6,871 research outputs found

### Non-Gaussianity from extragalactic point-sources

The population of compact extragalactic sources contribute to the
non-Gaussianity at Cosmic Microwave Background frequencies. We study their
non-Gaussianity using publicly available full-sky simulations. We introduce a
parametrisation to visualise efficiently the bispectrum and we describe the
scale and frequency dependences of the bispectrum of radio and IR
point-sources. We show that the bispectrum is well fitted by an analytical
prescription. We find that the clustering of IR sources enhances their
non-Gaussianity by several orders of magnitude, and that their bispectrum peaks
in the squeezed triangles. Examining the impact of these sources on primordial
non-Gaussianity estimation, we find that radio sources yield an important
positive bias to local fNL at low frequencies but this bias is efficiently
reduced by masking detectable sources. IR sources produce a negative bias at
high frequencies, which is not dimmed by the masking, as their clustering is
dominated by faint sources.Comment: 4pages, 2 figures, 2 tables. Contribution to the proceedings of the
International Conference on Gravitation and Cosmology, Goa, December 201

### Measurement of anisotropies in the large-scale diffuse gamma-ray emission

We have performed the first measurement of the angular power spectrum in the
large-scale diffuse emission at energies from 1-50 GeV. We compared results
from data and a simulated model in order to identify significant differences in
anisotropy properties. We found angular power above the photon noise level in
the data at multipoles greater than ~ 100 for energies 1< E <10 GeV. The excess
power in the data suggests a contribution from a point source population not
present in the model.Comment: Contribution to SciNeGHE 2010, Trieste, Italy, September 2010; 4
pages, 5 figure

### Constraints on primordial magnetic fields from the optical depth of the cosmic microwave background

Damping of magnetic fields via ambipolar diffusion and decay of
magnetohydrodynamical (MHD) turbulence in the post decoupling era heats the
intergalactic medium (IGM). Delayed recombination of hydrogen atoms in the IGM
yields an optical depth to scattering of the cosmic microwave background (CMB).
The optical depth generated at $z\gg 10$ does not affect the "reionization
bump" of the CMB polarization power spectrum at low multipoles, but affects the
temperature and polarization power spectra at high multipoles. Writing the
present-day energy density of fields smoothed over the damping scale at the
decoupling epoch as $\rho_{B,0}=B_{0}^2/2$, we constrain $B_0$ as a function of
the spectral index, $n_B$. Using the Planck 2013 likelihood code that uses the
Planck temperature and lensing data together with the WMAP 9-year polarization
data, we find the 95% upper bounds of $B_0<0.63$, 0.39, and 0.18~nG for
$n_B=-2.9$, $-2.5$, and $-1.5$, respectively. For these spectral indices, the
optical depth is dominated by dissipation of the decaying MHD turbulence that
occurs shortly after the decoupling epoch. Our limits are stronger than the
previous limits ignoring the effects of the fields on ionization history.
Inverse Compton scattering of CMB photons off electrons in the heated IGM
distorts the thermal spectrum of CMB. Our limits on $B_0$ imply that the
$y$-type distortion from dissipation of fields in the post decoupling era
should be smaller than $10^{-9}$, $4\times10^{-9}$, and $10^{-9}$,
respectively.Comment: 14 pages, 30 figures, calculations revised and updated, accepted for
publication in JCA

### Cosmological Constraints on Horndeski Gravity in Light of GW170817

The discovery of the electromagnetic counterpart to GW170817 severely
constrains the tensor mode propagation speed, eliminating a large model space
of Horndeski theory. We use the cosmic microwave background data from Planck
and the joint analysis of the BICEP2/Keck Array and Planck, galaxy clustering
data from the SDSS LRG survey, BOSS baryon acoustic oscillation data, and
redshift space distortion measurements to place constraints on the remaining
Horndeski parameters. We evolve the Horndeski parameters as power laws with
both the amplitude and power law index free. We find a 95% CL upper bound on
the present-day coefficient of the Hubble friction term in the cosmological
propagation of gravitational waves is 2.38, whereas General Relativity gives 2
at all times. While an enhanced friction suppresses the amplitude of the
reionization bump of the primordial B-mode power spectrum at $\ell < 10$, our
result limits the suppression to be less than 0.8%. This constraint is
primarily due to the scalar integrated Sachs-Wolfe effect in temperature
fluctuations at low multipoles.Comment: 23 pages, 10 figures. Version accepted for publication in JCA

### Cosmic String Power Spectrum, Bispectrum and Trispectrum

We use analytic calculations of the post-recombination gravitational effects
of cosmic strings to estimate the resulting CMB power spectrum, bispectrum and
trispectrum. We place a particular emphasis on multipole regimes relevant for
forthcoming CMB experiments, notably the Planck satellite. These calculations
use a flat sky approximation, generalising previous work by integrating string
contributions from last scattering to the present day, finding the dominant
contributions to the correlators for multipoles l > 50. We find a well-behaved
shape for the string bispectrum (without divergences) which is easily
distinguishable from the inflationary bispectra which possess significant
acoustic peaks. We estimate that the nonlinearity parameter characterising the
bispectrum is approximately f_NL \sim -20 (given present string constraints
from the CMB power spectrum. We also apply these unequal time correlator
methods to calculate the trispectrum for parrallelogram configurations, again
valid over a large range of angular scales relevant for WMAP and Planck, as
well as on very small angular scales. We find that, unlike the bispectrum which
is suppressed by symmetry considerations, the trispectrum for cosmic strings is
large. Our current estimate for the trispectrum parameter is tau_NL \sim 10^5,
which may provide one of the strongest constraints on the string model as
estimators for the trispectrum are developed

