measured in 256-millisecond
bins, the total fluence () and the spectral hardness ratios ( and
) contain information on clustering. Further, our analysis found
evidence of five different kinds of GRBs and that these groups have different
kinds of dispersions in terms of shape, size and orientation. In terms of
duration, fluence and spectrum, the five types of GRBs were characterized as
intermediate/faint/intermediate, long/intermediate/soft,
intermediate/intermediate/intermediate, short/faint/hard and
long/bright/intermediate.Comment: 17 pages, 12 figures, 6 table
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