94 research outputs found
CMB Anisotropies, Cosmological Parameters and Fundamental Physics: Current Status & Perspectives
I describe briefly the Cosmic Microwave Background (hereafter CMB) physics
which explains why high accuracy observations of its spatial structure are a
unique observational tool both for the determination of the global cosmological
parameters and to constrain observationally the physics of the early universe.
I also briefly survey the many experiments which have measured the anisotropies
of the CMB and led to crucial advances in observational Cosmology. The somewhat
frantic series of new results has recently culminated with the outcome of the
WMAP satellite which confirmed earlier results, set new standards of accuracy,
and suggested that the Universe may have reionised earlier than anticipated.
Many more CMB experiments are currently taking data or being planned, with the
Planck satellite on the 2007 Horizon poised to extract all the cosmological
information in the temperature anisotropies, and foray deeply into
polarisation.Comment: To appear in the proceedings of "Where Cosmology and Fundamental
Physics Meet", 23-26 June, 2003, Marseille, Franc
Large scale CMB anomalies from thawing cosmic strings
Cosmic strings formed during inflation are expected to be either diluted over
super-Hubble distances, i.e., invisible today, or to have crossed our past
light cone very recently. We discuss the latter situation in which a few
strings imprint their signature in the Cosmic Microwave Background (CMB)
Anisotropies after recombination. Being almost frozen in the Hubble flow, these
strings are quasi static and evade almost all of the previously derived
constraints on their tension while being able to source large scale
anisotropies in the CMB sky. Using a local variance estimator on thousand of
numerically simulated Nambu-Goto all sky maps, we compute the expected signal
and show that it can mimic a dipole modulation at large angular scales while
being negligible at small angles. Interestingly, such a scenario generically
produces one cold spot from the thawing of a cosmic string loop. Mixed with
anisotropies of inflationary origin, we find that a few strings of tension GU =
O(1) x 10^(-6) match the amplitude of the dipole modulation reported in the
Planck satellite measurements and could be at the origin of other large scale
anomalies.Comment: 23 pages, 11 figures, uses jcappub. References added, matches
published versio
The Structure and Dynamical Evolution of Dark Matter Halos
(Shortened) We use N-body simulations to investigate the structure and
dynamical evolution of dark matter halos in galaxy clusters. Our sample
consists of nine massive halos from an EdS universe with scale free power
spectrum and n = -1. Halos are resolved by ~20000 particles each, with a
dynamical resolution of 20-25 kpc. Large scale tidal fields are included up to
L=150 Mpc using background particles. The halo formation process can be
characterized by the alternation of two dynamical configurations: a merging
phase and a relaxation phase, defined by their signature on the evolution of
the total mass and rms velocity. Halos spend on average one 1/3 of their
evolution in the merging phase and 2/3 in the relaxation phase. Using this
definition, we study the density profiles and their change during the halo
history. The average density profiles are fitted by the NFW analytical model
with an rms residual of 17% between the virial radius Rv and 0.01 Rv. The
Hernquist (1990) profiles fits the same halos with an rms residual of 26%. The
trend with mass of the scale radius of these fits is marginally consistent with
that found by Cole & Lacey (1996): in comparison our halos are more centrally
concentrated, and the relation between scale radius and halo mass is slightly
steeper. We find a moderately large scatter in this relation, due both to
dynamical evolution within halos and to fluctuations in the halo population. We
analyze the dynamical equilibrium of our halos using the Jeans' equation, and
find that on average they are approximately in equilibrium within their virial
radius. Finally, we find that the projected mass profiles of our simulated
halos are in very good agreement with the profiles of three rich galaxy
clusters derived from strong and weak gravitational lensing observations.Comment: 20 pages, Latex, with all figures included. Modified to match the
published versio
Kurtosis of Large-Scale Cosmic Fields
An attractive and simple hypothesis for the formation of large-scale
structure is that it developed by gravitational instability from primordial
fluctuations with an initially Gaussian probability distribution. Non-linear
gravitational evolution drives the distribution away from the Gaussian form,
generating measurable skewness and kurtosis even when the variance of the
fluctuations is much smaller than unity. We use perturbation theory to compute
the kurtosis of the mass density field and the velocity divergence field that
arises during the weakly non-linear evolution of initially Gaussian
fluctuations. We adopt an Einstein--de~Sitter universe for the perturbative
calculations, and we discuss the generalization to a universe of arbitrary
. We obtain semi-analytic results for the case of scale-free, power-law
spectra of the initial fluctuations and final smoothing of cosmic fields with a
Gaussian filter. We also give an exact analytical formula for the dependence of
the skewness of these fields on the power spectrum index. We show that the
kurtosis decreases with the power spectrum index, and we compare our more
accurate results for the kurtosis to previous estimates from Monte Carlo
integrations. We also compare our results to values obtained from cosmological
N-body simulations with power-law initial spectra. Measurements of the skewness
and kurtosis parameters can be used to test the hypothesis that structure in
the universe formed by gravitational instability from Gaussian initial
