6,266 research outputs found
Stabilizing the Dilaton in Superstring Cosmology
We address the important issue of stabilizing the dilaton in the context of
superstring cosmology. Scalar potentials which arise out of gaugino condensates
in string models are generally exponential in nature. In a cosmological setting
this allows for the existence of quasi scaling solutions, in which the energy
density of the scalar field can, for a period, become a fixed fraction of the
background density, due to the friction of the background expansion. Eventually
the field can be trapped in the minimum of its potential as it leaves the
scaling regime. We investigate this possibility in various gaugino condensation
models and show that stable solutions for the dilaton are far more common than
one would have naively thought.Comment: 13 pages, LaTex, uses psfig.sty with 3 figure
Geometrical estimators as a test of Gaussianity in the CMB
We investigate the power of geometrical estimators on detecting
non-Gaussianity in the cosmic microwave background. In particular the number,
eccentricity and Gaussian curvature of excursion sets above (and below) a
threshold are studied. We compare their different performance when applied to
non-Gaussian simulated maps of small patches of the sky, which take into
account the angular resolution and instrumental noise of the Planck satellite.
These non-Gaussian simulations are obtained as perturbations of a Gaussian
field in two different ways which introduce a small level of skewness or
kurtosis in the distribution. A comparison with a classical estimator, the
genus, is also shown. We find that the Gaussian curvature is the best of our
estimators in all the considered cases. Therefore we propose the use of this
quantity as a particularly useful test to look for non-Gaussianity in the CMB.Comment: 9 pages, 6 postscript figures, submitted to MNRA
Some aspects of thermal inflation: the finite temperature potential and topological defects
Currently favoured extensions of the Standard Model typically contain `flaton
fields' defined as fields with large vacuum expectation values (vevs) and
almost flat potentials. If a flaton field is trapped at the origin in the early
universe, one expects `thermal inflation' to take place before it rolls away to
the true vacuum, because the finite-temperature correction to the potential
will hold it at the origin until the temperature falls below 1\TeV or so. In
the first part of the paper, that expectation is confirmed by an estimate of
the finite temperature corrections and of the tunneling rate to the true
vacuum, paying careful attention to the validity of the approximations that are
used. The second part of the paper considers topological defects which may be
produced at the end of an era of thermal inflation. If the flaton fields
associated with the era are GUT higgs fields, then its end corresponds to the
GUT phase transition. In that case monopoles (as well as GUT higgs particles)
will have to be diluted by a second era of thermal inflation. Such an era will
not affect the cosmology of GUT strings, for which the crucial parameter is the
string mass per unit length. Because of the flat Higgs potential, the GUT
symmetry breaking scale required for the strings to be a candidate for the
origin of large scale structure and the cmb anisotropy is about three times
bigger than usual, but given the uncertainties it is still compatible with the
one required by the unification of the Standard Model gauge couplings. The
cosmology of textures and of global monopoles is unaffected by the flatness of
the potential.Comment: 40 pages, LaTeX with epsf macro, 1 figure, preprint number correcte
Reconstructing the Inflaton Potential
A review is presented of recent work by the authors concerning the use of
large scale structure and microwave background anisotropy data to determine the
potential of the inflaton field. The importance of a detection of the
stochastic gravitational wave background is emphasised, and some preliminary
new results of tests of the method on simulated data sets with uncertainties
are described. (Proceedings of ``Unified Symmetry in the Small and in the
Large'', Coral Gables, 1994)Comment: 13 pages, uuencoded postscript file with figures included (LaTeX file
available from ARL), FERMILAB-Conf 94/189
On the regularity of the covariance matrix of a discretized scalar field on the sphere
We present a comprehensive study of the regularity of the covariance matrix
of a discretized field on the sphere. In a particular situation, the rank of
the matrix depends on the number of pixels, the number of spherical harmonics,
the symmetries of the pixelization scheme and the presence of a mask. Taking
into account the above mentioned components, we provide analytical expressions
that constrain the rank of the matrix. They are obtained by expanding the
determinant of the covariance matrix as a sum of determinants of matrices made
up of spherical harmonics. We investigate these constraints for five different
pixelizations that have been used in the context of Cosmic Microwave Background
(CMB) data analysis: Cube, Icosahedron, Igloo, GLESP and HEALPix, finding that,
at least in the considered cases, the HEALPix pixelization tends to provide a
covariance matrix with a rank closer to the maximum expected theoretical value
than the other pixelizations. The effect of the propagation of numerical errors
in the regularity of the covariance matrix is also studied for different
computational precisions, as well as the effect of adding a certain level of
noise in order to regularize the matrix. In addition, we investigate the
application of the previous results to a particular example that requires the
inversion of the covariance matrix: the estimation of the CMB temperature power
spectrum through the Quadratic Maximum Likelihood algorithm. Finally, some
general considerations in order to achieve a regular covariance matrix are also
presented.Comment: 36 pages, 12 figures; minor changes in the text, matches published
versio
Wavelets Applied to CMB Maps: a Multiresolution Analysis for Denoising
Analysis and denoising of Cosmic Microwave Background (CMB) maps are
performed using wavelet multiresolution techniques. The method is tested on
maps with resolution resembling the
experimental one expected for future high resolution space observations.
