1,451 research outputs found
Sub-degree CMB anisotropies from inflationary bubbles
It is well known that processes of first order phase transitions may have
occurred in the inflationary era. If one or more occurred well before the end
of inflation, the nucleated bubbles are stretched to large scales and the
primordial power spectrum contains a scale dependent non-Gaussian component
provided by the remnants of the bubbles. We predict the anisotropies in the
cosmic microwave background (CMB) induced by inflationary bubbles. We build a
general analytic model for describing a bubbly perturbation; we evolve each
Fourier mode using the linear theory of perturbations from reheating until
decoupling; we get the CMB anisotropies by considering the bubbly perturbation
intersecting the last scattering surface. The CMB image of an inflationary
bubble is a series of concentric isothermal rings of different color (sign of
) on the scale of the sound horizon at decoupling ( in
the sky); the resulting anisotropy is therefore strongly non-Gaussian. The mean
amplitude of for a bubble of size follows the known estimates
for linear perturbations, . In particular, bubbles with size corresponding to the seeds of
the observed large scale voids (tens of comoving Mpc) induce an interesting
pattern of CMB anisotropies on the sub-degree angular scale, to be further
investigated and compared with the forthcoming high resolution CMB maps
provided by the MAP and the Planck experiments.Comment: 10 pages, 5 postscript figures, accepted by Ap.
Digital Deblurring of CMB Maps II: Asymmetric Point Spread Function
In this second paper in a series dedicated to developing efficient numerical
techniques for the deblurring Cosmic Microwave Background (CMB) maps, we
consider the case of asymmetric point spread functions (PSF). Although
conceptually this problem is not different from the symmetric case, there are
important differences from the computational point of view because it is no
longer possible to use some of the efficient numerical techniques that work
with symmetric PSFs. We present procedures that permit the use of efficient
techniques even when this condition is not met. In particular, two methods are
considered: a procedure based on a Kronecker approximation technique that can
be implemented with the numerical methods used with symmetric PSFs but that has
the limitation of requiring only mildly asymmetric PSFs. The second is a
variant of the classic Tikhonov technique that works even with very asymmetric
PSFs but that requires discarding the edges of the maps. We provide details for
efficient implementations of the algorithms. Their performance is tested on
simulated CMB maps.Comment: 9 pages, 13 Figure
Imprints of Primordial Voids on the CMB
We generalize in several ways the results existing in the literature: a) we
make use of an exact general relativistic solution for a spherical, nearly
empty cavity in the matter dominated era to evaluate the null geodesics and the
Sachs-Wolfe effect; b) we evaluate the magnitude of the adiabatic fluctuations
of the photon-baryon plasma; c) we study the influence of the shell profile;
and d) we take into account the finite thickness of the last scattering surface
(LSS) and the influence of its position with respect to the void center. We
find empirically an analytic approximation to the Sachs-Wolfe effect for all
crossing geometries and we derive an upper limit of 25 Mpc
for the comoving radii of voids sitting on the LSS in order to achieve
compatibility with COBE's data. As a nearly empty void has an overcomoving
expansion of a factor of 4 between decoupling and the present, the
maximum allowed size at present is 100 Mpc. On the other
hand, the smallness of the comoving size relative to the sound horizon reduces
strongly the adiabatic effect by Silk damping and makes it negligible. Most of
the signature of primordial voids comes therefore from metric effects and
consists of subdegree spots blue or red depending on whether the center lies
beyond or within the LSS. In conclusion we refine and confirm earlier
constraints on a power law void spectrum originated in an inflationary phase
transition and capable of generating the observed large scale structure.Comment: 14 pages, 5 figures, submitted to Montly Notice
Astrophysical and Cosmological Information from Large-scale sub-mm Surveys of Extragalactic Sources
We present a quantitative analysis of the astrophysical and cosmological
information that can be extracted from the many important wide-area, shallow
surveys that will be carried out in the next few years. Our calculations
combine the predictions of the physical model by Granato et al. (2004) for the
formation and evolution of spheroidal galaxies with up-to-date phenomenological
models for the evolution of starburst and normal late-type galaxies and of
radio sources. We compute the expected number counts and the redshift
distributions of these source populations separately and then focus on
proto-spheroidal galaxies. For the latter objects we predict the counts and
redshift distributions of strongly lensed sources at 250, 350, 500, and 850
micron, the angular correlation function of sources detected in the surveys