conditions.Comment: 29 pp incl. 8 figs, uuencoded compressed postscript, submitted to
MNRAS, preprints CAMK/281, IASSNS-AST 94/3
Probing inflation with CMB polarization : weak lensing effect on the covariance of CMB spectra
CMB anisotropies are modified by the weak lensing effect of intervening large
scale structures on the photon path from the last scattering surface to the
observer. This has to be accounted for when observational data of sensitive
experiments are used to constrain cosmological models. A common approximation
to analyze the CMB angular power spectra is to include only the Gaussian part
of the lensing correction and to ignore the non-gaussian terms in the error
covariance matrix of the spectra. In order to investigate the validity of this
approximation, we computed these non-Gaussian terms by using a perturbative
expansion method. We present a graphical method to write down any N-point
correlation functions at any order in lensing. We use a pedagogical approach to
demonstrate that neglecting non-gaussian terms is an accurate approximation for
all polarizations but B, and it will remain so even for the analysis of very
sensitive post-Planck experiments. For the B polarization, non-gaussian
contributions up to order 4 must be taken into account.Comment: 29 pages, 13 figures, JCAP style. Improved to match published version
: annex A expanded, references added, few minor correction
Previrialization: Perturbative and N-Body Results
We present a series of N-body experiments which confirm the reality of the
previrialization effect. We also use weakly nonlinear perturbative approach to
study the phenomenon. These two approaches agree when the rms density contrast,
, is small; more surprisingly, they remain in agreement when . When the slope of the initial power spectrum is , nonlinear
tidal interactions slow down the growth of density fluctuations and the
magnitude of the suppression increases when (i.e. the relative amount of
small scale power) is increased. For we see an opposite effect: the
fluctuations grow more rapidly than in linear theory. The transition occurs at
when the weakly nonlinear correction to is close to zero and
the growth rate is close to linear. Our results resolve recent controversy
between two N-body studies of previrialization. Peebles (1990) assumed
and found strong evidence in support of previrialization, while Evrard \& Crone
(1992), who assumed , reached opposite conclusions. As we show here, the
initial conditions with are rather special because the nonlinear effects
nearly cancel out for that particular spectrum. In addition to our calculations
for scale-free initial spectra, we show results for a more realistic spectrum
of Peacock \& Dodds (1994). Its slope near the scale usually adopted for
normalization is close to , so is close to linear. Our results
retroactively justify linear normalization at 8 Mpc, while also
demonstrating the danger and limitations of this practice.Comment: Significantly revised, 25 pages, uuencoded compressed postscript,
figures included, to appear in Ap
Contribution of point sources to the soft gamma-ray Galactic emission
The nature of the soft gamma-ray (20-200 keV) Galactic emission has been a
matter of debate for a long time. Previous experiments have tried to separate
the point source contribution from the real interstellar emission, but with a
rather poor spatial resolution, they concluded that the interstellar emission
could be a large fraction of the total Galactic emission. INTEGRAL, having both
high resolution and high sensitivity, is well suited to reassess more precisely
this problem. Using the INTEGRAL core program Galactic Center Deep Exposure
(GCDE), we estimate the contribution of detected point sources to the total
Galactic flux.Comment: Proceedings of the 5th INTEGRAL Workshop, Munich 16-20 February 2004.
ESA SP-55
Probing CMB Non-Gaussianity Using Local Curvature
It is possible to classify pixels of a smoothed cosmic microwave background
(CMB) fluctuation map according to their local curvature in ``hill'', ``lake''
and ``saddle'' regions. In the Gaussian case, fractional areas occupied by
pixels of each kind can be computed analytically for families of excursion sets
as functions of threshold and moments of the fluctuation power spectrum. We
show how the shape of these functions can be used to constrain accurately the
level of non-Gaussianity in the data by applying these new statistics to an
hypothetical mixed model suggested by Bouchet et al. (2001). According to our
simple test, with only one 12.5x12.5 deg^2 map, Planck should be able to detect
with a high significance a non-Gaussian level as weak as 10% in temperature
standard deviation (rms) (5% in C_l), whereas a marginal detection would be
possible for MAP with a non-Gaussian level around 30% in temperature (15% in
C_l).Comment: 11 pages, 13 figures, submitted to MNRA
Moments of the Counts Distribution in the 1.2 Jy IRAS Redshift Survey
We derive the volume-averaged 2, 3, 4, and 5-point correlation functions from
the moments of the Count probability distribution function of a redshift survey
of IRAS galaxies, and find them all to be reasonably well-described by power
laws. Weak systematic effects with the sample size provide evidence for
stronger clustering of galaxies of higher luminosity on small scales.
Nevertheless, remarkably tight relationships hold between the correlation
functions of different order. In particular, the ``normalized" skewness defined
by the ratio varies at most weakly with
scale in the range . That is, is close to
constant () from weakly to strongly non-linear scales.
Furthermore, we find that the void probability function obeys a scaling
relation with density to great precision, in accord with the scale-invariance
hypothesis ().Comment: 38 pages, postscript file (1.3 Megabytes) . IAS preprint number AST
93/2
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