Semianalytic formulae of the variance of wavelet coefficients are given for the
Haar and Mexican Hat wavelet bases. Results are presented for the standard Cold
Dark Matter (CDM) model. Denoising of simulated maps is carried out by removal
of wavelet coefficients dominated by instrumental noise. CMB maps with a
signal-to-noise, , are denoised with an error improvement factor
between 3 and 5. Moreover we have also tested how well the CMB temperature
power spectrum is recovered after denoising. We are able to reconstruct the
's up to with errors always below in cases with
.Comment: latex file 9 pages + 5 postscript figures + 1 gif figure (figure 6),
to be published in MNRA
Isotropic Wavelets: a Powerful Tool to Extract Point Sources from CMB Maps
It is the aim of this paper to introduce the use of isotropic wavelets to
detect and determine the flux of point sources appearing in CMB maps. The most
suited wavelet to detect point sources filtered with a Gaussian beam is the
Mexican Hat. An analytical expression of the wavelet coefficient obtained in
the presence of a point source is provided and used in the detection and flux
estimation methods presented. For illustration the method is applied to two
simulations (assuming Planck Mission characteristics) dominated by CMB (100
GHz) and dust (857 GHz) as these will be the two signals dominating at low and
high frequency respectively in the Planck channels. We are able to detect
bright sources above 1.58 Jy at 857 GHz (82% of all sources) and above 0.36 Jy
at 100 GHz (100% of all) with errors in the flux estimation below 25%. The main
advantage of this method is that nothing has to be assumed about the underlying
field, i.e. about the nature and properties of the signal plus noise present in
the maps. This is not the case in the detection method presented by Tegmark and
Oliveira-Costa 1998. Both methods are compared producing similar results.Comment: 6 pages. Accepted for publication in MNRA
Filtering techniques for the detection of Sunyaev-Zel'dovich clusters in multifrequency CMB maps
The problem of detecting Sunyaev-Zel'dovich (SZ) clusters in multifrequency
CMB observations is investigated using a number of filtering techniques. A
multifilter approach is introduced, which optimizes the detection of SZ
clusters on microwave maps. An alternative method is also investigated, in
which maps at different frequencies are combined in an optimal manner so that
existing filtering techniques can be applied to the single combined map. The SZ
profiles are approximated by the circularly-symmetric template , with and , where the core radius and the overall amplitude of the effect
are not fixed a priori, but are determined from the data. The background
emission is modelled by a homogeneous and isotropic random field, characterized
by a cross-power spectrum with . The
filtering methods are illustrated by application to simulated Planck
observations of a patch of sky in 10 frequency
channels. Our simulations suggest that the Planck instrument should detect
SZ clusters in 2/3 of the sky. Moreover, we find the catalogue
to be complete for fluxes mJy at 300 GHz.Comment: 12 pages, 7 figures; Corrected figures. Submitted to MNRA
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