considered, the angular power spectra due to clustering of sources below the
detection limit in Herschel and Planck surveys. An optimal survey for selecting
strongly lensed proto-spheroidal galaxies is described, and it is shown how
they can be easily distinguished from the other source populations. We also
discuss the detectability of the imprints of the 1-halo and 2-halo regimes on
angular correlation functions and clustering power spectra, as well as the
constraints on cosmological parameters that can be obtained from the
determinations of these quantities. The novel data relevant to derive the first
sub-millimeter estimates of the local luminosity functions of starburst and
late-type galaxies, and the constraints on the properties of rare source
populations, such as blazars, are also briefly described.Comment: 16 pages, 10 figures. Accepted for publication on MNRA
Polarization Properties of Extragalactic Radio Sources and Their Contribution to Microwave Polarization Fluctuations
We investigate the statistical properties of the polarized emission of
extragalactic radio sources and estimate their contribution to the power
spectrum of polarization fluctuations in the microwave region. The basic
ingredients of our analysis are the NVSS polarization data, the multifrequency
study of polarization properties of the B3-VLA sample (Mack et al. 2002) which
has allowed us to quantify Faraday depolarization effects, and the 15 GHz
survey by Taylor et al. (2001), which has provided strong constraints on the
high-frequency spectral indices of sources. The polarization degree of both
steep- and flat-spectrum at 1.4 GHz is found to be anti-correlated with the
flux density. The median polarization degree at 1.4 GHz of both steep- and
flat-spectrum sources brighter than mJy is . The data by Mack et al. (2002) indicate a substantial mean Faraday
depolarization at 1.4 GHz for steep spectrum sources, while the depolarization
is undetermined for most flat/inverted-spectrum sources. Exploiting this
complex of information we have estimated the power spectrum of polarization
fluctuations due to extragalactic radio sources at microwave frequencies. We
confirm that extragalactic sources are expected to be the main contaminant of
Cosmic Microwave Background (CMB) polarization maps on small angular scales. At
frequencies GHz the amplitude of their power spectrum is expected to be
comparable to that of the -mode of the CMB. At higher frequencies, however,
the CMB dominates.Comment: 10 pages, A&A in pres
A multifrequency angular power spectrum analysis of the Leiden polarization surveys
The Galactic synchrotron emission is expected to be the most relevant source
of astrophysical contamination in cosmic microwave background polarization
measurements, at least at frequencies 30'. We
present a multifrequency analysis of the Leiden surveys, linear polarization
surveys covering essentially the Northern Celestial Hemisphere at five
frequencies between 408 MHz and 1411 MHz. By implementing specific
interpolation methods to deal with these irregularly sampled data, we produced
maps of the polarized diffuse Galactic radio emission with pixel size of 0.92
deg. We derived the angular power spectrum (APS) (PI, E, and B modes) of the
synchrotron dominated radio emission as function of the multipole, l. We
considered the whole covered region and some patches at different Galactic
latitudes. By fitting the APS in terms of power laws (C_l = k l^a), we found
spectral indices that steepen with increasing frequency: from a = -(1-1.5) at
408 MHz to a = -(2-3) at 1411 MHz for 10 < l < 100 and from a = -0.7 to a =
-1.5 for lower multipoles (the exact values depending on the considered sky
region and polarization mode). The bulk of this steepening can be interpreted
in terms of Faraday depolarization effects. We then considered the APS at
various fixed multipoles and its frequency dependence. Using the APSs of the
Leiden surveys at 820 MHz and 1411 MHz, we determined possible ranges for the
rotation measure, RM, in the simple case of an interstellar medium slab model.
Taking also into account the polarization degree at 1.4 GHz, we could break the
degeneracy between the identified RM intervals. The most reasonable of them
turned out to be RM = 9-17 rad/m^2.Comment: 18 pages, 14 figures. Astronomy and Astrophysics, in pres
Cosmic microwave background: polarization and temperature anisotropies from symmetric structures
I consider the case of anisotropies in the Cosmic Microwave Background (CMB)
from one single ordered perturbation source, or seed, existing well before
decoupling between matter and radiation. Such structures could have been left
by high energy symmetries breaking in the early universe.
I focus on the cases of spherical and cylindrical symmetry of the seed. I
give general analytic expressions for the polarization and temperature linear
perturbations, factoring out of the Fourier integral the dependence on the
photon propagation direction and on the geometric coordinates describing the
seed. I show how the CMB perturbations manifestly reflect the symmetries of
their seeds. CMB anisotropies are obtained with a line of sight integration.