### Scale-dependent bias of galaxies and mu-type distortion of the cosmic microwave background spectrum from single-field inflation with a modified initial state

We investigate the phenomenological consequences of a modification of the
initial state of a single inflationary field. While single-field inflation with
the standard Bunch-Davies initial vacuum state does not generally produce a
measurable three-point function (bispectrum) in the squeezed configuration,
allowing for a non-standard initial state produces an exception. Here, we
calculate the signature of an initial state modification in single-field
slow-roll inflation in both the scale-dependent bias of the large-scale
structure (LSS) and mu-type distortion in the black-body spectrum of the cosmic
microwave background (CMB). We parametrize the initial state modifications and
identify certain choices of parameters as natural, though we also note some
fine-tuned choices that can yield a larger bispectrum. In both cases, we
observe a distinctive k^-3 signature in LSS (as opposed to k^-2 for the
local-form). As a non-zero bispectrum in the squeezed configuration correlates
a long-wavelength mode with two short-wavelength modes, it induces a
correlation between the CMB temperature anisotropy on large scales with the
temperature-anisotropy-squared on very small scales; this correlation persists
as the small-scale anisotropy-squared is processed into mu-type distortions.
While the local-form mu-distortion turns out to be too small to detect in the
near future, a modified initial vacuum state enhances the signal by a large
factor owing to an extra factor of k_1/k. For example, a proposed
absolutely-calibrated experiment, PIXIE, is expected to detect this correlation
with a signal-to-noise ratio greater than 10, for an occupation number of about
0.5 in the observable modes. Relatively calibrated experiments such as Planck
and LiteBIRD should also be able to measure this effect, provided that the
relative calibration between different frequencies meets the required
precision. (Abridged)Comment: 14 pages, 6 figures. Matches version in PRD. Improved explanation in
Sec. IV; added references and corrected typo

### Non-Gaussianity from Inflation: Theory and Observations

This is a review of models of inflation and of their predictions for the
primordial non-Gaussianity in the density perturbations which are thought to be
at the origin of structures in the Universe. Non-Gaussianity emerges as a key
observable to discriminate among competing scenarios for the generation of
cosmological perturbations and is one of the primary targets of present and
future Cosmic Microwave Background satellite missions. We give a detailed
presentation of the state-of-the-art of the subject of non-Gaussianity, both
from the theoretical and the observational point of view, and provide all the
tools necessary to compute at second order in perturbation theory the level of
non-Gaussianity in any model of cosmological perturbations. We discuss the new
wave of models of inflation, which are firmly rooted in modern particle physics
theory and predict a significant amount of non-Gaussianity. The review is
addressed to both astrophysicists and particle physicists and contains useful
tables which summarize the theoretical and observational results regarding
non-Gaussianity.Comment: LaTeX file: 218 pages, 19 figures. Replaced to match the accepted
version in Physics Reports. A high-resolution version of Fig. 2 can be
downloaded from: http://www.pd.infn.it/~liguori/Non_Gaussianity

### Improved constraints on primordial non-Gaussianity for the Wilkinson Microwave Anisotropy Probe 5-yr data

We present new constraints on the non-linear coupling parameter fnl with the
Wilkinson Microwave Anisotropy Probe (WMAP) data. We use an updated method
based on the spherical Mexican hat wavelet (SMHW) which provides improved
constraints on the fnl parameter. This paper is a continuation of a previous
work by Curto et al. where several third order statistics based on the SMHW
were considered. In this paper, we use all the possible third order statistics
computed from the wavelet coefficient maps evaluated at 12 angular scales. The
scales are logarithmically distributed from 6.9 arcmin to 500 arcmin. Our
analysis indicates that fnl is constrained to -18 < fnl < +80 at 95% confidence
level (CL) for the combined V+W WMAP map. This value has been corrected by the
presence of undetected point sources, which adds a positive contribution of
Delta_fnl = 6 +- 5. Our result excludes at ~99% CL the best-fitting value
fnl=87 reported by Yadav & Wandelt. We have also constrained fnl for the Q, V
and W frequency bands separately, finding compatibility with zero at 95 % CL
for the Q and V bands but not for the W band. We have performed some further
tests to understand the cause of this deviation which indicate that systematics
associated to the W radiometers could be responsible for this result. Finally
we have performed a Galactic North-South analysis for fnl. We have not found
any asymmetry, i.e. the best-fitting fnl for the northern pixels is compatible
with the best-fitting fnl for the southern pixels.Comment: 6 pages, 4 figures, 4 tables. Accepted for publication in Ap

### Modeling Intra-Cluster Gas in Triaxial Dark Halos : An Analytical Approach

We present the first physical model for the non-spherical intra-cluster gas
distribution in hydrostatic equilibrium under the gravity of triaxial dark
matter halos. Adopting the concentric triaxial density profiles of the dark
halos with constant axis ratios proposed by Jing & Suto (2002), we derive an
analytical expression for the triaxial halo potential on the basis of the
perturbation theory, and find the hydrostatic solutions for the gas density and
temperature profiles both in isothermal and polytropic equations of state. The
resulting iso-potential surfaces are well approximated by triaxial ellipsoids
with the eccentricities dependent on the radial distance. We also find a
formula for the eccentricity ratio between the intra-cluster gas and the
underlying dark halo. Our results allow one to determine the shapes of the
underlying dark halos from the observed intra-cluster gas through the X-ray
and/or the Sunyaev-Zel'dovich effects clusters.Comment: accepted by ApJ, LaTex file, 22 pages, 8 postscript figure

- …