This treatment highlights the undulatory properties of the CMB. I show with
numerical examples how the polarization and temperature perturbations propagate
beyond the size of their seeds, reaching the CMB sound horizon at the time
considered. Just like the waves from a pebble thrown in a pond, CMB anisotropy
from a seed intersecting the last scattering surface appears as a series of
temperature and polarization waves surrounding the seed, extending on the scale
of the CMB sound horizon at decoupling, roughly in the sky. Each wave
is characterized by its own value of the CMB perturbation, with the same mean
amplitude of the signal coming from the seed interior.
These waves could allow to distinguish relics from high energy processes of
the early universe from point-like astrophysical sources, because of their
angular extension and amplitude. Also, the marked analogy between polarization
and temperature signals offers cross correlation possibilities for the future
Planck Surveyor observations.Comment: 21 pages, seven postscript figures, final version accepted for
publication in Phys.Rev.
Correlated Component Analysis for diffuse component separation with error estimation on simulated Planck polarization data
We present a data analysis pipeline for CMB polarization experiments, running
from multi-frequency maps to the power spectra. We focus mainly on component
separation and, for the first time, we work out the covariance matrix
accounting for errors associated to the separation itself. This allows us to
propagate such errors and evaluate their contributions to the uncertainties on
the final products.The pipeline is optimized for intermediate and small scales,
but could be easily extended to lower multipoles. We exploit realistic
simulations of the sky, tailored for the Planck mission. The component
separation is achieved by exploiting the Correlated Component Analysis in the
harmonic domain, that we demonstrate to be superior to the real-space
application (Bonaldi et al. 2006). We present two techniques to estimate the
uncertainties on the spectral parameters of the separated components. The
component separation errors are then propagated by means of Monte Carlo
simulations to obtain the corresponding contributions to uncertainties on the
component maps and on the CMB power spectra. For the Planck polarization case
they are found to be subdominant compared to noise.Comment: 17 pages, accepted in MNRA
Quintessence and cosmic acceleration
A cosmological model with perfect fluid and self-interacting quintessence
field is considered in the framework of the spatially flat
Friedmann-Robertson-Walker (FRW) geometry. By assuming that all physical
quantities depend on the volume scale factor of the Universe, the general
solution of the gravitational field equations can be expressed in an exact
parametric form. The quintessence field is a free parameter. With an
appropriate choice of the scalar field a class of exact solutions is obtained,
with an exponential type scalar field potential fixed via the gravitational
field equations. The general physical behavior of the model is consistent with
the recent cosmological scenario favored by supernova Type Ia observations,
indicating an accelerated expansion of the Universe.Comment: 6 pages, 3 figures, to appear in Int. J. Mod. Phys.
Estimating the spectral indices of correlated astrophysical foregrounds by a second-order statistical approach
We present the first tests of a new method, the Correlated Component Analysis
(CCA) based on second-order statistics, to estimate the mixing matrix, a key
ingredient to separate astrophysical foregrounds superimposed to the Cosmic
Microwave Background (CMB). In the present application, the mixing matrix is
parameterized in terms of the spectral indices of Galactic synchrotron and
thermal dust emissions, while the free-free spectral index is prescribed by
basic physics, and is thus assumed to be known. We consider simulated
observations of the microwave sky with angular resolution and white stationary
noise at the nominal levels for the PLANCK satellite, and realistic foreground
emissions, with a position dependent synchrotron spectral index. We work with
two sets of PLANCK frequency channels: the low frequency set, from 30 to 143
GHz, complemented with the Haslam 408 MHz map, and the high frequency set, from
217 to 545 GHz. The concentration of intense free-free emission on the Galactic
plane introduces a steep dependence of the spectral index of the global
Galactic emission with Galactic latitude, close to the Galactic equator. This
feature makes difficult for the CCA to recover the synchrotron spectral index
in this region, given the limited angular resolution of PLANCK, especially at
low frequencies. A cut of a narrow strip around the Galactic equator (|b|<3
deg), however, allows us to overcome this problem. We show that, once this
strip is removed, the CCA allows an effective foreground subtraction, with
residual uncertainties inducing a minor contribution to errors on the recovered
CMB power spectrum.Comment: 9 pages, 5 figures and 1 table accepted by MNRA
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